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)

Passage of a Depression – Animation

UK Synoptic Charts: Pressure and Wind Skills Exercise

Skills Exercise



You will need an Atlas for these activities

  1. Identify on the map a country experiencing HIGH pressure.                                                
  2. What type of air pressure is arriving in the UK?                                                                                            
  3. What is the lowest value of pressure shown on the map?                                                                           
  4. What is the air pressure over Greenland?                                                                                                     
  5. What is the air pressure over Northern Africa?                                                                                
  6. Identify an area that would have STRONG winds (the isobars will be close together)                                                                                                                    
  7. Identify an area that would have WEAK winds (the isobars will be far apart)                                 

 

EXTENSION– can you draw simple arrows to show the pattern of wind over the UK on the small inset map? (recall that wind moves from high to low and is deflected to the left in the Northern hemisphere) 

 

Synoptic Weather Charts – Questions

Synoptic weather charts

The figure below shows the synoptic pressure chart at midnight on Wednesday, 17 May.

1. Name the pressure feature running from the Baltic States (Estonia, Latvia and Lithuania) south to the Adriatic Sea.

2. What is the name of the pressure feature extending south from Iceland into Scotland, Wales and England?

3. What is type of pressure feature to the south-west of Greenland?

4. From which direction would you expect the wind direction to be blowing over south-west England?

5. Compare the pattern of isobars to the south-west of Iceland, with the pattern of isobars in the bay of Biscay.

    (a) In which area might you expect faster wind speeds?

    (b) Explain your answer, with reference to the isobar patterns.

6. Imagine that the pressure system, observed at 1033 millibars, to the west of Ireland moved east in the next 12 hours and then remained over the United Kingdom for several days.

    (a) What is the technical term given to this type of pressure system which remains stationary over the country?

    (b) Outline the weather patterns and hazards associated if this was to happen in June or July.

    (c) What differences in weather features might occur if this happened in November or December?

rainfall

Web page reproduced with the kind permission of the Met Office

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

Interpreting Isotherms

Isotherms

The figure below is a map of isotherms, showing the average mean temperatures for January over the UK, based on average values for 1961–90.

1. Explain the reasons why Newquay is warmer than Ayr in January.

2. With reference to the Environmental Lapse Rate, outline why temperatures at Okehampton are lower than at Newquay.

3. In northern England, temperatures on the west coast near Keswick are similar to those at Middlesbrough on the east coast. Explain how the föhn effect might influence these temperature patterns.

4. (a) Outline how physical factors affect the shape of the 4 °C isotherm in the River Severn estuary and valley north of Gloucester.
    (b) How does this pattern affect agriculture?

5. The 4 °C isotherm also bulges around cities such as London and Bristol. Explain the human factors which have caused these cities to be milder in January than rural areas with a similar latitude, such as Marlborough.

isotherms

Web page reproduced with the kind permission of the Met Office.

 

Shipping Forecast

Guide to shipping forecasts

This is a brief guide to the main terms used in the shipping forecasts that are broadcast on BBC Radio 4, and also as a reference to the main shipping areas (known as the coastal waters) surrounding the British Isles.

Shipping bulletin

Weather bulletins for shipping are broadcast daily on BBC Radio 4 at the following times: 0048 and 0520 (long wave and FM), 1200 and 1755 (normally long wave only). The bulletins consist of a gale warning summary, general synopsis, sea-area forecasts and coastal station reports. In addition, gale warnings are broadcast at the first available programme break after receipt. If this does not coincide with a news bulletin, the warning will be repeated after the next news bulletin.

A glossary of terms used in the shipping bulletin

Latest shipping forecast/gale warning or as a map

Forecasts for inshore waters

In addition, some bulletins include a forecast for all UK inshore waters, as distinct from the coastal waters. This can be heard on BBC Radio 4 at the end of programmes (approximately 0048), and on BBC Radio 3 at 0535. The forecast covers the area up to 12 miles offshore and is for the period up to 1800 the next day. It includes a general synopsis, the forecast of wind direction and force, visibility and weather. The broadcast on Radio 4 also includes the latest available reports of wind direction and force, visibility, sea-level pressure and tendency for approximately 20 stations around the UK.

Latest inshore waters forecast

Beaufort scale

 

 

 

Specifications and equivalent speeds

Web page reproduced with the kind permission of the Met Office

 

Shipping Forecast weather system excercise notes and worksheet.

Weather Systems Video

Weather Systems Teachers’ Notes

Teachers’ notes to accompany Weather Systems

Teaching objectives/Learning outcomes

By the end of the lesson, pupils will know and understand:

  • the characteristics of depressions and fronts and the sequence of associated weather
  • the characteristics of anticyclones and the contrast between those in summer and in winter.

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.

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.

Part A – Anticyclones and Depressions

Part B – Fronts

Part C – Life cycle of a depression

Part D – Depression cross-section and weather sequence

Exercises

Five worksheets with exercises are provided to consolidate learning.

Suggestions for home work

Any of the worksheet activities can be completed.

Reproduced with the kind permission of the Met Office

Weather Systems

Anticyclones, Depressions and Fronts

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

  • Understand the characteristics of depressions and fronts and the sequence of associated weather
  • Understand the characteristics of anticyclones and the contrast between those in summer and in winter. 

Part A – Anticyclones and Depressions

Part B – Fronts

Part C – Life cycle of a Depression

Part D – Depression cross-section and weather sequence

Teachers’ notes

fronts diagram

Part A


Anticyclones, Depressions and Fronts

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 complete Extension 1 or worksheet 2.

Part B


Anticyclones, Depressions and Fronts

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

high pressure diagram

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

low pressure diagram

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

isobaric through

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

ridge

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 diagrams below depict the formation of an occlusion.

diagrams below depict the formation of an occlusion.

The diagram below shows the occlusion in cross section.

The diagram below shows the occlusion in cross section.

What to do next

Now you can go on to Part C – Life cycle of a Depression.

Anticyclones, Depressions and Fronts

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. map of UK 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. air temp

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. Occlusion chart

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 Now you can go on to Part D – Depression cross-section and weather sequence.

Anticyclones, Depressions and Fronts

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.

Cross-section through a classic Depression

WEATHER ASSOCIATED WITH THE PASSAGE OF A CLASSIC DEPRESSION
 Ahead of the warm frontPassage of the warm frontWarm sectorPassage of the cold frontCold sector
Pressure
starts to fall steadilycontinues to fallsteadiesstarts to risecontinues to rise
Temperature
quite cold, starts to risecontinues to risequite mildsudden dropremains cold
Cloud cover
cloud base drops and thickens (cirrus and altostratus)cloud base is low and thick (nimbostratus)cloud may thin and breakclouds thicken (sometimes with large cumulonimbus)clouds thin with some cumulus
Wind speed and direction
speeds increase and direction backsveers and becomes blustery with strong gustsremain steady, backs slightlyspeeds increase, sometimes to gale force, sharp veerwinds are squally
Precipitation
none at first, rain closer to front, sometimes snow on leading edgecontinues, and sometimes heavy rainfallrain turns to drizzle or stopsheavy rain, sometimes with hail, thunder or sleetshowers

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