How Does the Weather Affect You

Use this table through the week to record how the weather has an impact on your life:

 

Sunday

Monday

Tuesday

Wednesday

Thursday

Friday

Saturday

What was the Weather Like?

 

 

 

 

 

 

 

How did the weather affect what you wore?

 

 

 

 

 

 

 

How did the weather affect what you ate?

 

 

 

 

 

 

 

How did the weather affect what you drank?

 

 

 

 

 

 

 

How did the weather affect how you travelled?

 

 

 

 

 

 

 

How did the weather affect your health or how you felt?

 

 

 

 

 

 

 

How did the weather affect your school, work or leisure activities?

 

 

 

 

 

 

 

Classroom Behaviour and the Weather

Behaviour and the weather

Overview

This project aims to extend students’ ideas and knowledge on correlation using the Weather Observations Website (WOW) website. It focuses on looking at the possible link between the weather and behaviour in schools

The project is more suited for KS4 pupils but a high ability KS3 class could probably cope with its content. It involves pupils drawing scatter graphs or using spreadsheets if they have access to computers.

The ideas here can be taught in a few lessons using these resources or they can be made into a mini project lasting longer.

Teachers can adapt the ideas to suit their needs and tasks can be extended.

For example pupils could design a survey to collect information on behaviour in their own school and gather local weather data using the WOW website. It has possible cross curricula links with maths.

Objectives

To develop knowledge and understanding on correlation between two variables.

To investigate if there is a link between behaviour in schools and the weather.

To use the WOW website to gather data on past weather observations.

To design a survey to collect information on behaviour in your school.

To gain experience in recording data in tables and spreadsheets.

To build on pupils’ ability to draw and interpret graphs.

Introduction

In this task you are going to analyse the weather data for a certain town and establish if there is a correlation between weather and behaviour. For instance, do pupils behave better or worse if it is windy?

The behaviour of the pupils was judged by their teachers over four weeks in the month of March and their behaviour was given a score by their teachers on a 1 to 8 scale.

Behaviour scale

The behaviour scale is determined by the teacher with 1 being excellent behaviour from the class and 8 being behaviour that is seen to be unacceptable from that class for that teacher.

 Behaviour no. Behaviour shown
 1 No interruptions from the class
 2 Very few interruptions to the lesson
 3 When they are completing their own work some pupils get distracted
 4 A few pupils start to distract each other and lose focus for longer periods
 5 Level of noise starts to increase and more off task behaviour is seen
 6 Pupils are distracted from their work and find it difficult to work
 7 Lots of interruptions to the lesson from a range of pupils both in their own work and when listening to the teacher
 8 Constant interruptions to the lesson, unable to work in the lesson

Worksheet exercise

Ask the students to use the worksheet to draw a graph. If time and resources permit they can gather their own data from WOW. Alternatively they can use the data from the completed worksheet.

Worksheet 1

Answers for Teachers

Extracting the weather data from WOW 

[the WOW website has changed a little since these instructions were written but it should be clear how to access the data]
 

1. Go to the WOW website address and search for station 3034 or St-Athan.

2. Click on the St-Athan weather station on the map.

3. Click on ‘View Full Observation’.

4. Click on the Graph tab.

5. Click on the ‘Show Filters’ tab and then the ‘Filter Options’ drop down box. Select ‘Air Temperature’ and ‘Wind Speed’, set the date range to the first week of observations, then click Update Graph.

5. To obtain the wind speed readings – go to the correct day and estimate the wind speed reading at 12:00. Fill this in the table of results.

The reading on 27/07/2020 at 12:00 is 9 mph. So write 9 mph for the wind speed.

6. Repeat this for each day of the week and then reset the date range for the next week. Do this by clicking the ‘Show Filters’ tab and then the ‘Filter Options’ drop down box, then ‘Update graph’.

Web page reproduced with permission from the Met Office.

Pressure and Rainfall

Investigating the Link Between Between Pressure and Rainfall

Teachers Notes

Here is some data collected by a weather station on the outskirts of Edinburgh, at the start of 2019.

Date

Atmospheric Pressure (hPa)

Rainfall (mm)

10/12/2018

1025

0.0

11/12/2018

1020

0.0

12/12/2018

1019

0.0

13/12/2018

1022

0.0

14/12/2018

1017

0.0

15/12/2018

988

1.0

16/12/2018

1005

5.1

17/12/2018

1005

0.3

18/12/2018

996

1.5

19/12/2018

995

0.3

20/12/2018

995

0.5

21/12/2018

1000

0.5

22/12/2018

1014

0.0

23/12/2018

1027

0.0

24/12/2018

1032

0.3

25/12/2018

1026

0.3

26/12/2018

1023

0.0

27/12/2018

1023

0.0

28/12/2018

1022

0.0

29/12/2018

1030

2.3

30/12/2018

1030

0.3

31/12/2018

1026

0.0

01/01/2019

1044

0.0

02/01/2019

1043

0.0

03/01/2019

1041

0.0

04/01/2019

1039

0.0

05/01/2019

1034

0.0

06/01/2019

1031

1.0

07/01/2019

1024

0.0

08/01/2019

1033

0.0

09/01/2019

1031

0.0

Using this data, draw a graph of rainfall against pressure.

graph paper

Now use this information to complete the following sentences:

The most it rained in one day was _______________mm.

It didn’t rain at all on ____________ days.

The highest pressure recorded was ______________hPa (a hPa is the same as a millibar).

The lowest pressure recorded was _______________hPa.

Does it always rain when the pressure is low? Use figures to justify your answer.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Does it ever rain when the pressure is high? Use figures to justify your answer.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Many weather apps assume that if the pressure is low, it will rain. Does your graph justify this assumption?

_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

 

 

Extension:

Here are the weather maps for 4 of the days when it rained: the first 3 show when the pressure was low and the 4th shows when the pressure was high and it rained.

1)

2)

3)

4)

Air Masses: Case Studies

Pick one of the synoptic charts below.  Can you work out where the wind over the UK is coming from? Try to ignore any fronts, and don’t think about how things might have changed in the past or be about to change in the future. 

Now answer the following questions: 

What is the wind direction over the UK?_______________

What is the air mass affecting the UK?_________________

Describe the weather, in terms of wind speed, direction, temperature, cloud and precipitation.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Would you expect any difference in the weather between day and night? __________________________________________________________________________________________________

Would you expect any difference in the weather between the sea/ the windward coast and inland regions? ____________________________________________________________________________________________________________________________________________________________

Synoptic Chart February 2015

February 2015

Synoptic Chart February 2018

February 2018

Synoptic Chart May 2020

May 2020

Synoptic Chart October 2011

October 2011

Syoptic Chart October 2017

October 2017

November 2010 synoptic chart

November 2010

Depressions, Anticyclones and Fronts

Passage of a Depression – interactive animation

Worksheet to accompany the animation.

For 11+

Depressions from our Weather and Climate teacher’s guide

Pop-up depression

Cold and Warm fronts activities for differentiation and revision

Finding weather features on a simple synoptic chart

Red sky at Night, Shepherd’s Delight worksheet and Teacher’s Notes – a resource looking at how our prevailing wind direction means this saying is largely true.

Depressions Taboo

For 14+

Weather systems PowerPoints and cross section practice

Using WOW data to investigate a depression passing across the UK with  worksheets for students including isoline drawing practice.

Anticyclones, depressions and fronts with student worksheets 

Depressions and anticyclones with a synoptic chart exercise

A case study of orographic rainfall in Scotland.

What is the weather? Work out what the weather is like at several UK locations based on some simplified weather maps.

Interpreting weather charts basic information on synoptic charts, with Isotherm map exercise and Synoptic chart exercise.

Isotherm and Isobar drawing exercise based on a depression on our contour resources page.

For 16+

Weather Projects

Introduction

Project ideas:

1. How accurate are weather forecasts for my local area?
2. A survey of how the temperature changes in my back garden
3. An analysis of temperature patterns across a town/city
4. How do wind patterns vary around a large building?
5. How do temperatures vary inside and outside a woodland area?
6. How much precipitation is intercepted in a woodland area?
7. How does the weather change as a depression/warm front/cold front passes over?
8. A study of the shelter effect of trees/hedges
9. How do air temperatures change as you move up a hillside?
10. How do temperatures change as you move inland from the sea/coast

Introduction

These pages give practical advice for pupils and teachers on weather-related projects that could be undertaken. In addition, there is general guidance and advice on equipment that pupils can use at home, at school or out in the field.

General points for teachers when giving advice on weather-related projects

It is always a good idea to encourage more able pupils by adding in the variables of seasonal change or different pressure patterns. Even the simplest project, such as Project 1 on weather forecasts, can be extended to see if the forecasts become more accurate under high pressure.

Practice runs beforehand are ideal and strongly recommended – they give pupils valuable practice with unfamiliar equipment and can help to both identify and iron out potential problems at certain sites. From experience, this gives pupils scope for making extremely good points in the evaluation section of their project.

A 10- to 14-day collection period is advised for many of these projects. Less than 10 days can cause problems with abnormal readings. If the pupils are prepared to take readings for up to 21 days, then let them do so.

The use of maximum-minimum thermometers is the one area where erroneous data can be produced. In theory, their use should be straightforward, but in practice, pupils may not read from the right place, or reset the thermometers. These points should be stressed, especially if their friends or family are making the readings – do not assume that parents know how to use the maximum-minimum thermometer either.

Measuring precipitation using a manufactured rain gauge is no problem, but these can be expensive. In any case, many pupils prefer to make their own, but their design can lead to difficulties. Refer the pupils to Met Office guidelines on the correct size and conversion formulae (they are also in good textbooks). Pupils frequently use large plastic bottles, but both these and milk bottles may not be wide enough, so suggest a funnel is placed inside to make a wider opening – ideally it should be at least 125 mm in diameter. The collecting vessels should be designed to allow regular emptying and should be robust enough to withstand regular handling. If they split, leakage will occur and ruin the results. Pupils should be made aware of all these points and that even family pets can cause damage to the vessels.

With some of these projects, especially numbers 2, 4, 5, 6 and 8, pupils might want to consider the use of a control station. This can be used to spot sources of irregularities, and faulty equipment can be recalibrated. The school’s weather station or Stevenson screen is ideal for this. Having such a control will allow pupils to comment on their evaluation about having a real scientific method, and checking for sources of error in their observations.

If a standard household thermometer is being used, remember that it can take up to 15 minutes to settle and record the actual temperature at the site. When measuring wind speed, pupils should remember that gusts and lulls can occur. Holding up the anemometer for up to a minute or two can help to overcome this, and an average speed calculated for that period. If readings are being made alongside a busy road, the pupils should also remember that large vehicles can cause sudden gusts.

If there is no access to a good quality anemometer, you can buy ventimeters from sailing shops. These can give good readings.

1. How accurate are weather forecasts for my local area?

Equipment needed

A simple thermometer, anemometer (and compass?) and cloud recognition chart.

Pupil’s notes

This project involves collecting weather data each day, for a 10- to 14-day period, and comparing your readings with forecasts in the local newspaper or on web sites. Around midday you should record the air temperature, weather conditions, cloud cover, cloud type, wind speed and wind direction. If you have an automatic weather station at your school, you can use these readings and make your own observations on clouds and weather conditions. You should keep the weather forecasts that have been made, compare them each day with your readings, and then work out how accurate the forecasts have been. At the end of the time period, you can work out the overall accuracy level, and then suggest reasons for any differences.

accuracy

Teacher’s notes

In essence, this is a very simple project, but one which able pupils can extend by explaining the discrepancies between observations and forecasts, e.g. fronts moving faster than expected, the impact of local topography and the shelter effect of hills.

2. A survey of how the temperature changes in my back garden

Equipment needed

At least two thermometers – one for ground temperatures, and the other for air temperatures at 1.2 metres above the ground (possibly on a post). An anemometer (and compass?) would also be useful.

Pupil’s notes

This is a detailed survey of how temperature changes in your garden. You need to collect data each day (or even twice a day) for a 10- to 14-day period, recording the air and ground temperatures. You could also make a note of cloud cover and wind speed/direction. Cloud cover will help you explain unusual changes, e.g. temperatures may drop if skies have been clear at night. Similarly, knowing wind speed and especially direction, will help you explain temperature changes in terms of the prevailing air mass. If you are only measuring data at one place, you should take care to avoid shaded areas of your garden. You could set up several measuring points and see how temperature varies around your back garden, and then draw a chloropleth or isoline map to show the differences and patterns. Having more than one collection point would also allow you to calculate a daily average for your garden. Another extension would be collecting data twice a day, e.g. at 8 a.m. and 6 p.m.

Teacher’s notes

Pupils should use maximum-minimum thermometers and a fairly sensitive anemometer, but great care is needed in resetting the thermometers. More able candidates could collect weather maps from a broadsheet newspaper or the images and charts from the Met Office web site, and then relate the temperature changes to the passage of frontal systems across the area. Dramatic temperature changes can also occur under a blocking anticyclone where temperature inversions and ground frosts regularly develop. Pupils should therefore be encouraged to take careful note of the cloud cover and whether a ground frost occurs.

3. An analysis of temperature patterns across a town/city

Equipment needed

A digital thermometer or probe.

Pupil’s notes

Temperature changes across an urban area, and this project involves looking at these subtle changes, caused by different types of buildings or open spaces. You should make a transect across the urban area (from north to south, or east to west) taking readings at regular intervals every 500 metres, or you can choose a variety of locations all over the urban area. Ideally, you should have 10-15 sites which you can visit on foot, by bike or in a parent’s car. At each site, you should record the air temperature, holding your digital thermometer at the same height above the ground at each site. You should also make a description of the site – densely built up, low-density housing with gardens, open space, etc. You should repeat your survey at the same time over the next two or three days. Remember, you are not really after an average for each site, but checking whether the temperature changes in the same way at each site at different times. You can extend this project by visiting each site early in the morning, around noon and in the late afternoon.

Teacher’s notes

Help may be needed in deciding on the choice of observation site and direction of transect. The timing of the transect is also an important consideration, as urban heat islands are often most sharply defined in the early evening. Also remember that strong winds can equalise differences, so suggest that calm days are chosen. A basic household thermometer, or maximum-minimum thermometer should not be used, unless of course it is left at each site – help from school friends and relatives is a possibility. Digital thermometers will be the most accurate. Pupils could also collect weather maps from a broadsheet newspaper or the images and charts from the Met Office web site. Dramatic temperature changes can occur under a high-pressure system with little cloud cover, so that temperature inversions develop. Pupils should therefore be encouraged to take careful note of the cloud cover at the time of their transect. Very interesting patterns can be found if the transect crosses the central business district or a river valley. If the survey is being undertaken in a small town or large village, this project could be extended using data-collection points in the rural areas, so that differences between urban and rural areas are noticeable.

4. How do wind patterns vary around a large building?

Equipment needed

A good anemometer (and compass?).

Pupil’s notes

Wind speeds and directions can vary dramatically around buildings, especially tall tower blocks, large sports stadia or public buildings such as cathedrals. The wind can increase and swirl in unusual eddies as the air passes over and around the obstructions. You should identify a number of sites – 10 or 12 would be ideal – and try to get an even coverage around the building. Visit each site in turn, making a note of the wind direction and speed. You should try to visit the sites on mornings and/or afternoons for several days (possibly for up to a week). Although you can take an average wind speed and average direction for each data collection, it is even more interesting to notice the changes around the building, and you could answer the following questions. Where are wind speeds above average, and below average? Are the strongest winds always in the same place? In addition, the wind direction readings might help you spot where eddies are strongest.

well graph

Teacher’s notes

The key to this project is having a sensitive enough and/or fairly sophisticated anemometer. Some of the basic ones will not adequately measure light breezes. Having said this, very good results can be obtained near tower blocks, and more able pupils might be able to study the venturi effects produced, or the problems these faster winds cause, e.g. blowing litter around and low-level pollution. This project is very effective in winter and spring when low-pressure systems cross the country. It can be less effective under high pressure, so if the pupils are making these surveys in the summer holidays they should be made aware of these difficulties. This will prevent the embarrassment of them returning to school in August or September saying that the project did not work, or that there were never any winds! Speeds should be recorded in metres per second rather than by the Beaufort scale.

5. How do temperatures vary inside and outside a woodland area?

Equipment needed

Several maximum-minimum thermometers. At each site, you will need one for ground temperatures, and another for air temperatures at 1.2 metres above the ground (possibly on a post). You could use a digital thermometer rather than use several maximum-minimum thermometers. You can also use a light meter (see pupil’s notes).

Pupil’s notes

Air and ground temperatures will vary inside and outside a wood because of the vegetation and shade. To see how these change, choose one area inside the wood, and another up to 100 or 200 metres away, well out of shade. You should measure ground and air temperatures at each site over a 10- to 14-day period. If you are using a maximum-minimum thermometer, just one visit a day will be needed, whereas a digital thermometer will need reading each day at about 8 a.m. and at 6 p.m. It would also be useful to record the weather patterns and cloud cover at the time of the readings, as this may help explain unusual patterns, e.g. low temperatures early in the morning under clear skies. If you are using a digital thermometer, you could make a transect across the wood, taking readings every 50 metres or so. The vegetation also filters the solar radiation so that light intensity changes inside a wood. This can be measured using a light intensity meter or the light meter on a camera – if the latter is chosen, set the aperture to f8 and point the camera at the same object each time (a clipboard will suffice). The shutter speed will give a surrogate measure of light intensity, as the faster the shutter speed, the greater the light intensity.

Teacher’s notes

In order to obtain decent results, a fairly large copse or wood should be chosen, and the pupil should check that they can gain access beforehand. Maximum-minimum thermometers are ideal, but if they are not available, a digital thermometer can be used to record ‘real-time’ temperatures. This will entail the pupil visiting the sites at roughly the same time each day over the period – again an important fact that they need to be aware of before starting. From experience, maximum-minimum thermometers give more flexibility. More able candidates could also collect weather maps from a broadsheet newspaper or the images and charts from the Met Office web site. These will help explain any dramatic temperature changes that might occur under a blocking anticyclone, where temperature inversions and ground frosts regularly develop. Pupils should therefore be encouraged to take careful note of the cloud cover and whether there is a ground frost when they make their observations. If the readings are being made in a large wood, it is a good idea to encourage pupils to choose a variety of sites within the wood. Another extension is to compare the measurements from a wooded area with a variety of non-woodland sites, e.g. back garden, at school or in a built-up area. This could also lead to a more detailed project on temperature differences between urban and rural areas.

6. How much precipitation is intercepted in a woodland area?

Equipment needed

A simple rain gauge or collecting device. A simple thermometer might also be used (see pupil’s notes).

Pupil’s notes

Trees intercept rainfall, so this project is a variation on Project 5, whereby you need to place a number of rain gauges in and outside a woodland. You should choose a number of sites inside the wood, and at least one up to 100 or 200 metres away, well out of any shade. You should then measure the amount of rainfall at each site over a 10- to 14-day period. You could make readings several times a day if there is heavy rain. If so, it might be useful to monitor the temperature as well as cloud cover and type – these readings will help you work out if the rain is associated with the passage of a warm or cold front, etc. If your school has an automatic weather station with an electronic rain gauge, you can use this to work out the approximate time of your storm, the intensity and its duration. This will all help you answer questions such as whether more or less interception takes place in longer or heavier storms.

Kids rain gauge

Teacher’s notes

Potentially this can be a very good project, but problems can occur, chiefly with vandalism or the knocking over of the rain gauges. In addition, the summer months should be avoided as, in theory, there should be less rain! This is a good project at Easter or during the late spring when the trees are in leaf and there is a greater potential for interception. More-able pupils might nevertheless want to see how interception varies during the year, or in different seasons, and from experience, some very good projects have been undertaken on this topic. Another practical difficulty is that in very heavy storms, leaves are often battered down by the fast-falling raindrops. The best results are often obtained in steady rain.

7. How does the weather change as a depression/warm front/cold front passes over?

Equipment needed

Thermometers (preferably maximum-minimum), an anemometer (plus compass?), a cloud recognition chart and a barometer.

Pupil’s notes

Subtle changes occur in weather patterns as mature depressions move across Britain, especially with the passage of warm and cold fronts (plus occluded fronts), as well as the warm and cold sectors. You can observe these changes by setting up an observation station in your back garden or by using the school weather station or Stevenson screen. If you are making observations at home, take care to avoid shady areas in your back garden. To do this project effectively you should keep a close eye on weather charts in local or national newspapers, or the images on web sites, in order to see roughly when the depression and fronts will cross your home region. You will then need to carefully monitor the changes in air pressure, air temperatures, cloud cover, cloud type, wind speed, wind direction and weather conditions. Ideally, you should try to make recordings every two hours during a two- or even three-day period as the low-pressure system passes over. Satellite images and synoptic charts on the Met Office web site could be printed off to help explain the changes you observed in the weather patterns.

temp rain chart

Teacher’s notes

This is another project where data collection is quite straightforward, although accurate thermometers are needed, hence the preference for a digital one. Having said this, the regularity of making observations is crucial. Taking readings just twice or three times a day may not be sufficient. It is important that the pupils look at, and keep, the synoptic charts and weather maps from the broadsheets or web sites. More-able pupils should be able to see whether their changes fit the textbook models, and then explain any discrepancies. Another extension would be to add a rain gauge to measure precipitation as the fronts pass over. The regularity of data collection, every two hours, can be a difficulty, especially the night and early morning readings. Therefore, the data from the school’s automatic weather station can be substituted, with the pupils still collecting primary data by noting cloud cover, cloud type and weather conditions.

8. A study of the shelter effect of trees/hedges

Equipment needed

A digital thermometer or several thermometers, preferably maximum-minimum. At each site you will need two thermometers – one for ground temperatures, and the other for air temperatures at 1.2 metres above the ground (possibly on a post).

Pupil’s notes

Trees and hedges can have a shelter effect, causing temperatures, especially close to the ground, to change in a subtle way. For this project you should choose an area with woodland or one with thick, mature hedges. You could use your garden if it is quite large. Set up a line of evenly spaced measuring points where, if you are using maximum-minimum thermometers, you should place your measuring posts. Six or eight posts moving away from the hedge, or if possible on both sides of the hedge will be needed. Remember to ask permission to place these beforehand! If you are using a digital thermometer, place wooden pegs in the ground so you always measure at the same place. Take readings over a 10- to 14-day period at each observation post – if you are using a maximum- minimum thermometer, only one daily reading is needed, but if you are using a digital thermometer, you need to take readings around 8 a.m. and 6 p.m. It is also useful to make a note of cloud cover, as temperatures can fall very low under clear skies. Remember that it is the differences between air and ground temperatures at each site and as you move away from the obstacle, that are important, so take great care to read your thermometers accurately, and do not round up the temperatures on digital displays.

Teacher’s notes

This can be a really good project in a rural area or for pupils who live on farms. The choice of a back garden is adequate, as long as it is a good-sized one. If so, this could be combined with Project 2, to produce an isoline map of temperatures across a back garden, showing the shelter affect. More-able candidates could also collect weather maps from a broadsheet newspaper or the images and charts from the Met Office web site. These will help explain any dramatic temperature changes that might occur under a blocking anticyclone where temperature inversions and ground frosts regularly develop. Pupils should therefore be encouraged to take careful note of the cloud cover and whether there is a ground frost when they make their observations.

9. How do air temperatures change as you move up a hillside?

Equipment needed

A digital thermometer, while an anemometer and hygrometer are optional extras (see pupil’s notes).

Pupil’s notes

Air temperature decreases steadily as altitude increases, therefore a transect up a hillside or upland area can identify these changes. You will need 10-12 sites up the hillside, or along a main road. Ideally they should be at regular height intervals, so plot these beforehand using an Ordnance Survey map. Visit each site on foot, by bike, or in a parent’s car, and at each location accurately measure the air temperature, taking care not to round up the temperatures on the digital displays and trying to hold the digital thermometer at the same height above the ground at each location. You may find it useful to make a note of wind speeds and directions, because these may influence the changes, e.g. a cold down-valley wind. When you have finished, you can draw a scattergraph, showing the temperature changes, or thermal gradient, for your transect. You should repeat your transect several times, so that you can draw a series of thermal gradients, seeing whether the changes are always at the same rate between each site. It would also be worthwhile knowing the relative humidity for the area – this is because the amount of water vapour in the air can influence the rate of temperature change (ask your teacher to explain this!). So if you have access to a hygrometer it would be worth noting the readings. If not, use the information from your school’s automatic weather station or Stevenson screen. Some web sites also carry readings on relative humidity that you could use as well.

Teacher’s notes

This can be a very stimulating and interesting project, and a fruitful extension would be to measure temperatures on both the leeward and windward sides of the upland area. On the leeward slopes, a simple Föhn effect can sometimes be observed. It is essential that pupils do not round up the readings to whole degrees – going to two decimal places is a real bonus! More-able candidates should also be encouraged to gather the weather maps from a broadsheet newspaper or the images and charts from the Met Office web site for the days when they are making their transect. These will help explain any dramatic temperature changes that might occur under a blocking anticyclone where temperature inversions might affect the results, especially at the foot of the slope, so that for a while temperature increases with altitude. More-able pupils will also link humidity data with the lapse rates, and whether the saturated adiabatic lapse rate or the dry adiabatic lapse rate prevails.

10. How do temperatures change as one moves inland from the sea/coast?

Equipment needed

A digital thermometer and an anemometer (plus compass?).

Pupil’s notes

Air temperature changes as you move inland away from the sea, a large lake or reservoir. Water bodies can have a cooling effect in the summer months, and a warming effect in the winter. However, the patterns are influenced by the onshore or offshore breezes. This project requires a transect to be made inland away from the water body or coast. You will need 10-12 sites, possibly along a main road running away from the coast. Ideally they should be at regular height intervals, so plot these beforehand using an Ordnance Survey map. Visit each site on foot, by bike, or in a parent’s car, and at each location accurately measure the air temperature and the wind speed and direction. Take care not to round up the temperatures on the digital displays, and try to hold the digital thermometer and anemometer at the same height above the ground at each location. When you have finished, you can draw a scattergraph, showing the temperature changes, as well as a wind rose, for your transect. You should repeat your transect several times at roughly the same time of day, so that you can draw a series of thermal gradients, seeing whether the changes are always at the same rate between each site, and whether they differ depending on the type of breeze and its strength. Alternatively, you could repeat your transect several times a day to see the daily (diurnal) changes as the land or sea warms up and cools down.

Teacher’s notes

From experience, this is another good project for the summer months, or the mid-winter, and some very interesting patterns can occur under high pressure. Very good results can also be found if the transect is repeated at different times of the day, or year. It is important though for pupils to recognise the subtle differences between local breezes and the more-general prevailing winds – local breezes can create interesting small-scale patterns. Once again, pupils should be encouraged to gather the weather maps from a broadsheet newspaper or the images and charts from the Met Office web site for the days when they are making their transect. These will help to relate the micro-scale changes to the macro-scale patterns.

Web page reproduced with the kind permission of the Met Office

Case Study – Heatwave

The heatwave of 2003

More than 20,000 people died after a record-breaking heatwave left Europe sweltering in August 2003. The period of extreme heat is thought to be the warmest for up to 500 years, and many European countries experienced their highest temperatures on record.

Physical Impacts

Effects of the heatwave

Immediate responses to the heatwave

What happened to cause the heatwave?

Physical Impacts

Low river flows and lake levels
The River Danube in Serbia fell to its lowest level in 100 years. Bombs and tanks from World War 2, which had been submerged under water for decades, where revealed, causing a danger to people swimming in the rivers. Reservoirs and rivers used for public water supply and hydro-electric schemes either dried up or ran extremely low.

Forest fires
The lack of rainfall meant very dry conditions occurred over much of Europe. Forest fires broke out in many countries. In Portugal 215,000 hectares area of forest were destroyed. This is an area the same size as Luxembourg. It is estimated millions of tonnes of topsoil were eroded in the year after the fires as the protection of the forest cover was removed. This made river water quality poor when the ash and soil washed into rivers.

The satellite image shown in Fig. 1 shows forest fires in southern Portugal and Spain in September 2003. The fires are shown by the red dots and smoke is in white.

Melting glaciers
Extreme snow and glacier-melt in the European Alps led to increased rock and ice falls in the mountains.

Effects of the heatwave

About 15,000 people died due to the heat in France, which led to a shortage of space to store dead bodies in mortuaries. Temporary mortuaries were set up in refrigeration lorries. There were also heat-related deaths in the UK (2,000), Portugal (2,100), Italy (3,100), Holland (1,500) and Germany (300).

Human effects

  • Heat-stroke — normally we sweat, and this keeps us cool on hot days. On very hot days our bodies may not be able to keep cool enough by sweating alone, and our core body temperature may rise. This can lead to headaches, dizziness and even death.
  • Dehydration — this is the loss of water from our bodies. It can cause tiredness and problems with breathing and heart rates.
  • Sunburn — damage to the skin which can be painful and may increase the risks of getting skin cancer.
  • Air pollution — it is thought that one third of the deaths caused by the heatwave in the UK were caused by poor air quality.
  • Drowning — some people drowned when trying to cool off in rivers and lakes.

The Met Office provides the Department of Health with heatwave warnings (Heat-Health Watch) to prepare the NHS, health professionals, carers and the general public for the effects of extreme heat.

Summers as hot as 2003 could happen every other year by the year 2050 as a result of climate change due to human activities.

Environment and social effects

  • Water supplies — drinking water supplies were affected in some parts of the UK and hosepipe bans introduced.
  • Tourism — many parts of the UK reported increased levels of tourism as people decided to holiday in the UK while the weather was unusually dry and hot.
  • Agriculture — many chickens, pigs and cows died during the heat in Europe and crops failed in the dry conditions. This led to higher food prices. It is thought to have cost European farming 13.1 billion euros.
  • Transport — some railway tracks buckled in the heat. The London Underground became unbearable. Some road surfaces melted. Low river levels prevented some boats from sailing.
  • The London Eye closed on one day as it became too hot in the cabins.
  • Energy — two nuclear power plants to close down in Germany. These rely on water for cooling in the power generation process.

In picturesThe

Fig 1. Satellite image.
Fig 1. Satellite image.
A river with low levels of water
A river with low levels of water
A forest fire
A forest fire
Family playing on the beach
Family playing on the beach

Immediate responses to the heatwave

  • France requested aid from the European Union to deal with the effects.
  • Public water supply shortages occurred in several countries, including the UK and Croatia, which led to a temporary ban on using hose pipes.
  • TV news, internet and newspapers informed the public on how to cope with the heat — drinking plenty of water, wearing cool clothing, and staying in the shade in the middle of the day.
  • Network Rail in the UK imposed speed restrictions for trains when the temperature was above 30 °C. This was to help avoid trains derailing when railway lines might have buckled
  • Workers around Europe altered their working hours. Some refuse collectors started earlier to pick up rapidly decomposing rubbish from the streets.

What happened to cause the heatwave?

Weather chart

Fig 2. Weather Chart for midday on 5 August 2003.
Fig 2. Weather Chart for midday on 5 August 2003.

It shows an area of high pressure over most of Western Europe. Air is moving around the high in a clockwise direction, bringing a hot, dry tropical continental air mass to the UK at this time. This pattern occurred for much of the rest of the month. High pressure areas usually bring little cloud and warm conditions in summer.

You can find out more about weather charts in the weather section of the Met Office website.

Satellite imagery
The satellite images below confirm there is very little cloud over most of Europe.

Fig 3. Satellite Image of north-west Europe at 2 p.m. on 5 August.
Fig 3. Satellite Image of north-west Europe at 2 p.m. on 5 August.

Fig. 3 shows a visible satellite image of north-west Europe at 2 p.m. on 5 August. Visible satellites show what you would see if you were in space looking down at Earth. White areas show were there is cloud, the brighter the shading the deeper the cloud. The dark areas show cloud free areas. On Figure 12, the darker areas over most of Europe show the area has thin or little cloud.

Fig 4. Satellite Image for north-west Europe at 2 p.m. on 5 August.
Fig 4. Satellite Image for north-west Europe at 2 p.m. on 5 August.

Fig. 4 shows an infrared satellite image for north-west Europe at 2 p.m. on 5 August. Infrared satellite images measure the temperature of the cloud or ground surface. The dark areas show surfaces that are warm and where there is no cloud. The whiter shading indicates cold cloud. The darker the shading of the land, the hotter it is.

 

You can find out more about satellites on the MetLink website.

Maximum temperatures
Many parts of Europe saw their temperature records broken during this summer, including the UK. A sweltering 39 °C was recorded in Brogdale in Kent on 10 August 2003, a record high which still stands today.

European rainfall
Rainfall over much of Europe was below what is normally expected during the months of June, July and August. The long-lasting high pressure system tended to reduce the amount of rain that fell.

As a result of the European heatwave:

  • A joint Met Office/Department of Health project called the Heat-Health Watch now gives advanced warning of UK hot. weather. It operates every summer from 1 June to 15 September.
  • The French government has made efforts to improve its prevention, surveillance and alert system for people such as the elderly living alone.

Further information on the Met Office main site
Met Office Event Summary

Further information on other websites
BBC News articles on the August 2003 European heatwave

The Met Office is not responsible for the content of external internet sites. Inclusion of a link on this site does not decree any endorsement by the Met Office of the content or the website.

Web page reproduced with the kind permission of the Met Office

CloudWheel Cutout

We have made a cloud wheel that can be cut out and used to identify clouds. Simply download the pdf, cut out the two circles, fasten together with a split pin and use to identify clouds.

Download Cloudwheel >>

CloudWheel Cut Out

Or, if you need a simpler version, use our Cloud bookmark >>

Also, why not try our interactive Cloud Key >>

Or, you can buy a laminated cloud identification key, produced in conjunction with the Field Studies Council, from our shop.

Clouds

Useful links

Download a Cloud Wheel or bookmark as a cloud identification chart.

Experiments demonstrate clouds forming in the Classroom from Physics Education, 2012, Catalyst article on Cloud SeedingPhysics Review article on Clouds, or have a look at our Experiments and Demonstrations page for experiments which demonstrate how clouds can look dark from below but white from above, or how to make a hygrometer to measure air humidity.

For a deeper understanding of how and where clouds form, have a look at our exercise using height/ temperature graphs to investigate atmospheric stability, lapse rates and cloud formation with a worksheet for students with an introductory PowerPoint or this paper.

Other useful links

What causes clouds
Types of clouds
Low clouds
Medium clouds
High clouds
What influences the colour of clouds?
Why do clouds stop growing upwards?
Why are there no clouds on some days?
Measuring clouds
The formation of precipitation
The nature of clouds
Types of cloud
Cumulus
Cumulonimbus
Stratus
Stratocumulus
Altocumulus
Altostratus
Nimbostratus
Cirriform clouds
Short-answer questions

What causes clouds

A cloud is defined as ‘a visible aggregate of minute droplets of water or particles of ice or a mixture of both floating in the free air’. Each droplet has a diameter of about a hundredth of a millimetre and each cubic metre of air will contain 100 million droplets. Because the droplets are so small, they can remain in liquid form in temperatures of -30 °C. If so, they are called supercooled droplets.

Clouds at higher and extremely cold levels in the atmosphere are composed of ice crystals – these can be about a tenth of a millimetre long.

Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals. For this to happen, the parcel of air must be saturated, i.e. unable to hold all the water it contains in vapour form, so it starts to condense into a liquid or solid form. There are two ways by which saturation is reached.

(a) By increasing the water content in the air, e.g. through evaporation, to a point where the air can hold no more.

(b) By cooling the air so that it reaches its dew point – this is the temperature at which condensation occurs, and is unable to ‘hold’ any more water. Figure 1 shows how there is a maximum amount of water vapour the air, at a given temperature, can hold. In general, the warmer the air, the more water vapour it can hold. Therefore, reducing its temperature decreases its ability to hold water vapour so that condensation occurs.

Graph plotting temperature and vapour pressure
Fig 1: There is a maximum amount of water vapour the air, at a given temperature, can hold

Method (b) is the usual way that clouds are produced, and it is associated with air rising in the lower part of the atmosphere. As the air rises it expands due to lower atmospheric pressure, and the energy used in expansion causes the air to cool. Generally speaking, for each 100 metres which the air rises, it will cool by 1 °C, as shown in Figure 2. The rate of cooling will vary depending on the water content, or humidity, of the air. Moist parcels of air may cool more slowly, at a rate of 0.5 °C per 100 metres.

graph plotting height and temperature
Fig 2: For each 100 metres which the air rises, it will cool by 1 °C

Therefore, the vertical ascent of air will reduce its ability to hold water vapour, so that condensation occurs. The height at which dew point is reached and clouds form is called the condensation level.

There are five factors which can lead to air rising and cooling.

1. Surface heating. The ground is heated by the sun which heats the air in contact with it causing it to rise. The rising columns are often called thermals.

2. Topography. Air forced to rise over a barrier of mountains or hills. This is known as orographic uplift.

3. Frontal. A mass of warm air rising up over a mass of cold, dense air. The boundary is called a ‘front’.

4. Convergence. Streams of air flowing from different directions are forced to rise where they meet.

5. Turbulence. A sudden change in wind speed with height creating turbulent eddies in the air.

Another important factor to consider is that water vapour needs something to condense onto. Floating in the air are millions of minute salt, dust and smoke particles known as condensation nuclei which enable condensation to take place when the air is just saturated.

Types of clouds

In 1803 a retail chemist and amateur meteorologist called Luke Howard proposed a system which has subsequently become the basis of the present international classification. Howard also become known by some people as ‘the father of British meteorology’, and his pioneering work stemmed from his curiosity into the vivid sunsets in the late 18th century following a series of violent volcanic eruptions. They had ejected dust high up into the atmosphere, thereby increasing the amount of condensation nuclei, and producing spectacular cloud formations and sunsets.

Howard recognised four types of cloud and gave them the following Latin names.

Cumulus  heaped or in a pile

Stratus  in a sheet or layer

Cirrus  thread-like, hairy or curled

Nimbus  a rain bearer

If we include another Latin word altum meaning height, the names of the 10 main cloud types are all derived from these five words and based upon their appearance from ground level and visual characteristics.

The cloud types are split into three groups according to the height of their base above mean sea level. Note that ‘medium’ level clouds are prefixed by the word alto and ‘high’ clouds by the word cirro (see Table 1). All heights given are approximate above sea level in mid-latitudes. If observing from a hill top or mountain site, the range of bases will accordingly be lower.

Table 1: The 10 main cloud type
Low clouds
Surface – 7,000 ft
Medium clouds
7,000 – 17,000 ft
High clouds
17,000 – 35,000 ft
Cumulus
Altocumulus
Cirrus
Cumulonimbus
Altostratus
Cirrostratus
Stratus
Nimbostratus
Cirrocumulus
Stratocumulus
 
 

 

 

Low clouds

Cumulus (Cu)
Height of base: 1,200-6,000 ft
Colour: White on its sunlit parts but with darker undersides.
Shape: This cloud appears in the form of detached heaps. Shallow cumulus may appear quite ragged, especially in strong winds, but well formed clouds have flattened bases and sharp outlines. Large cumulus clouds have a distinctive ‘cauliflower’ shape.
Other features: Well developed cumulus may produce showers.

Cumulus clouds
Fig 3: Cumulus
photo © R.K.Pilsbury

Cumulonimbus (Cb)
Height of base: 1,000-5,000 ft
Colour: White upper parts with dark, threatening undersides.
Shape: A cumulus-type cloud of considerable vertical extent. When the top of a cumulus reaches great heights, the water droplets are transformed into ice crystals and it loses its clear, sharp outline. At this stage the cloud has become a cumulonimbus. Often, the fibrous cloud top spreads out into a distinctive wedge or anvil shape.
Other features: Accompanied by heavy showers, perhaps with hail and thunder. By convention Cb is usually reported if hail or thunder occur, even if the observer does not immediately recognise the cloud as Cb (it may be embedded within layers of other cloud types).

Cumulonimbus clouds
Fig 4: Cumulonimbus
photo © R.K.Pilsbury

Stratus (St)
Height of base: surface-1,500 ft
Colour: Usually grey.
Shape: May appear as a layer with a fairly uniform base or in ragged patches, especially during precipitation falling from a cloud layer above. Fog will often lift into a layer of stratus due to an increase in wind or rise in temperature. As the sun heats the ground the base of stratus cloud may rise and break becoming shallow cumulus cloud as its edges take on a more distinctive form.
Other features: If thin, the disc of the sun or moon will be visible (providing there are no other cloud layers above). If thick, it may produce drizzle or snow grains.

Stratus clouds
Fig 5: Stratus
photo © C.S.Broomfield

Stratocumulus (Sc)
Height of base: 1,200-7,000 ft
Colour: Grey or white, generally with shading.
Shape: Either patches or a sheet of rounded elements but may also appear as an undulating layer. When viewed from the ground, the size of individual elements will have an apparent width of more than 5° when at an elevation greater than 30° (the width of three fingers at arm’s length).
Other features: May produce light rain or snow. Sometimes the cloud may result from the spreading out of cumulus, giving a light shower.

stratocumulus clouds
Fig 6: Stratocumulus
photo © J.F.P Galvin

Medium clouds

Altocumulus (Ac)
Height of base: 7,000-17,000 ft
Colour: Grey or white, generally with some shading.
Shape: Several different types, the most common being either patches or a sheet of rounded elements but may also appear as a layer without much form. When viewed from the ground, the size of individual elements will have an apparent width of 1 to 5° when at an elevation greater than 30° (the width of one to three fingers at arm’s length). Even if the elements appear smaller than this the cloud is still classified altocumulus if it shows shading.
Other features: Occasionally some slight rain or snow, perhaps in the form of a shower may reach the ground. On rare occasions, a thunderstorm may occur from one type of Ac known as altocumulus castellanus – so called because in outline, the cloud tops look like a series of turrets and towers along a castle wall.

altocumulus clouds
Fig 7: Altocumulus
photo © C.S.Broomfield

Altostratus (As)
Height of base: 8,000-17,000 ft
Colour: Greyish or bluish.
Shape: A sheet of uniform appearance totally or partly covering the sky.
Other features: Sometimes thin enough to reveal the sun or moon vaguely, as through ground glass. Objects on the ground do not cast shadows. May give generally light rain or snow, occasionally ice pellets, if the cloud base is no higher than about 10,000 ft.

altostratus clouds
Fig 8: Altostratus
photo © C.S.Broomfield

Nimbostratus (Ns)
Height of base: 1,500-10,000 ft
Colour: Dark grey.
Shape: A thick, diffuse layer covering all or most of the sky.
Other features: Sun or moon always blotted out. Accompanied by moderate or heavy rain or snow, occasionally ice pellets. Although classed as a medium cloud, its base frequently descends to low cloud levels. May be partly or even totally obscured by stratus forming underneath in precipitation.

nimbostratus clouds
Fig 9: Nimbostratus
photo © C.S.Broomfield

High clouds

Cirrus (Ci)
Height of base: 17,000-35,000 ft
Colour: Composed of ice crystals, therefore white.
Shape: Delicate hair-like filaments, sometimes hooked at the end; or in denser, entangled patches; or occasionally in parallel bands which appear to converge towards the horizon.
Other features: The remains of the upper portion of a cumulonimbus is also classified as cirrus.

cirrus
Fig 10: Cirrus
photo © R.K.Pilsbury

Cirrocumulus (Cc)
Height of base: 17,000-35,000 ft
Colour: Composed of ice crystals, therefore white.
Shape: Patches or sheet of very small elements in the form of grains or ripples or a honeycomb. When viewed from the ground, the size of individual elements will have an apparent width of less than 1° when at an elevation greater than 30° (no greater than the width of a little finger at arm’s length).
Other features: Sometimes its appearance in a regular pattern of ‘waves’ and small gaps may resemble the scales of a fish, thus giving rise to the popular name ‘mackerel sky’ (this name may also be attributed to high altocumulus clouds).

cirrocumulus
Fig 11: Cirrocumulus
photo © R.K.Pilsbury

Cirrostratus (Cs)
Height of base: 17,000-35,000 ft
Colour: Composed of ice crystals, therefore white.
Shape: A transparent veil of fibrous or smooth appearance totally or partly covering the sky.
Other features: Thin enough to allow the sun to cast shadows on the ground unless it is low in the sky. Produces halo phenomena, the most frequent being the small (22°) halo around the sun or moon ≬ a little more than the distance between the top of the thumb and the little finger spread wide apart at arm’s length.

cirrostratus
Fig 12: Cirrostratus
photo © R.K.Pilsbury

Condensation trails (contrails)
These are thin trails of condensation, formed by the water vapour rushing out from the engines of jet aircraft flying at high altitudes. They are not true clouds, but can remain in the sky for a long time, and grow into cirrus clouds.

contrails
Fig 13: Cirrus with contrails
photo © S D Burt

What influences the colour of clouds?

Light from both the sky and from clouds is sunlight which has been scattered. In the case of the sky, the molecules of air (nitrogen and oxygen) undertake the scattering, but the molecules are so small that the blue part of the spectrum is scattered more strongly than other colours.

The water droplets in the cloud are much larger, and these larger particles scatter all of the colours of the spectrum by about the same amount, so white light from the sun emerges from the clouds still white.

Sometimes, clouds have a yellowish or brownish tinge – this is a sign of air pollution.

Why do clouds stop growing upwards?

Condensation involves the release of latent heat. This is the ‘invisible’ heat which a water droplet ‘stores’ when it changes from a liquid into a vapour. Its subsequent change of form again releases enough latent heat to make the damp parcel of air warmer than the air surrounding it. This allows the parcel of air to rise until all of the ‘surplus’ water vapour has condensed and all the latent heat has been released.

Therefore, the main reason which stops clouds growing upwards is the end of the release of latent heat through the condensation process. There are two other factors which also play a role. Faster upper atmospheric winds can plane off the tops of tall clouds, whilst in very high clouds, the cloud might cross the tropopause, and enter the stratosphere where temperatures rise, rather than decrease, with altitude. This thermal change will prevent further condensation.

Why are there no clouds on some days?

Even when it is very warm and sunny, there might not be any clouds and the sky is a clear blue. The usual reason for the absence of clouds will be the type of pressure, with the area being under the influence of a high pressure or anticyclone. Air would be sinking slowly, rather than rising and cooling. As the air sinks into the lower part of the atmosphere, the pressure rises, it becomes compressed and warms up, so that no condensation takes place. In simple terms, there are no mechanisms for clouds to form under these pressure conditions.

Measuring clouds

The cloud amount is defined as ‘the proportion of the celestial dome which is covered by cloud.’ The scale used is eighths, or oktas, with observers standing in an open space or on a rooftop to get a good view or panorama of the sky.

Complete cloud cover is reported as 8 oktas, half cover as 4 oktas, and a completely clear sky as zero oktas. If there is low-lying mist or fog, the observer will report sky obscured.

The reporter will also report the amount of each cloud level – 2 oktas of cumulus and 3 oktas of cirrus, etc.

The frequent passage of depressions across the United Kingdom means that the most commonly reported cloud amount is, not surprisingly, 8 oktas. A clear blue sky, i.e. zero oktas, is less common, as often on hot, sunny days, there are small wispy layers of cirrostratus or fine tufts of thin cirrus at high altitudes.

The formation of precipitation

Cooling, condensation and cloud formation is the start of the process which results in precipitation. But not all clouds will produce raindrops or snowflakes – many are so short-lived and small that there are no opportunities for precipitation mechanisms to start.

There are two theories that explain how minute cloud droplets develop into precipitation.

10.1 The Bergeron Findeisen ice-crystal mechanism

If parcels of air are uplifted to a sufficient height in the troposphere, the dew-point temperature will be very low, and minute ice crystals will start to form. The supercooled water droplets will also freeze on contact with these ice nuclei.

The ice crystals subsequently combine to form larger flakes which attract more supercooled droplets. This process continues until the flakes fall back towards the ground. As they fall through the warmer layers of air, the ice particles melt to form raindrops. However, some ice pellets or snowflakes might be carried down to ground level by cold downdraughts.

10.2 Longmuir’s collision and coalescence theory

This applies to ‘warm’ clouds, i.e. those without large numbers of ice crystals. Instead they contain water droplets of many differing sizes, which are swept upwards at different velocities so that they collide and combine with other droplets.

It is thought that when the droplets have a radius of 3 mm, their movement causes them to splinter and disintegrate, forming a fresh supply of water droplets.

This theory allows droplets of varying sizes to be produced, and as shown in the table below, each will have a different terminal (or falling) velocity. 

 

 

Particle radius (mm)
Terminal velocity (m/s)
Cloud
 
 
 
0.001
0.005
0.01
0.5
0.0001
0.0025
0.01
0.25
Drizzle
 
0.1
0.25
0.7
2.0
Rain
 
 
 
 
0.5
1.0
1.5
2.0
2.5
3.9
6.5
8.1
8.8
9.1

Table 2: The terminal velocities of different particle sizes

10.3 Man-made rain

In recent years, experiments have taken place, chiefly in the USA, China and the former USSR, adding particles into clouds that act as condensation or freezing nuclei. This cloud seeding involves the addition into the atmosphere from aircraft of dry ice, silver iodide or other hygroscopic substances. These experiments have largely taken place on the margins of farming areas where rainfall is needed for crop growth, or to divert rain from major events such as the 2008 Beijing Olympics.

The nature of clouds

A classification of clouds was introduced by Luke Howard (1772-1864) who used Latin words to describe their characteristics.
  • Cirrus – a tuft or filament (e.g. of hair)
  • Cumulus – a heap or pile
  • Stratus – a layer
  • Nimbus – rain bearing
clouds
C.S. Broomfield (© Crown Copyright)
There are now ten basic cloud types with names based on combinations of these words (the word ‘alto’, meaning high but now used to denote medium-level cloud, is also used).

Clouds form when moist air is cooled to such an extent that it becomes saturated. The main mechanism for cooling air is to force it to rise. As air rises it expands – because the pressure decreases with height in the atmosphere – and this causes it to cool. Eventually it may become saturated and the water vapour then condenses into tiny water droplets, similar in size to those found in fog, and forms cloud. If the temperature falls below about minus 20 °C, many of the cloud droplets will have frozen so that the cloud is mainly composed of ice crystals.

The main ways in which air rises to form cloud

  1. Rapid local ascent when heated air at the earth’s surface rises in the form of thermal currents (convection).
  2. Slow, widespread, mass ascent where warm moist air is forced to rise above cold air. The region between warm and cold air is called a ‘front’.
  3. Upward motion associated with turbulent eddies resulting from the frictional effect of the earth’s surface.
  4. Air forced to rise over a barrier of mountains or hills.

The first of these tends to produce cumulus-type clouds, whereas the next two usually produce layered clouds. The last can produce either cumulus-type cloud or layered cloud depending upon the state of the atmosphere. The range of ways in which clouds can be formed and the variable nature of the atmosphere give rise to the enormous variety of shapes, sizes and textures of clouds.

Types of cloud

The ten main types of cloud can be separated into three broad categories according to the height of their base above the ground: high clouds, medium clouds and low clouds.

High clouds are usually composed solely of ice crystals and have a base between 18,000 and 45,000 feet (5,500 and 14,000 metres).

  • Cirrus – white filaments
  • Cirrocumulus – small rippled elements
  • Cirrostratus – transparent sheet, often with a halo

Medium clouds are usually composed of water droplets or a mixture of water droplets and ice crystals, and have a base between 6,500 and 18,000 feet (2,000 and 5,500 metres).

  • Altocumulus – layered, rippled elements, generally white with some shading
  • Altostratus – thin layer, grey, allows sun to appear as if through ground glass
  • Nimbostratus – thick layer, low base, dark. Rain or snow falling from it may sometimes be heavy

Low clouds are usually composed of water droplets – though cumulonimbus clouds include ice crystals – and have a base below 6,500 feet (2,000 metres).

  • Stratocumulus – layered, series of rounded rolls, generally white with some shading
  • Stratus – layered, uniform base, grey
  • Cumulus – individual cells, vertical rolls or towers, flat base
  • Cumulonimbus – large cauliflower-shaped towers, often ‘anvil tops’, sometimes giving thunderstorms or showers of rain or snow

Most of the main cloud types can be subdivided further on the basis of shape, structure and degree of transparency.

Cumulus

Cumulus clouds are often said to look like lumps of cotton wool. With a stiff breeze, they march steadily across the sky; their speed of movement gives a clue to their low altitude. Cumulus clouds occasionally produce light showers of rain or snow.

Cumulus clouds over water

© Steve Jebson

Cumulus clouds over land

© Steve Jebson

Typically, the base of cumulus clouds will be about 2,000 feet (600 metres) above ground in winter, and perhaps 4,000 feet (1,200 metres) or more on a summer afternoon. Individual clouds are often short-lived, lasting only about 15 minutes. They tend to form as the ground heats up during the day and become less frequent as the sun’s heat wanes towards evening.

The cause of small cumulus clouds is usually convection. Heat from the sun warms the ground, which in turn warms the air above. If a ‘parcel’ of warm air is less dense than the cooler air around it or above it, the ‘parcel’ of air starts to rise – this is known as a ‘thermal’. As it rises it expands and cools, and, if cooled sufficiently, the water vapour condenses out as tiny cloud droplets. A cumulus cloud is born.

The air within the cloud will continue to rise until it ceases to be buoyant. On some sunny days there is insufficient moisture or instability for moisture to form.

In hilly regions, a high, south-facing slope acts as a good source of thermals, and therefore of cumulus. Occasionally, a power station or factory will produce a cloud of its own. 

When air rises in thermals there must be compensating downdraughts nearby. These create the clear areas between cumulus clouds and make it easier for glider pilots to find the thermals that they can use to gain height.

Cumulonimbus

Just as cumulus is heaped cloud, so cumulonimbus is a heaped rain cloud (nimbus means rain).

Cumulonimbus
© N. Elkins

In many ways the rain-bearing variety can be considered as a bigger, better-organised version of the cumulus. A cumulonimbus may be 10 km across and extend 10 km above the ground. This compares with a cumulus cloud which is typically a few hundred metres across and reaches a height of only a few kilometres. Instead of a ball of cotton wool, a cumulonimbus will resemble a huge cauliflower of sprouting towers and bulging turrets.

But there is one important structural difference in that the uppermost levels of the cumulonimbus have turned to ice and become fibrous in appearance, whereas cumulus clouds are composed entirely of water droplets. This icy section at the top may flatten out into an ‘anvil’ shape when the cloud is fully developed. When it reaches this stage, the base is usually dark, and there will be showers of rain or, sometimes, hail. In winter, the showers may be of sleet or snow. The showers are often quite heavy and may be accompanied by lightning and thunder.

Sometimes cumulonimbus will be ’embedded’ or half hidden among other clouds. On other occasions they will be well separated and the ‘anvil’ may well be visible many miles away. Cumulonimbus clouds may be seen at any time of the day, but are most common inland during the afternoon in spring and summer, and frequently occur in the tropics. They develop where convection is at its strongest and most organised.

The lifetime of a cumulonimbus is usually less than one hour.

There are exceptions though. The ‘Hampstead storm’ of 14 August 1975 was an example of a cumulonimbus cloud that managed to keep regenerating itself over one small area of London. About 170 mm of rain fell in three hours, causing severe flooding.

Stratus

Stratus over hills

© Jim Galvin

Stratus over buildings

© A. Bushell

Stratus is a low-level layer cloud (not to be confused with altostratus and cirrostratus, which are much higher). In appearance, it is usually a featureless grey layer. Sometimes, when a sheet of stratus is affecting an area, the cloud base will be right down to the ground and will cause fog. However, the usual base is between the ground and 1,000 feet (300 metres), which means that hilltops may be obscured by cloud. Sometimes stratus will produce drizzle or light snow, particularly over hills.

Perhaps the most important indication of its low altitude is its apparent rapid movement across the sky in any wind stronger than a flat calm. For example, a stratus cloud at 500 feet (150 metres) moving at 20 miles per hour will appear to move much faster than altostratus with its base at 10,000 feet (3,000 metres) moving at 60 miles per hour.

An approximate guide to the height of stratus may be gained by measuring the relative humidity and subtracting it from 100. The resulting number gives some idea of the height of the low cloud in hundreds of feet. For example, 94% relative humidity would indicate that the stratus is about 600 feet (180 metres) above the ground. 

Stratus forms as the result of condensation in moist air at low levels due to cooling. The cooling may be caused in a number of ways:

  1. lifting of air over land due to hills or ‘bumping’ over rough ground;
  2. warm air moving over a cold sea. If the cloud moves in over the land, it will readily cover any relatively high ground. In some cases, the base of the cloud falls to the sea surface, causing fog. This may drift in over the coast and is called sea fog, though it goes by the name of haar in the north and east of Scotland and fret in the east of England;
  3. temperature falling over land at night. The air may have been brought inland during the day on a sea breeze. There needs to be some wind, otherwise the cooling may lead to radiation fog.

Stratocumulus

Stratocumulus clouds usually form between 1,000 and 6,500 feet (300 and 2,000 metres).

Stratocumulus
© Jim Galvin

Stratocumulus will often give a sheet of almost total cloud cover, with perhaps one or two breaks. The cloud elements are rounded and almost join up. Occasionally, the sheet is composed of a series of more or less parallel rolls, which often, but not always, lie ‘across the wind’. Stratocumulus sometimes produces light falls of rain or snow.

Stratocumulus is formed by weak convection currents, perhaps triggered by turbulent airflows aloft. The convection affects a shallow zone because dry, stable air above the cloud sheet prevents further upward development.

Sometimes there are huge sheets of stratocumulus covering thousands of square kilometres around the flanks of a high pressure system, especially over the oceans. The weather below such sheets tends to be dry, but it may be rather dull if the cloud is two or three thousand feet thick.

Altocumulus

Altocumulus clouds usually form between 6,500 and 17,000 feet (2,000 and 5,000 metres) and are referred to as medium level clouds.

Altocumulus
© Steve Jebson

In most cases, there is little difference between the properties of stratocumulus and altocumulus, since both are composed of water droplets and are normally limited in vertical extent. The deciding factor between stratocumulus and altocumulus normally comes down to height as both types are formed in the same way.

Altocumulus also provides a sort of dappled pattern, but, since it is at a greater altitude, the cloud elements look smaller. One significantly different form is altocumulus castellanus, which is like a vigorous medium-level cumulus , sometimes with rain falling from their base, known as trailing virga. This type of cloud is sometimes an indication that thunderstorms will follow

Altostratus

Altostratus clouds normally have a base between 8,000 and 17,000 feet (2,500 and 5,000 metres).

Altostratus © C. S. Broomfield

Altostratus appears as a uniform sheet either totally or partially covering the sky. Sometimes it is thin enough to just reveal the sun or moon. The sun appears as if through ground glass but shadows are not visible on the ground. Sometimes, if the base is below 10,000 feet (3,000 metres) it may give light rain or snow.

 

Nimbostratus

Nimbostratus
© C. S. Broomfield

Nimbostratus clouds are found between 1,500 and 10,000ft (450 and 3,000 metres).

Nimbostratus forms a thick, diffuse layer of dark grey cloud covering all or most of the sky, which always obscures the sun or moon. It is accompanied by moderate or heavy rain or snow, occasionally ice pellets. Although classed as a medium cloud, its base frequently descends to low cloud levels. Nimbostratus may be partly or even totally obscured by stratus forming underneath in precipitation.

Cirriform clouds

Cirriform clouds (i.e. clouds from the cirrus family) are found at high altitude, usually above 20,000 feet (6,000 metres). They are composed of ice crystals. Three types of cloud make up the group: cirrus, cirrostratus and cirrocumulus.

CirrusCirrus itself is very common in the British Isles and throughout most of the world. It is thin, wispy and white in appearance, and its name, coming from the Latin word for ‘tuft of hair’, gives a good description of the cloud. Another name for the cloud, ‘mares tails’, also conjures up an accurate image. Cirrus may be hooked or straight depending on the airflow aloft. Sometimes it comes as a very dense patch which is left over from the ‘anvil’ cloud of a cumulonimbus that has disappeared. On other occasions, cirrus may be quite extensive when associated with a jet stream – the cloud can then be seen moving across the sky, despite its great altitude. Aircraft condensation trails are a form of man-made cirrus. They can sometimes be seen in ‘historical’ films, to the delight of film buffs who enjoy spotting technical inaccuracies.

CirrostratusCirrostratus is a fairly uniform sheet of thin cloud through which the sun or moon can be seen. Sometimes, if the cloud is thin, a bright ring of light (called a halo) surrounds the sun or moon. A layer of cirrostratus is often an indication of a deterioration in the weather.

CirrocumulusCirrocumulus is often present in small amounts along with cirrus, but rarely does it dominate the sky. On those occasions when it is widespread, a beautiful spectacle is created, especially at sunset. The individual clouds appear very small – often tiny rows of roughly spherical pear-like cloud elements. Sometimes they occur in undulating patterns like tiny ripples.

This information sheet is based on a series of articles written by Dick File that appeared in The Guardian. Web page reproduced with the kind permission of the Met Office

Short-answer questions

1. Make concise definitions of the following terms.
(a) Condensation.
(b) Dew point.
(c) Supercooled.
(d) Humidity.

2. Explain the two ways by which parcels of air can reach saturation.

3. Outline the five factors that will cause parcels of air to
rise and cool.

4. Match up the descriptions in list B with the correct term
in list A:
List A: Cumulus; Cirrus; Stratus; Nimbus.
List B: Rain bearer; Heaped; Thread-like or hairy; Sheets or layers.

5. Which of the following are correct statements?
(i)   Low clouds form up to 10,000
feet above the surface.
(ii)  High clouds form between 17,000
and 35,000 feet above the surface.
(iii) Altocumulus and altostratus are two types
of high cloud.
(iv) Nimbostratus is a medium-level cloud.
(v)  Cumulonimbus is a low cloud.

6. Describe the likely characteristics of the following cloud
types.
(a) Cumulus
(b) Stratus
(c) Cirrus

7. With which cloud formations would you associate the phrase
‘mackerel sky’?

8. What weather conditions might follow the appearance of altocumulus
castellanus?

9. What are contrails? What clouds might they produce over time?

10. Why do most clouds appear white?

11. What prevents clouds from building up to very high levels
in the troposphere?

12. Under what conditions might you find warm, sunny weather,
but no clouds forming?

13. Outline how clouds are measured by observers.

14. Which amount of cloud cover is most commonly observed in
the British Isles? Explain why?

15. Why is it quite rare to observe zero oktas of cloud cover?

16. Explain the two theories that explain how cloud droplets
turn into precipitation.

17. What is cloud seeding?

Web page reproduced with the kind permission of the Met Office

Cumulus clouds over water
Cumulus clouds over land
Stratus over hills
Stratus over buildings

Severe Storms

Including hail, downdraughts and cloudbursts

hailstructureIt was reported in The Times newspaper on 15 April 1986 that a hailstorm lashing Dhaka, the capital of Bangladesh, had killed nearly 50 people and injured more than 400. The storm had brought winds of about 60 mph and hailstones weighing up to 2 lb (nearly 1 kg). Houses had been flattened, communications disrupted and the windscreens of more than 700 cars shattered. In such conditions, an umbrella was no use whatsoever; even a riot shield may not have provided adequate protection! According to Dick File, in Weather Facts (Oxford University Press, 1991), this storm (which struck on 14 April 1986) killed 92 people and produced hailstones that weighed 1.02 kg.

The heaviest hailstones to fall on the United Kingdom did so at Horsham, West Sussex, on 5 September 1958 and weighed 140 g. They were almost the size of a tennis ball. When they hit the ground, they were travelling at speeds in excess of 100 mph (50 m/s). If you find this surprising, do a little calculation, using the formula:
V2 = u2 + 2as where u is the initial speed, v the terminal speed, a the acceleration (in this case, due to gravity) and s the distance travelled. For a hailstone falling from a height of 500 m through still air, v = 100 m/s! The impact of a missile the size of a tennis ball travelling this fast is much more serious than that of a cricket ball hit for six.

Should you ever get the chance, collect some large hailstones and cut them in half. You may find a layered structure, with alternate layers of clear and opaque ice (as in the picture on the right, which shows a section of a hailstone viewed by transmitted light). The layers are acquired in different parts of the storm clouds. As hailstones fall, they collect tiny water droplets, which flow around them and freeze. If no air is trapped, the ice is clear.

The storm which struck the Wokingham area of Berkshire on 9 July 1959 produced hailstones more than 2.5 cm in diameter. This storm was studied in detail by Professor Frank Ludlam of Imperial College and his team of co-workers, who produced a striking three-dimensional model of the airflow with-in the storm and explained how large multi-layered hailstones may form in such weather systems.

diagram of severe storm 3
This three-dimensional model is taken from a paper entitled Airflow in convective storms by K.A.Browning and F.H.Ludlam published in the April 1962 issue of the Quarterly Journal of the Royal Meteorological Society (Volume 88, pp.117-135).

In the diagram on the right, streamlines of air in which condensation occurred are shaded. The surface areas affected by rain and hail are shown by, respectively, grey and black shading. Heights are shown in thousands of feet. Precipitation formed in air which entered the storm near position H. As shown, the precipitation was carried across relative to the storm to around 13-15,000 feet, whereupon it fell and re-entered the strong updraught near posit-ion O. Some precipitation particles reached altitu-des of 30,000 feet or more and grew into large hail-stones before falling again, forward of the strong updraught, near position H’. The storm moved from left to right, with rain on its left flank and a squally ‘gust front’ (shown as a cold front) on its right flank. Behind the storm, chilled air reached the ground.

severe storm diagram

The diagram on the right shows a vertical section through a typical severe hailstorm (moving from right to left) and is also taken from Ludlam’s 1961 article in Weather. Compare this diagram with the three-dimensional model above

The paths of the air are drawn as if the storm was stationary. They are, therefore, relative streamlines. The dashed lines are trajectories of small hailstones. The thick full line shows the trajectory of a large hailstone.

To some extent, the features shown on this vertical section occur also in vigorous cumulo-nimbus systems which do not produce large hail. Students can look out for mamma, the udder-like cloud feature that hangs under the anvil and other parts of the cloud. How are mamma formed? Students can also observe gust fronts and measure the temperature drop that occurs when a storm passes. It is often several degrees Celsius. Perhaps, with the help of someone who has a car, they can map areas of rain and hail relative to moving storms.

severe storm diagram 2
This diagram has been taken from The microburst hazard to aircraft by J.McCarthy and R.J.Serafin, published in Weatherwise in 1984 (Volume 37, pp.120-127)

In severe storms, downdraughts may be as strong as 30-40 m/s and reach the ground as ‘down-bursts’. These are dangerous, as the diagram above shows. Downbursts spread out near the ground. An aeroplane that flies into such an outflow first encounters an increasing head-wind (at 1 and 2), which adds to the speed of the speed of the air flowing over the aircraft’s wings and thus increases lift. At 3, however, the strength of the downdraught begins to reduce the altitude of the aircraft; and at 4 and 5 the aircraft experiences both a tail-wind (which reduces air speed and lift) and a downward force from the downdraught. Over the years, there have been many air disasters caused this way, especially in North America.

Cloudbursts

On 15 August 1952, the village of Lynmouth in North Devon was devastated by a torrent of water which poured off Exmoor; 34 people died. On 29 May 1920, in and around the Lincolnshire town of Louth, 22 people died when water from a storm over the Wolds caused the River Lud, normally a small stream, to rise 5 m above its normal level. In Dorset and Somerset, there have been similar occurrences; and in all cases, severe storms caused the havoc. When such storms occur in the British Isles, the wind in the upper troposphere is typically from the south-west, with the wind in the lower troposphere from a north-easterly point (and pressure low to the south and south-west). If this flow is lifted orographically, the storm may become stationary and deposit several inches of rain in a short time. Thus, it is places below slopes that face northwards or north-eastwards that are most at risk.

Hail Prevention

To frighten away the evil spirits that caused hail, primitive tribes used to shoot arrows into storm clouds; and Christians have tried to exorcise these spirits by ringing church bells (a dangerous practice because of lightning strikes on bell towers). Not only arrows, but also cannon-balls, artillery shells and rockets have been fired into storm clouds, but all to no avail. Though there is some evidence that cloud seeding may help to reduce the size of hailstones, there is nothing we can yet do to prevent the formation of severe storms.