## Drawing Pressure and Temperature Contours (Harder)

### Drawing Isobars

Isobars are lines of constant pressure. Drawing the isobars reveals features (eg highs, lows, ridges and troughs) which help us understand the weather.

When trying to draw isobars, remember the following

– You are trying to draw the isobars of pressure for the values below the
graph

– The symbols on the map give the observed pressure and wind speed and
direction. Remember that the wind is blowing from the tail of the arrow
to the centre. The bars on the tail of the arrow tell you the wind speed:

– The wind blows almost parallel to the isobars (they are usually blow
slightly more towards the centre of the low pressure area). If you stand
with your back to the wind in the northern hemisphere the pressure is

– Isobars tend to be parallel to each other, don’t wiggle and never cross.

– The closer the isobars are to each other, the stronger the wind. You can
use the bars on the tail of the weather station symbol to give you the
Beaufort force of the wind. Look at the scale at the top of the map (this is
called the ‘geostrophic scale’). The distance from the left hand edge of
the scale to the force at the symbol gives you the spacing between 2mbar
isobars at that point.

Draw the 1004, 1008, 1012 and 1016 isobars (noting that they have been recorded in shorthand, such that 1004 becomes 04 etc.) remembering that the wind tends to blow parallel to the isobars. Can they see what sort of a weather system it is, and where the fronts might be?

Draw the 2.5, 4.5, 6.5, 8.5 and 10.5 isotherms (lines of equal temperature), remembering that sometimes, if there isn’t any data in a particular place, you have to make an educated guess about what the contour might look like. Is it more obvious now what the structure of the weather system is?

Shade the areas of rain, heavy rain and drizzle in the map below. Is it more obvious now what the structure of the weather system is?

## What is the Weather

For each location marked on each map, what do you expect the weather to be doing? Think about whether it is windy, what the wind direction is, whether it is raining and whether it is warmer or colder.

Galway __________________________________________________________________

Aberdeen __________________________________________________________________

London __________________________________________________________________

Glasgow __________________________________________________________________

Norwich __________________________________________________________________

Exeter __________________________________________________________________

Tiree __________________________________________________________________

Carlisle __________________________________________________________________

Norwich __________________________________________________________________

Tiree __________________________________________________________________

Leeds __________________________________________________________________

Brighton __________________________________________________________________

## Eva – A Case Study of a Depression

### A Case Study of a Weather System using WOW data

Go to the WOW website wow.metoffice.gov.uk

Use the pop up calendar to select 24th December 2015 and the drop down box to choose 0600 to 0659 in the morning.

Use the Layers menu on the right to select wind speed/ direction.

Use the Filters menu to select only official observations.

Where is the wind coming from?

Is the wind weak or strong?

Change the time to 0800-0859. How does the wind look now?

Change the time to 1200-1259. How does the wind look now?

Capture the image, stick it into a work book

##### NOW USE THE LAYERS MENU TO LOOK AT TEMPERATURE.

At 0600-0659, what are the temperatures across the U.K.? (You may need to click on some dots to work out exactly what the temperature is)

Capture the image, stick it into a work book at draw a line roughly dividing colder and warmer temperatures. What is a line dividing cold and warm temperatures called?

Now look at the temperature between 0800-0859. What is the pattern of temperatures?

Now look at the temperature between 1200-1259. What is the pattern of temperatures?

Capture the image, stick it into a work book at draw a line dividing colder and warmer temperatures.

Now compare this image and the line you have drawn on it with the wind image you captured earlier. What do you notice?

##### CHALLENGE YOURSELF:

Find the Met Office Weather Station in Glasgow/ Bishopton, and click on it. Click on ‘View Full Observation’ and use the Graph view and the calendars to select from
23/12/15 to 25/12/15. How does the air temperature change?

##### NOW USE THE LAYERS MENU TO SELECT RAINFALL RATE AT 0600-0659

You’ll also need to add ‘WOW observations’ from the Filter menu because the official observations don’t include rainfall. Where is it raining?

Go back in time, hour by hour, what happens to where the rain is?

##### NOW USE THE LAYERS MENU TO SELECT ‘PRESSURE’.

Investigate how that changes through the day.

##### CHALLENGE YOURSELF:

Find the Met Office Weather Station in Glasgow/ Bishopton again, and click on it. Click on ‘View Full Observation’ and use the Graph view and the calendars to select from 23/12/15 to 25/12/15. How does the mean sea level pressure change?

##### CHALLENGE YOURSELF:

Capture the pressure image from 1200-1259 on 24th December 2015.
Can you sketch pressure contours for 992, 996, 1000, 1004, 1008 and 1012hPa?

Summarise what you think is going on with the weather on the 24th December 2015.

## Red Sky at Night, Shepherd’s Delight

### “Red sky at night, shepherd’s delight. Red sky in the morning, shepherd’s warning” (or sailors rather than shepherds, depending on where in the country you are).

Is there any truth in this saying and, if so, why?

COMMON MISCONCEPTION: SOMETIMES THERE IS CONFUSION ABOUT THIS SAYING. IT IS TRUE THAT WHEN THE AIR IS ‘DIRTY’ – EITHER WITH POLLUTANTS FROM INDUSTRY, VEHICLES ETC OR WITH VOLCANIC ASH, SUNSETS AND SUNRISES TEND TO BE MORE SPECTACULAR. THESE CONDITIONS ARE ENHANCED BY HIGH PRESSURE, WHICH WE ASSOCIATE WITH CLEAR SKIES AND THEREFORE WARM, SUNNY WEATHER IN THE SUMMER, COLD, CRISP WEATHER IN THE WINTER (AND GLOOMY OVERCAST WEATHER IN THE LATE WINTER AND SPRING). HOWEVER, THIS IS NOT THE EXPLANATION OF THIS SAYING, BECAUSE HIGH PRESSURE TENDS TO LAST FOR SEVERAL DAYS.

You can find the Students’ Worksheet here.

Low pressure weather systems (depressions) bring us rain, strong winds and sometimes snow on high ground. Sometimes, they also bring thunder, lightning and even tornadoes to the UK. Neither shepherds nor sailors like this sort of weather.

1) Red sky at night

Imagine you are standing on the ground with the Sun setting (remember the Sun rises in the east and sets in the west).

Our weather tends to come from the west. Let’s consider a weather system which has already passed us first, moving from west to east – the diagram shows the clouds associated with the weather system and the direction they are moving.

The wiggly red lines represent the Sun’s light – if the light can reach you, then you can see the Sun. At sunset and sunrise the Sun appears red.

a) Looking at the diagram, are you going to be able to see the sunset? ___Yes______________

b) Why? _Light can get from the Sun to you_____________

Light is reflected by the water droplets in a cloud. This means that some of the light heading towards the cloud is reflected back in the direction it came from. If this light can reach you, then you’ll see the cloud lit up by the setting Sun.

c) Looking at the diagram, are you going see the cloud lit up by the setting Sun? ___Yes____________

d) Why? ____The sunlight will reflect back from the cloud to you______

e) Shade the side of the cloud which will appear red.

Now consider a weather system which hasn’t reached us yet:

f) Looking at the diagram, are you going to be able to see the sunset? ____No______________

g) Why? __The cloud is in the way – sunlight can’t reach you________

h) Looking at the diagram, are you going see the cloud lit up by the setting Sun? ___No_____

i) Why? ___The side of the cloud lid up by the Sun is on the further side, which you cant see.____

j) Shade the side of the cloud which will appear red.

Weather systems are usually a few days apart – if a weather system has just passed us, we probably won’t get another one the same day. If a weather system is in sight, it will probably bring bad weather to us shortly.

In your own words, explain how these diagrams explain “red sky at night, shepherd’s delight”.

__If a storm has just passed, you’ll be able to see the sunset and the cloud illuminated by the sun, giving a red sky. If a storm has just passed, it’s unlikely that there will be another one tomorrow, so the weather is likely to be good. On the other hand, if you can’t see the setting sun or the illuminated side of the cloud, the sky won’t look red at sunset – this is the case when the cloud from an approaching storm is between you and the Sun, and bad weather is on the way.___

## Red Sky Worksheet

### “Red sky at night, shepherd’s delight. Red sky in the morning, shepherd’s warning” (or sailors rather than shepherds, depending on where in the country you are).

Is there any truth in this saying and, if so, why?

Low pressure weather systems (depressions) bring us rain, strong winds and sometimes snow on high ground. Sometimes, they also bring thunder, lightning and even tornadoes to the UK. Neither shepherds nor sailors like this sort of weather.

1) Red sky at night

Imagine you are standing on the ground with the Sun setting (remember the Sun rises in the east and sets in the west).

Our weather tends to come from the west. Let’s consider a weather system which has already passed us first, moving from west to east – the diagram shows the clouds associated with the weather system and the direction they are moving.

The wiggly red lines represent the Sun’s light – if the light can reach you, then you can see the Sun. At sunset and sunrise the Sun appears red.

a) Looking at the diagram, are you going to be able to see the sunset? ______________________

b) Why? ____________________________________________________

Light is reflected by the water droplets in a cloud. This means that some of the light heading towards the cloud is reflected back in the direction it came from. If this light can reach you, then you’ll see the cloud lit up by the setting Sun.

c) Looking at the diagram, are you going see the cloud lit up by the setting Sun? _______________

d) Why? ____________________________________________________

e) Shade the side of the cloud which will appear red.

Now consider a weather system which hasn’t reached us yet:

f) Looking at the diagram, are you going to be able to see the sunset?

g) Why?

h) Looking at the diagram, are you going see the cloud lit up by the setting Sun?

i) Why?

j) Shade the side of the cloud which will appear red.

Weather systems are usually a few days apart – if a weather system has just passed us, we probably won’t get another one the same day. If a weather system is in sight, it will probably bring bad weather to us shortly.

k) In your own words, explain how these diagrams explain “red sky at night, shepherd’s delight”.

2) Red sky in the morning

Going through the same thinking as above, complete the following diagrams to show

a) A Sunrise with a weather system which has already gone passed us.

– First draw the Sun rising in the East

– Next draw the cloud and the arrow showing which way it is moving

– Next draw lines showing light coming from the Sun and reflecting from the cloud.

b) A Sunrise with a weather system which is about to reach us.

– First draw the Sun rising in the East

– Next draw the cloud

– Next draw lines showing light coming from the Sun and reflecting from the cloud.

In your own words, explain how these diagrams explain “red sky in the morning, shepherd’s warning”.

## Weather Systems

Key Stage 4 – A series of downloadable lesson plans and teacher’s notes prepared on weather systems for GCSE geography.

Produced by Tony Cassidy

## Fronts

### Introduction

A front is the boundary between two different types of air mass. In our latitudes a front usually separates warm, moist air from the tropics and cold, relatively dry air from polar regions.

Fronts move with the wind, so in the UK this is normally from west to east because our prevailing winds are from the west or southwest. At a front, the heavier cold air undercuts the less dense warm air, causing the warm air to rise up over the wedge of cold air.

As the air rises there is cooling and condensation, thus leading to the formation of clouds and rainfall. Consequently, fronts tend to be associated with cloud and rain. Three types of front can be identified; warm fronts, cold fronts and occluded fronts.

### Warm front

A warm front marks the leading edge of a warm air mass. The presence of a warm front means that the warm air is advancing and rising up over the 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.

Cloud extends well ahead of the front, becoming thicker as the front approaches, accompanied by falling pressure. Rain then starts to fall, usually becoming heaviest on the front itself.

The passage of the front is followed by a rise in temperature and humidity and a veer in the wind, while the pressure also stops falling. Although the rain dies out, it often stays cloudy.

On windward coasts and hills, the cloud base may be low enough to give fog and thick enough to produce drizzle. Inland and to the lee of hills, the cloud may break, allowing for some warm sunshine.

### Cold front

This marks the leading edge of colder air. The presence of a cold front means that 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 therefore replacing warmer air at the surface.

Pressure begins to fall increasingly rapidly as the front approaches and rain usually starts not long before it arrives, becoming heavy for a short time. This is often accompanied by an increase in and a backing of the wind. In some cases, there may also be hail and thunder.

The passage of the front is usually marked by a sharp change from falling to rising pressure and a veer in the wind.

As the rain dies away, the cloud lifts and breaks and, although there is sunshine, the air temperature falls.

After a time, cumulus clouds begin to form, often bringing showers. Sometimes the showers may become heavy, perhaps even accompanied by hail and thunder. On the other hand, in some cases, pressure will rise rapidly after a cold front has passed and this causes there to be few, if any, showers.

### Occluded front or occlusion

Occlusions are slightly more complex than warm or cold fronts. They occur because cold fronts travel more quickly than warm fronts and eventually this results in the cold front ‘catching up’ with the warm front.

This causes the warm air to be undercut and lifted up from the surface.

The characteristics of an occlusion are similar to those of a cold front in that the rain belt is narrow.

Cloud lifts and breaks after the front has moved through and there may be a change in temperature and a veer in the wind, though these tend to be small.

### Trough

Whereas fronts separate air masses, which are different in temperature, troughs generally develop in cold air and are characterised by an increase in the frequency and intensity of showers. Pressure begins to fall as the trough approaches and often rises sharply once it has passed. As with fronts, the wind tends to back ahead of the trough and to veer immediately behind it. There are also particularly strong gusts of wind in the showers

Summary

• Fronts form as the result of ‘conflict’ between warm and cold air
• Fronts are the boundary between two air masses
• The most significant weather occurs on fronts, since that is where the air is rising fastest

### Introduction

A depression is an area of low pressure and is associated with unsettled weather. This is due to the fact that the air within the depression is rising, causing it to cool and the water vapour within it to condense into clouds. This rising air within a depression causes an area of low pressure at the surface. The deeper the depression (or low), the more unsettled the weather.

Consequently, the weather associated with a depression is often cloudy, wet and windy. However, weather is not uniformly distributed around a depression. Different parts of it have very different types of weather, which also vary through its lifetime. The most significant weather (cloud and precipitation) occurs in discrete lines or fronts. In the northern hemisphere winds blow anticlockwise around areas of low pressure; this is reversed in the southern hemisphere.

### Development of a depression

#### Stage 1 – Origin and infancy

The depression usually starts life as a wave, shown on a chart by ‘buckling’ on a front. At this stage, the air is warm to the south of the front and relatively cold to the north of it. The weather in the warm air can vary from fine and sunny to cloudy, sometimes with drizzle and perhaps even with fog. The type of cloud is layered or stratiform and is not very thick. In the cold air, there is usually some cloud, but it tends to be more broken, appearing as discrete speckles on a satellite image. The cloud is cumuliform and can often be large enough to produce showers. Figure 7 shows an example of a ‘wave’ on a synoptic chart, whilst Figure 8 shows the infrared satellite image for the same time.

• In warm air, the weather can vary from warm and sunny to dull and drizzly
• Colder air is more showery, but with some sunshine too.

#### Stage 2 – Maturity

As the depression develops, the pressure around it falls, leading to more tightly packed isobars. This causes winds to be stronger and, at the same time, the buckle in the front becomes much more marked (see Figure 9), with distinct warm and cold fronts being formed. Warm air is pushed towards the north while colder air drives southwards. The region between the warm and cold front is called the warm sector. The cloud near the fronts thickens and the frontal zone becomes broader, which means that rain is more prolonged, becoming heavier nearer the front. However, it is often the case that one front is more active than the other. At this stage, the heaviest rain occurs near the centre of the low. Figure 9 shows an example of a mature ‘depression’ on a synoptic chart, whilst Figure 10 shows the infrared satellite image for the same time.

• The weather on or near a front is usually cloudy, with precipitation that may vary from virtually nothing on a weak front to a torrential downpour on a particularly active one

#### Stage 3 – Occlusion

The fronts move at a speed indicated by the separation between the isobars along them, although the speed of the warm front is about two-thirds of this. Consequently, the cold front is usually faster than the warm front. Cold air is denser than warm air, which it replaces at the surface, causing the warm air to lift and the warm sector to become progressively smaller. The cold air increasingly undercuts the warm air, initially from near the centre of the low, leading to the development of an occluded front, or occlusion (see Figure 11).

The rainfall usually becomes more sporadic on an occlusion, with the heaviest rain occurring near the triple point (see Figure 11), where all three types of front meet. By this stage, a depression is now in its mature stage, the pressure of its centre stops falling and starts to rise. Cold air has been brought well to the south, often over areas with higher surface temperatures. This can lead to particularly heavy showers, some of which may be thundery. Showers sometimes become organised into lines, which can be indicated on the weather chart by troughs (see Figure 11). Figure 11 shows an example of an occluded depression on a synoptic chart, whilst Figure 12 shows the infrared satellite image for the corresponding time.

• Troughs are organised lines of precipitation, which can often be quite heavy

#### Stage 4 – 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 of the front.

### Weather associated with a classic depression

Every depression is different and hence the weather associated with each depression is also unique. However the weather associated with the passage of a classic depression does follow some general trends. Table 1 details the changes associated with the passage of both warm and cold fronts, whilst Figure 14 shows a cross section through a mature depression.

TABLE 1: WEATHER ASSOCIATED WITH THE PASSAGE OF A CLASSIC DEPRESSION
AHEAD OF THE WARM FRONTPASSAGE OF THE WARM FRONTWARM SECTORPASSAGE OF THE COLD FRONTCOLD SECTOR
Temperaturequite cold, starts to risecontinues to risequite mildsudden dropremains cold
Cloud covercloud 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 directionspeeds increase and direction backsveers and becomes blustery with strong gustsremain steady, backs slightlyspeeds increase, sometimes to gale force, sharp veerwinds are squally
Precipitationnone 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

For more background information about mid-latitude weather systems and their associated weather, watch our weather systems video.

### Polar lows

Polar lows form in cold air, mainly in winter or spring, and tend to be quite small. They usually originate from eddies that form to the lee of high ground. They produce showers that are wintry in nature and which sometimes become aligned into troughs. When they come across land, they can produce quite large amounts of snowfall. Figure 15 shows an example of a polar low on a synoptic chart, whilst Figure 16 shows the infrared satellite image for the corresponding time.

### Thundery lows

Thundery lows form over hot land in summer and can produce a large number of thunderstorms. These thunderstorms can also become aligned into troughs, giving spells of particularly intense downpours with hail, occasionally accompanied by tornadoes or waterspouts.

Figure 17 shows an example of thundery lows over France and Spain on a synoptic chart. Figures 18 and 19 show satellite images for the same day, showing the development of thunderstorms during the day.

### Lee lows

Lee lows form to the lee of high ground when strong winds are blowing directly against a ridge. They don’t produce any particular type of weather, but winds around them can be very unpredictable in both speed and direction. Figures 20 and 21 show an example of a lee low over the Gulf of Genoa on both a synoptic chart and satellite image.

### Anticyclones

An anticyclone is a region of high pressure. This is the result of the air in the atmosphere subsiding towards the earth’s surface. This subsidence, or sinking motion, leads to the air becoming drier and warmer. In the northern hemisphere winds blow clockwise around areas of high pressure, this is reversed in the southern hemisphere.

When anticyclones form over land, the skies above are often clear of cloud. During the summer, this means long, sunny days and clear nights. In winter, the longer nights mean that temperatures fall lower, with frost often forming, which may persist all day. The falls in temperature overnight and light winds can lead to fog forming.

When anticyclones are over the sea, the weather can vary from fine and sunny to overcast cloud. This cloud may be thick enough to give drizzle and may fall low enough to produce fog. This happens most often during spring and is least frequent in autumn. If the anticyclone extends over both land and sea, cloud and fog can spread across coastal regions, sometimes reaching quite far inland.

• In summer over land, days are warm or hot and sunny, nights are clear.
• Over the sea, it is sometimes quite cloudy. Drizzle, mist or fog are most likely early in the summer.
• In winter over land, days are often dull and misty, perhaps even foggy. Sunny days tend also to be cold and dry. Nights can be clear and frosty, or become foggy or cloudy.
• Over the sea, the winter weather is much the same as during the summer. Drizzle, mist or fog become more likely as spring approaches.

Now why not have a go at answering some questions about the weather shown on a weather chart.

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

## Watch our Weather Systems Video with Pete Inness of Reading University

In this series Pete Inness explains what weather systems are, why they happen and how we can understand weather systems.

## 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

## 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

### Anticyclones, Depressions and Fronts

#### 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.

### 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.

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

#### 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.

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

#### 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.

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. 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.

#### 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.

#### 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.

 WEATHER ASSOCIATED WITH THE PASSAGE OF A CLASSIC DEPRESSION Ahead of the warm front Passage of the warm front Warm sector Passage of the cold front Cold sector Pressure starts to fall steadily continues to fall steadies starts to rise continues to rise Temperature quite cold, starts to rise continues to rise quite mild sudden drop remains cold Cloud cover cloud base drops and thickens (cirrus and altostratus) cloud base is low and thick (nimbostratus) cloud may thin and break clouds thicken (sometimes with large cumulonimbus) clouds thin with some cumulus Wind speed and direction speeds increase and direction backs veers and becomes blustery with strong gusts remain steady, backs slightly speeds increase, sometimes to gale force, sharp veer winds are squally Precipitation none at first, rain closer to front, sometimes snow on leading edge continues, and sometimes heavy rainfall rain turns to drizzle or stops heavy rain, sometimes with hail, thunder or sleet showers

What to do next

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

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