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+

Global Atmospheric Circulation

16. Anticyclones

Weather and Climate: a Teachers’ Guide

Pathway: Extending Weather

Air MassesPressure and WindWater in the AtmosphereAnticyclones

Lesson overview: In this lesson we look at the weather associated with anticyclones in summer and winter and its potential impacts.

Anticyclones are high-pressure weather systems caused by descending air.  They rotate clockwise as outflowing air at ground level is deflected by the Coriolis effect and bring light winds and warm sunny days in the summer, but also crisp winter days and endless gloom in spring.  Large high-pressure systems can sit in place for long periods of time and deflect low-pressure weather around the UK.  This can lead to protracted periods of stable weather with little precipitation.

Learning objectives:

  • To understand what an anticyclone is.

  • To be able to distinguish between the weather in a winter and summer anticyclone.

  • To be able to explain why we get high pressure in an anticyclone.

  • To understand the positive and negative impacts of anticyclones in summer and winter

Key Teaching Resources

Anticyclones PowerPoint
Anticyclones PowerPoint (easier)
Anticyclones Worksheet
Anticyclones Worksheet (easier)
Anticyclones Homework

Teacher CPD/ Extended Reading

Anticyclones_More for Teachers

Alternative or Extension Resources

Further resources for teaching about anticyclones, depressions and fronts. 

Weather and Climate: a Teachers’ Guide

11. Hot Deserts

Weather and Climate: a Teachers’ Guide

Pathway: Climate

 Climate ZonesPast Climate ChangePolar Climate – Hot Deserts

Lesson overviewIn this lesson we look at the characteristics and locations of hot deserts and the adaptations of animals and vegetation found there.

Hot deserts have less than 250mm precipitation per year and daytime temperatures that may approach 50oC.   Hot deserts cover 14.2% of the Earth’s surface, their distribution largely determined by the global atmospheric circulation.   Physical factors such as altitude or latitude determine deserts with the most extreme values of precipitation/temperature. Their characteristics and distribution will change over long periods of time due to the Milankovitch cycles and Continental Drift.  Climate change is projected to have complex effects and the size of arid regions is expected to grow. The plants and animals that live in the deserts around the world have adapted to cope with the extreme climate.

Learning objectives:

  • To be able to describe the characteristics and location of at least one hot desert.

  • To understand why hot deserts are hot and dry.

  • To be able to draw and interpret a climate graph of a hot desert.

  • To understand animal and plant adaptations to the hot desert climate.

Key Teaching Resources

Hot Deserts PowerPoint
Hot Deserts PowerPoint (easier)
Hot Deserts Worksheet
Hot Deserts Making Plants and Animals
Hot Deserts Making Plants and Animals – Info sheet
Hot Deserts homework

Teacher CPD/ Extended Reading

Hot Deserts_More for Teachers

Alternative or Extension Resources

Using GIS to investigate temperature extremes

Weather and Climate: a Teachers’ Guide

10. Polar Climate

Weather and Climate: a Teachers’ Guide

Pathway: Climate

Atmospheric and Oceanic CirculationClimate ZonesPast Climate Change Polar Climate

Lesson overviewIn this lesson we investigate why polar climates are so extreme and how it can be too cold to snow in Antarctica.

The extremes of Polar weather – the lowest temperatures on Earth and little precipitation – vary significantly throughout the year.  Antarctica has the harshest conditions, recording an annual mean low of nearly -60oC due to its elevation and distance from the sea.  Low levels of precipitation define Polar regions as deserts and it can be too dry to snow. The effects of climate change are amplified in these regions through feedback mechanisms which in turn affect global climate. 

Learning objectives:

  • To understand why it is cold in Polar regions.

  • To understand why our Polar regions are classified as deserts.

  • To be able to calculate mean temperature and total precipitation and interpret a climate graph

Key Teaching Resources

Polar Climate PowerPoint
Polar Climate PowerPoint (easier)
Polar Climate Worksheet
Polar Climate Worksheet (easier)
Beast from the East homework

Teacher CPD/ Extended Reading

Polar Climate – More for Teachers

Alternative or Extension Resources

Using GIS to investigate temperature extremes

Weather and Climate: a Teachers’ Guide

Weather Symbols and Synoptic Charts

Interpreting weather charts 

Introduction

Weather systems
Fronts
Relationship between isobars and wind
Understanding station plots on a weather map
Plotting a station plot

Introduction

Weather charts consist of curved lines drawn on a geographical map in such a way as to indicate weather features. These features are best shown by charts of atmospheric pressure, which consist of isobars (lines of equal pressure) drawn around depressions (or lows) and anticyclones (or highs). Other features on a weather chart are fronts and troughs. These are drawn to highlight the areas of most significant weather, but that does not mean that there is nothing of significance elsewhere on the chart.

Weather systems
High pressure or anticyclones

Anticyclones are areas of high pressure, whose centres are often less well defined than depressions, and are associated with quiet, settled weather. Winds blow in a clockwise direction around anticyclones in the northern hemisphere, this is reversed in the southern hemisphere.

Fig 1: An anticyclone

 

 

Low pressure or depressions

Depressions are areas of low pressure, usually with a well-defined centre, and are associated with unsettled weather. Winds blow in an anticlockwise direction around depressions in the northern hemisphere, this is reversed in the southern hemisphere.

Fig 2: A depression

 Fronts

Early weather charts consisted simply of station plots and isobars, with the weather being written as comments, like ‘Rain, heavy at times’. During the 1920s, a group of Scandinavian meteorologists, known collectively as the Bergen School, developed the concept of representing the atmosphere in terms of air masses. Since the air masses could be considered as being in conflict with each other, the term ‘front’ was used to describe the boundary between them. Three types of front were identified which depend on the relative movement of the air masses.

Cold Front

A cold front marks the leading edge of an advancing cold air mass. On a synoptic chart a cold front appear as a blue line with triangles. The direction in which the triangles point is the direction in which the front is moving.

Warm Front

A warm front marks the leading edge of an advancing warm air mass. On a synoptic chart a warm front appears as a red line with semi-circles. The direction in which the semi-circles point is the direction in which the front is moving.

Occlusion (or occluded front)

Occlusions form when the cold front of a depression catches up with the warm front, lifting the warm air between the fronts into a narrow wedge above the surface. On a synoptic chart an occluded front appears as a purple line with a combination of triangles and semi-circles. The direction in which the symbols point is the direction in which the front is moving.


Troughs

Fronts describe thermal characteristics. They also happen to be where there is significant precipitation. However, precipitation is not confined to fronts. Drizzle in warm sectors or showers in cold air occur fairly randomly, but occasionally, lines of more organised precipitation can develop. These are called troughs.

Isobars

Isobars are lines joining places with equal mean sea-level pressures (MSLP).

Fig 3: Identification of weather systems, isobars and front

Weather systems and fronts

Relationship between isobars and wind

Fig 4: Relationship between isobars and wind speed[/captio

Understanding station plots on a weather map

Fig 5: An example of a plotted chart

Good quality observations are one of the basic ‘tools of the trade’ for a weather forecaster.

The weather conditions at each individual station can be represented on a surface chart by means of station plot.

This means that information which would take up a lot of space if written on to a chart can be displayed in a quick easy to understand format.

Figure 5 shows an example of a plotted chart.

The land station plot can represent all the elements reported from that station, these typically include:

  • Air temperature
  • Dew-point temperature
  • Wind speed
  • Wind direction
  • Visibility
  • Atmospheric pressure and three-hour tendency
  • Cloud amounts
  • Cloud types
  • Cloud heights
  • Present weather
  • Past weather

Traditionally station plots for manned observing sites were based around a central station circle. However, increasingly, automatic weather observations are replacing these and being plotted on weather charts. To differentiate between the two, automatic observations are plotted around a station triangle. Each element of the observation, with the exception of wind, is plotted in a fixed position around the station circle or triangle so that individual elements can be easily identified.

Fig 6: Plotting positions on a station circle
Fig 7: A typical coded manual observation
Fig 8: A typical coded automatic observation

Plotting a station plot

Total cloud amount

The total amount of the sky covered by cloud is expressed in oktas (eighths) and is plotted within the station circle for manned observations or station triangle for automatic stations, by the amount of shading.

The symbols used for both manual and automatic observations are shown below.

Fig 9: Symbols for manual cloud cover
Fig 10: Symbols for automatic cloud cover

Wind speed and direction

The surface wind direction is indicated on the station plot by an arrow flying with the wind. Direction is measured in degrees from true North. Therefore a wind direction of 180 is blowing from the south. The wind speed is given by the number of ‘feathers’ on the arrow. Half feathers represent 5 knots whilst whole feathers indicate 10 knots. A wind speed of 50 knots is indicated by a triangle. Combinations of these can be used to report wind speed to the nearest 5 knots. The symbols used are as follows.

Fig 11: Symbols for wind speed

Air temperature

Air temperature is plotted to the nearest whole degree Celsius, i.e. 23 would indicate 23 degrees Celsius.

Dew point temperature

Dew point temperature is plotted to the nearest whole degree Celsius, i.e. 18 would indicate a dew point of 18 degrees Celsius.

Pressure

Pressure is recorded in millibars and tenths and the last three digits are plotted. Therefore 1003.1 would be plotted as 031 and 987.1 would be plotted as 871.

Present weather

In total the Met Office has 100 codes for recording the current weather at the time of the observation. Different types of weather are represented using different weather symbols, a key to which can be found below.

Fig 12: Symbols for present weather

 

Fig 13: Symbols for present weather

 

Fig 14: Symbols for present weather

 

Fig 15: Symbols for present weather

 

Fig 16: Symbols for present weather

 

Past weather

A simplified version of the present weather plots is used to indicate past weather.

Fig 17: Symbols for past weather

 

Pressure Tendency

Pressure trend shows how the pressure has changed during the past three hours, i.e rising or falling, and pressure tendency shows by how much it has changed. The tendency is given in tenths of a millibar, therefore ’20’ would indicate a change of two millibars in the last three hours. Pressure tendency is indicated by the following symbols.

Fig 18: Symbols for pressure tendency

 

Visibility

Visibility, which is how far we can see, is given in coded format, in either meters or kilometres. Visibilities below five kilometres are recorded to the nearest 100 metres, whilst those above five kilometres are given to the nearest kilometre.

For visibilities equal to and less than five km:

Fig 22: Example plot
Table 1: Codes for visibilities of less than five kilometres
Code
Distance (km)
Code
Distance (km)
Code
Distance (km)
00
<0.0
19
1.9
38
3.8
01
0.1
20
2.0
39
3.9
02
0.2
21
2.1
40
4.0
03
0.3
22
2.2
41
4.1
04
0.4
23
2.3
42
4.2
05
0.5
24
2.4
43
4.3
06
0.6
25
2.5
44
4.4
07
0.7
26
2.6
45
4.5
08
0.8
27
2.7
46
4.6
09
0.9
28
2.8
47
4.7
10
1.0
29
2.9
48
4.8
11
1.1
30
3.0
49
4.9
12
1.2
31
3.1
50
5.0
13
1.3
32
3.2
51
Not Used
14
1.4
33
3.3
52
Not Used
15
1.5
34
3.4
53
Not Used
16
1.6
35
3.5
54
Not Used
17
1.7
36
3.6
  
18
1.8
37
3.7
  

For visibilities greater than five km:

Table 2: Codes for visibilities of more than five kilometres
Code
Distance (km)
Code
Distance (km)
56
6
73
23
57
7
74
24
58
8
75
25
59
9
76
26
60
10
77
27
61
11
78
28
62
12
79
29
63
13
80
30
64
14
81
35
65
15
82
40
66
16
83
45
67
17
84
50
68
18
85
55
69
19
86
60
70
20
87
65
71
21
88
70
72
22
89
>70

Low cloud type

The type of low cloud present is provided in coded format, using the symbols below.

Fig 19: Symbols for low cloud type

Medium cloud type

The type of medium cloud present is provided in coded format, using the symbols below.

Fig 20: Symbols for medium cloud type

High cloud type

The type of high cloud present is provided in coded format, using the symbols below.

Fig 21: symbols for high cloud type

Cloud height

Cloud heights are measured in hundreds or thousands of feet. The way these are plotted varies depending on whether the station is an automatic or manned observing site.

For automatic stations, indicated by a station triangle, the following codes are used.

Table 3: Cloud heights for automatic stations
Code
Height in feet
00
<100
05
500
10
1000
15
1500
20
2000
50
5000
60
6000

For manned stations, indicated by a station circle, the following codes are used.

Table 4: Cloud heights for manned stations
Code
Height in feet
0
0-149
1
150-299
2
300-599
3
600-999
4
1,000-1,999
5
2,000-2,999
6
3,000-4,999
7
5,000-6,499
8
6,500-7,999
9
8,000 or above
/
Cloud height unknown

Gust speed

Gust speeds are measured in knots and proceeded by the letter G. Gust speeds are normally only recorded if they exceed 25 knots and are plotted as whole knots, i.e. G35 indicates a gust of 35 knots.

Example

The decode of this station plot is as follows:

Fig 22: Example plot
Type of observation:
Manned
Total cloud amount:
8 oktas
Wind Speed:
28-32 knots
Wind direction:
South-westerly
Air temperature:
23 degrees Celsius
Dew point temperature:
18 degrees Celsius
Pressure:
1004.2 millibars
Present weather:
Continuous moderate rain
Past weather:
Rain
Pressure tendency:
Falling 0.5 millibars in the past three hours
Visibility:
6km
Low cloud type:
Stratus
Low cloud amount:
6 oktas
Low cloud height:
1000 feet
Medium cloud type:
Altostratus
High cloud type:
Cirrus
Gust speed:
45 knots

 

Exercise

Why not try decoding the following observational plots.



Web page reproduced with the kind permission of the Met Office

Weather Systems Video

Weather Charts

By the end of the lesson, you will be able to:

  • Understand isobars and their relationship with wind speed
  • Identify various pressure systems and fronts on a weather chart
  • Interpret and produce plotted weather symbols.

Part A – Isobars, pressure and wind

Part B – Identifying pressure systems and fronts

Part C – Plotted weather charts

Teachers’ notes

Teachers’ Notes

Resources required

Computers with Internet access would be desirable. Alternatively if Internet access is not available, printed copies of student sheets and worksheets should be made.

Prior knowledge required

A basic background of weather and climate.

Teaching activities

Students can visit the following pages to gain a basic background into the topics covered:

Weather charts

Interpreting Weather Maps

The information on the student sheets can be delivered by the teacher and activities completed individually. Alternatively students can work through the whole lesson themselves.

Exercises

Three worksheets with exercises are provided to consolidate learning.

A series of additional exercises are provided for more able students, or those who have already studied pressure systems and fronts in more detail prior to this lesson.

Suggestions for homework

Any of the worksheet activities can be completed. Alternatively students can collect weather charts from the Internet or a newspaper and repeat the exercises using these.

 

Part A – Isobars, pressure and wind

Isobars are lines joining points of equal pressure, similar to contours, which are shown on weather charts. Charts showing isobars are useful because they can help to identify anticyclones and depressions. Pressure is measured in millibars and isobars are normally drawn at intervals of 4 millibars. Pressure values are corrected to Mean Sea Level Pressure (MSLP) before being plotted on a map, this ensures that altitude does not affect the mapping.

Isobars are also helpful because the help us to understand the direction and strength of the wind in a particular area. Where isobars are very close together, for example near a depression, they indicate strong winds. Where the isobars are more widely spaced, near an anticyclone for example, they indicate light winds.

The wind will blow almost parallel to the isobars. Around an anticyclone the winds will blow slightly across the isobars, away from the centre of the anticyclone. In depressions, the wind will blow slightly across the isobars towards the centre of the low pressure.

Buys’ Ballot’s Law states that if you stand with you back to the wind in the northern Hemisphere, low pressure will be on your left. This means that you can work out the wind direction at different locations on a weather chart.

What to do next

Using this information on isobars you should now be able to complete worksheet 1.

Then you can complete extension exercise 1 or go on to Part B – Identifying pressure systems and fronts.

Part B – Identifying pressure systems and fronts

Anticyclones

An anticyclone, also known as a ‘high’ can be identified on a weather chart as an often large area of widely spaced isobars, where pressure is higher than surrounding areas. In the Northern Hemisphere winds blow in a clockwise direction around high pressure. The highest pressure occurs at the centre and is known as the ‘high pressure centre’. Anticyclones can bring warm and sunny weather in summer, but cold and foggy weather in winter.

Depressions

A depression, also known as a ‘low’ can be recognised on a weather chart by an area of closely spaced isobars, often in a roughly circular shape, where pressure is lower than surrounding areas. They are often accompanied by fronts. In the Northern Hemisphere winds blow around depressions in an anticlockwise direction. The lowest pressure occurs at the middle of a depression, this is known as the ‘low pressure centre’. Depressions are often associated with strong winds and heavy rain and are nearly always accompanied by fronts.

depressions

Troughs

Troughs are elongated extensions of areas of low pressure. They bring similar weather to that associated with depressions.

Ridges

Ridges are elongated extensions of areas of high pressure. They bring similar weather to that associated with anticyclones.

 

Col

A col can be identified as an area of slack pressure between two anticyclones and two depressions.

The following diagram summarises the appearance on a weather chart of the main types of pressure systems.

Cold fronts and warm fronts

Cold fronts can be identified on weather charts as bold lines with triangles. These are blue when displayed on colour charts. The points of the triangle indicate the direction in which the front is moving. A cold front indicates a change in air mass, where warmer air is being replaced by colder air. They often bring short spells of heavy rainfall in the form of showers and squally winds, and are accompanied by a decrease in temperature, a veer in wind direction and a change to brighter showery conditions.

Warm fronts can be identified on weather charts as bold lines with semi-circles or humps. These are coloured red when displayed on colour charts. The direction of the humps indicates the direction in which the front is moving. A warm front indicates a change from a colder to a warmer air mass. They often bring spells of prolonged and sometimes heavy rainfall, with strong winds.

Occluded fronts

Occluded fronts can be identified on weather charts as bold lines with sets of triangles and semi-circles. These are coloured purple on coloured weather charts. The direction in which the symbols face indicates the direction in which the front is travelling. Occlusions are formed when the cold front overtakes the warm front, therefore they have similar characteristics to a cold front, but less intense.

Warm Sector

The warm sector of a depression is located behind the warm front and ahead of the cold front. It often brings mild temperatures but the weather can be overcast with drizzle.

What to do next

Using this information you should now be able to complete worksheet 2.

Then you can complete extension exercise 2 or go on to Part C.

 Part C – Plotted weather charts

The following image is an example of a UK plotted weather chart.

Plotted surface charts are made up of individual ‘station circles’. Each individual stations observation is put into graphical format so that it is simple to understand, can be put on a chart and be compared to its neighbours.

The diagram opposite shows the basic station circle, including temperature, pressure, weather, cloud cover, wind speed and direction. Some elements, e.g. weather and cloud cover, are put into a graphical code to make them more obvious. The Met Office uses a much more complex station circle but the one below is a simplified version using the main weather elements.

Wind direction is indicated by a line coming from the centre of the station circle. The line indicates the direction from which the wind is blowing.Wind speed and direction

On the end of the wind direction line are ‘feathers’, these indicate the wind speed. Half feathers represent 5 knots whilst whole feathers indicate 10 knots. A wind speed of 50 knots is indicated by a triangle. Combinations of these can be used to report wind speed to the nearest 5 knots.

The table below shows the symbols used to indicate different wind speeds.

Cloud cover

Cloud cover is indicated by the shading of the centre of the station circle. The table below shows the meanings of the different symbols.

Temperature

Temperature is plotted to the nearest degree and is located in the top left-hand corner of the station plot.

Pressure

Pressure is plotted in the top right-hand corner of the station plot.

Weather

In total the Met Office has 99 codes for recording the current weather at the time of the observation. However these can be simplified down. Different types of weather are represented using different weather symbols, a key to which can be found below.

What to do next

Using this information you should now be able to complete worksheet 3.

Then you can complete extension exercise 3 .

Worksheet 1 – Reading pressure values from a surface pressure chart

The image below is an extract from a surface pressure chart, on which isobars, fronts and pressure systems have already been drawn. Download a copy of the worksheet here. If working on a PC, print out a copy of this page. Then study the chart, and complete the table below, by entering the approximate pressure at each of the labelled locations.

Worksheet 2 – Surface chart analysis

The image below shows a surface pressure chart, on which isobars, fronts and pressure systems have already been drawn. Download a copy of the worksheet here. Then study the chart, and identify and label the following items:

Cold front Col
Warm front Trough
Occlusion High pressure centre
Trough of low pressure Low pressure centre
Ridge of High pressure Warm sector

If you need help refer back to Part B – Identifying pressure systems and fronts.

Worksheet 3 – Station circle plots

The following three questions contain examples of plotted station circles. Download a copy of the worksheet here. Then study each of these plots and complete the tables below with details of the temperature, weather, pressure, cloud cover, wind speed and wind direction.

The following three questions contain tables of weather data. Study each of these tables and plot the details of the temperature, weather, pressure, cloud cover, wind speed and wind direction on to the station circle provided.

If you need help refer back to Part C – Plotted weather charts.

Worksheet 1 – extension exercise

The following diagram shows a series of plotted pressure values. Download a copy of the worksheet here. Complete the diagram by drawing isobars at intervals of 4 millibars, including 992, 996, 1000, 1004 and 1008.

 

Worksheet 2 – extension exercise

Study the chart below. This chart is for mid-November. Download a copy of the worksheet here. Then using your knowledge of the characteristics of anticyclones, depressions and fronts in winter, complete the table below with approximate readings.

There is no one correct answer. Your values should simply indicate the typical values and the variations between each location. For example, should location A be warmer or colder than location B? Should location E have stronger or lighter winds than location D?


 

Worksheet 3 – extension exercise

Study the chart below. This chart is for mid November. Download a copy of the worksheet here. Then using your knowledge of the characteristics of anticyclones, depressions and fronts in winter, construct a station circle for each of the locations marked on the chart.

There is no one correct answer. You values should simply indicate the typical values and the variations between each location. For example should location A be warmer or colder than location B? Should E have stronger or lighter winds than location D?

If you have already completed extension exercise 2, you will simply need to convert your table of results into station circle plots.

Location A
Location B

 
Location C

 
Location D

 
Location E

 
Location F

 
Location G

 
Location H

 

Web page reproduced with the kind permission of the Met Office

In Depth – Understanding Weather

Understanding weather

Air masses

Atmosphere

Clouds

El Niño and La Niña

High pressure or anticyclone

Low pressure or depression

Temperature differences

Weather fronts

Weather systems

Wind

Air masses

Air masses are parcels of air that bring distinctive weather features to the country. An air mass is a body or ‘mass’ of air in which changes in temperature and humidity within them are relatively slight. That is to say the air making up the mass is very uniform. in temperature and humidity.

An air mass is separated from an adjacent body of air by a weather front. An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.

Atmosphere

A thin layer of mixed gases which covers the Earth and helps it from becoming too hot or too cold. Its circulation, the heat (terrestrial radiation) and light (solar radiation) which pass through it, and the processes which go on in it, all affect the climate. The atmosphere is about 800 km (500 miles) deep and is made up of 21% oxygen, 78% nitrogen, 0.037% carbon dioxide, and other gases including hydrogen, helium, neon, argon, krypton, xenon, and water vapour.

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

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 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 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

blue sky with clouds

 

El Niño and La Niña

The tropical Pacific Ocean has a warming and cooling cycle. This cycle is a completely natural event and usually lasts between three to seven years. When the waters become warmer it is called El Niño, and when they become cooler it is called La Niña. During the cycle, the temperature of the ocean can change by around 3 °C between the warmest and coolest times.

Fishermen off the South American coast have known about this natural event for hundreds of years. When it happens, they see a huge fall in the numbers of fish caught. But scientists are only just beginning to understand how the event affects Earth’s weather and climate.

El Niño and La Niña

High pressure or anticyclone

In an anticyclone (also referred to as a ‘high’) the winds tend to be light and blow in a clockwise direction. Also the air is descending, which inhibits the formation of cloud. The light winds and clear skies can lead to overnight fog or frost. If an anticyclone persists over northern Europe in winter, then much of the British Isles can be affected by very cold east winds from Siberia. However, in summer an anticyclone in the vicinity of the British Isles often brings fine, warm weather.

Low pressure or depression

In a depression (also referred to as a ‘low’), air is rising. As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. Consequently, the weather in a depression is often cloudy, wet and windy (with winds blowing in an anticlockwise direction around the depression). There are usually frontal systems associated with depressions.

Temperature differences

Temperature affects other weather elements including air pressure, cloud formation, humidity and precipitation.

Factors affecting temperature:

  • Latitude – warmer closer to the equator and cooler moving away towards the poles
  • Altitude – getting colder as the land gets higher
  • Distance from the sea – temperatures inland are higher than the coast during the summer and lower than the coast during winter. This is because land heats up and cools down more quickly than the sea
  • North-facing slopes in the southern hemisphere and south-facing slopes in the northern hemisphere receive more sunlight than the opposite slopes and are warmer
  • Wind – generally makes the air feel cooler

Weather fronts

A weather front is simply the boundary between two air masses.

There are three different types of weather front. These are:

  1. Cold front
  2. Warm front
  3. Occluded front (also called an occlusion)

Cold front

This is the boundary between warm air and cold air and is indicative of cold air replacing warm air at a point on the Earth’s surface

On a synoptic chart a cold front appears blue

cold front symbol

 

The presence of a cold front means cold air is advancing and pushing underneath warmer air. This is because the cold air is ‘heavier’ or denser, than the warmer air. Cold air is thus replacing warm air at the surface. The symbols on the front indicate the direction the front is moving.

The passage of a cold front is normally marked at the earth’s surface by a rise of pressure, a fall of temperature and dew-point, and a veer of wind (in the northern hemisphere). Rain occurs in association with most cold fronts and may extend some 100 to 200 km ahead of or behind the front. Some cold fronts give only a shower at the front, while others give no precipitation. Thunder may occur at a cold front.

Warm front

This is the boundary between cold air and warm air and is indicative of warm air replacing cold air at a point on the Earth’s surface

On a synoptic chart a warm front appears red

warm front symbol

 

The presence of a warm front means warm air is advancing and rising up over cold air. This is because the warm air is ‘lighter’ or less dense, than the colder air. Warm air is thus replacing cold air at the surface. The symbols on the front indicate the direction the front is moving.

As a warm front approaches, temperature and dew-point within the cold air gradually rise and pressure falls at an increasing rate. Precipitation usually occurs within a wide belt some 400 km in advance of the front. Passage of the front is usually marked by a steadying of the barometer, a jump in temperature and dew-point, a veer of wind (in the northern hemisphere), and a cessation or near cessation of precipitation.

Occluded front

These are more complex than cold or warm fronts. An occlusion is formed when a cold front catches up with a warm front

When a cold front catches up with a warm front the warm air in the warm sector is forced up from the surface

On a synoptic chart an occluded front appears purple

occluded front symbol

 

Weather systems

Weather can change on a daily basis especially at middle to high latitudes where it is controlled by weather systems, depressions and anticyclones.
On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure), depressions (areas of low pressure), troughs and ridges which are associated with particular kinds of weather.

Wind

windsock

The movement of air around the earth from high pressure to low pressure is what brings about winds. The direction given for the wind refers to the direction from which it comes. For example, a westerly wind is blowing from the west towards the east.

Measurements of wind strength are made at 10 metres (33 feet) above the ground. A specified height has to be used because the wind speed decreases towards the ground. In this country winds are measured in knots (nautical miles per hour). However, forecast winds are often given in miles per hour (where 1 knot is equivalent to 1.15 mph) or in terms of the Beaufort Scale.

There are rapid variations in the wind – these are referred to as gusts. Gusts are higher inland than over the sea or windward coasts, although the mean wind speeds tend to be lower inland. Typically, gusts can be 60% higher than the mean speed, although in the middle of cities this can reach 100%. Northerly winds tend to be gustier than southerly ones. In general, the weather is strongly influenced by the wind direction, so information about the wind provides an indication of the type of weather likely to be experienced.

  • Northerly winds tend to bring relatively cold air from polar regions to the British Isles. Similarly, southerly winds tend to bring relatively warm air from the tropics
  • As cold polar air moves southwards over an increasingly warm sea, the heating of the air by the sea causes cumulus clouds to form. These clouds may grow sufficiently for showers to develop and, consequently, winds from the north-west, north or north-east usually bring cold, showery weather to the British Isles
  • Warm air from the tropics moving northwards over the sea is cooled from below. Sometimes the cooling is sufficient for sea fog or a thin layer of stratus to form. The cloud can become thick enough for drizzle, especially on windward coasts and over high ground. In general, winds from the west or south-west are associated with overcast, wet weather
  • Winds from the south and south-east mainly occur in summer and these bring warm, dry weather. However, southerly winds can sometimes bring hot, thundery weather
  • Easterly winds in winter bring very cold air to the British Isles. The characteristics and path of the air determine whether it is cloudy (with perhaps rain, sleet or snow) or fine and sunny. In summer, an easterly wind will mean it is cool on the east coast but warm elsewhere, usually with clear skies

windsock

Web page reproduced with the kind permission of the Met Office

Anticyclones, Depressions and Fronts

LESSON PLAN: Introduction to Anticyclones, Depressions and Fronts
Key Stage 4 – GCSE
Subject Geography


 

Length 1 lesson

Teaching Objectives/Learning Outcomes
By the end of the lesson, pupils will know and understand:
Characteristics of depressions and fronts and the sequence of associated weather
Characteristics of anticyclones and the contrast between those in summer and in winter.

Resources Required
None.

Prior Knowledge Required
A basic knowledge of weather and climate

Teaching Activities
The following web pages have related resources at a similar level:

Weather Systems

Student Charts

Weather Systems

Exercises
4 worksheets with exercises are provided to consolidate learning.
A series of extension exercises are provided for more able students, or those who have already studied the topics covered in more detail prior to this lesson.

Plenary – A quiz is available, which brings together all the topics covered. The can be used to examine whether the objectives of the lessons have been met.

Suggestions for Home Work
Any of the worksheet activities can be completed as homework.

PART A – ANTICYCLONES AND DEPRESSIONS

High Pressure Systems

A high pressure system, also known as an anticyclone occurs when the weather is dominated by stable conditions. Under an anticyclone air is descending, forming an area of higher pressure at the surface. Because of these stable conditions, cloud formation is inhibited, so the weather is usually settled with only small amounts of cloud cover. In the Northern Hemisphere winds blow in a clockwise direction around an anticyclone. As isobars are normally widely spaced around an anticyclone, winds are often quite light.
Anticyclones can be identified on weather charts as an often large area of widely spaced isobars, where pressure is higher than surrounding areas.

Winter Anticyclones

In winter the clear, settled conditions and light winds associated with anticyclones can lead to frost and fog. The clear skies allow heat to be lost from the surface of the earth by radiation, allowing temperatures to fall steadily overnight, leading to air or ground frosts. Light winds along with falling temperatures can encourage fog to form; this can linger well into the following morning and be slow to clear. If high pressure becomes established over Northern Europe during winter this can bring a spell of cold easterly winds to the UK.

Summer Anticyclones

In summer the clear settled conditions associated with anticyclones can bring long sunny days and warm temperatures. The weather is normally dry, although occasionally, very hot temperatures can trigger thunderstorms. An anticyclone situated over the UK or near continent usually brings warm, fine weather.

Low Pressure Systems

A low pressure system, also known as a depression occurs when the weather is dominated by unstable conditions. Under a depression air is rising, forming an area of low pressure at the surface. This rising air cools and condenses and helps encourage cloud formation, so the weather is often cloudy and wet. In the Northern Hemisphere winds blow in anticlockwise direction around a depression. Isobars are normally closely spaced around a depressions leading to strong winds.
Depressions can be identified on weather charts as an area of closely spaced isobars, often in a roughly circular shape, where pressure is lower than surrounding areas. They are often accompanied by fronts.

What to do next
Using this information on pressure systems you should now be able to complete worksheet 1. Then you can move on to extension exercise 1 or worksheet 2

  • PART B – FRONTS

A front is a boundary between two different types of air masses, these are normally warm moist air masses from the tropics and cooler drier air masses from polar regions. Fronts move with the wind so over the UK they normally move from west to east. The notes below provide information about the most common types of fronts. The descriptions given apply to active well developed fronts, weaker fronts may not display all the characteristics or they may be less well defined.

Warm Fronts

A warm front indicates that warm air is advancing and rising up over the colder air. This is because the warm air is ‘lighter’ or less dense, than the cold air. Therefore warm fronts occur where warmer air is replacing cooler air at the surface. As the warm front approaches there is a gradual deterioration in the weather. Clouds gradually lower from higher cirrus, through altostratus, to stratus and nimbostratus at the front. There is often a prolonged spell of rainfall which is often heavy. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it remains cloudy.

Temperatures rise behind the warm front and winds turn clockwise, also known as a wind ‘veer’. Pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage.

The following diagram shows the formation of a warm front in diagrammatic form.

PART B – FRONTS A front is a boundary between two different types of air masses, these are normally warm moist air masses from the tropics and cooler drier air masses from polar regions. Fronts move with the wind so over the UK they normally move from west to east. The notes below provide information about the most common types of fronts. The descriptions given apply to active well developed fronts, weaker fronts may not display all the characteristics or they may be less well defined. Warm Fronts A warm front indicates that warm air is advancing and rising up over the colder air. This is because the warm air is ‘lighter’ or less dense, than the cold air. Therefore warm fronts occur where warmer air is replacing cooler air at the surface. As the warm front approaches there is a gradual deterioration in the weather. Clouds gradually lower from higher cirrus, through altostratus, to stratus and nimbostratus at the front. There is often a prolonged spell of rainfall which is often heavy. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it remains cloudy. Temperatures rise behind the warm front and winds turn clockwise, also known as a wind ‘veer’. Pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage. The following diagram shows the formation of a warm front in diagrammatic form.

The following diagram shows a cross section through a warm front, with associated cloud, temperature and weather changes.

anticyclones_depressions_fronts

 

 

 

 

 

 

 

 

Cold Fronts

A cold front indicates that cold air is advancing and pushing underneath warmer air at the surface. This occurs because the cold air is ‘heavier’ or denser than the warm air. Therefore cold fronts occur where cooler air is replacing warmer air at the surface. The passage of weather associated with a cold front is much shorter lived than that with a warm front. As there is often a lot of cloud in the warmer air ahead of the cold front, there is often little indication of the approaching cold front. As the front passes temperatures fall and there is often a short spell of very heavy rain, sometimes with inbedded thunderstorms and cumulonimbus clouds. Behind the front the weather is much brighter with broken clouds but occasional showers. Winds veer with the passage of the cold front and are often strong and gusty, especially near showers. Pressure rises throughout the approach and passage of the cold front.

The following diagram shows the formation of a cold front in diagrammatic form.

anticyclones_depressions_fronts_3

 

 

 

 

 

 

 

 

 

 

 

The following diagram shows a cross section through a cold front, with associated cloud, temperature and weather changes.

anticyclones depressions fronts

 

 

 

 

 

 

Occlusions

In a mature depression the warm front normally precedes the cold front. Cold fronts generally travel much quicker than warm fronts, and eventually it will catch up with the warm front. Where the two fronts meet, warm air is lifted from the surface and an occlusion is formed. An occlusion can be thought of as having similar characteristics to both warm and cold fronts. The weather ahead of an occlusion is similar to that ahead of a warm front, whilst the weather behind is similar to that behind a cold front.

The following diagrams depict the formation of an occlusion

anticyclones_depressions_fronts_5

anticyclones_depressions_fronts_6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What to do next
You can now move on to Part C – Life Cycle of a Depression.

 

PART C – LIFE CYCLE OF A DEPRESSION

A Norwegian scientist called Vilhelm Bjerknes devised a simple model which described how depressions developed from the meeting of warm and cold air. The model had four stages which are detailed below.

Origin and Infancy

Initially a warm air mass such as one from the tropics, meets a cooler air mass, such as one from the polar regions. Depressions which affect the UK normally originate over the Atlantic Ocean.

anticyclones depressions

Maturity

The warm air rises up over the colder air which is sinking. A warm sector develops between the warm and cold fronts. The mature stage of a depression often occurs over the UK.

anticyclones depressions fronts

Occlusion

The cold front travels at around 40 to 50 miles per hour, compared to the warm front which travels at only 20 to 30 miles per hour. Therefore the cold front eventually catches up with the warm front. When this occurs an occlusion is formed.

anticyclones depressions fronts

Death

Eventually the frontal system dies as all the warm air has been pushed up from the surface and all that remains is cold air. The occlusion dies out as temperatures are similar on both sides. This stage normally occurs over Europe or Scandinavia.

What to do next
You can now move on to Part D – Depression cross-section and weather sequence

PART D – DEPRESSION CROSS SECTION AND WEATHER SEQUENCE

 

Cross-section through a Classic Depression

Most depressions have a warm and cold front, more mature depressions may also have an occluded front. The diagram below shows a cross-section through a depression, showing the warm and cold fronts and an indication of the associated weather.

anticyclones_depressions

table

What to do next

Using this information on the passage of depressions you should now be able to complete worksheet 3 and worksheet 4.

 

Web page reproduced with the kind permission of the Met Office