Cold and Warm Fronts Activities

1) SORT THE FOLLOWING STATEMENTS ABOUT COLD AND WARM FRONTS INTO THE THINKING ORGANISER BELOW:

a) Cold and warm air meet

b) Cold air pushes into warm air

c) Warm air pushes into cold air

d) Cold air is below warm air

e) Cloud

f) Rain

g) Thunder and lightning

h) Cirrus cloud

i) Stratus cloud

j) Cumulonimbus cloud

k) Usually move from west to east

 

Diagram of cold and warm front

2) ADD THE CORRECT WORD TO THE PARAGRAPH USING ONLY WARM OR COLD:

___________ and __________ fronts are where two different air masses meet. At a __________ front, cold air is pushing in to warm air. At a __________ front, warm air is pushing in to cold air.
__________ air always rises over __________ air. Cloud forms on both __________ and __________ fronts. The weather is usually more extreme on the __________ front, with heavier rain and possibly thunder, hail and lightning. Cirrus cloud is the first sign of a __________ front approaching. Where a __________ front is near the ground, you can expect to see featureless sheets of low, grey stratus cloud from which rain can fall. __________ fronts move faster than __________ fronts, leading to the formation of an occluded front. __________ fronts are marked with a semi-circle, pointing in the
direction the front is moving. __________ fronts are marked with a triangle, pointing in the direction the front is moving.

3) TRUE OR FALSE?
 Statement True? False?
 Cold air pushes under warm air at a cold front  
 The weather is usually worse on the warm front  
 Warm air pushes into and over cold air at a warm front  
 You get cumulonimbus or thunder clouds on a cold front  
 The temperature is colder at a cold front than at a warm front  
 Cold fronts are marked by a line with triangles on it  
 Cold fronts usually travel from west to east  
 Warm fronts move faster than cold fronts 

Rewrite any of the statements that you think are false so they become a true statement.

4) COMPARE AND CONTRAST
 Question or statement Cold front Warm front
 Symbol on a synoptic chart?  
 Colour on a synoptic chart?  
 Clouds are called…  
 Intensity of rainfall?  
 The air mass which is pushing in, is…

Depressions – Identifying Features

Identify the weather features on the synoptic (weather) chart below:weather chart and photos

Pop-up Depression

Depression
Depression with folding instructions

Passage of a Depression – Account

Passage of a depression

Use the following help sheet to write a fictional story about the passing of a depression. Imagine you are in the field taking observations and measurements. You should use all the statements included below.

 Coming of the warm front At the warm front Within the warm sector At the cold front Within the cold sector
 Barometer – shows pressure is falling Barometer – continues to show the pressure is falling. No significant changes on the barometer. Suddenly the pressure on the barometer begins to rise. Pressure continues to rise.
 Thermometer – shows the temperature is steady Thermometer – shows the temperature is rising. Feeling a little sticky in this heavy coat! Still quite mild. Time for a hat, and to button up that coat, the temperature is falling, it’s becoming chilly. It remains cold.
 Cirrus clouds high in the sky, they begin to descend and thicken becoming altostratus. Becomes darker, the clouds are low and cover the sky like blankets – nimbostratus. Starting to see breaks in the cloud, a little drizzle now – nothing heavy. Angry looking clouds, towering and grey, begin to form – cumulonimbus. The clouds have thinned out now – some cumulus.
 No need for the umbrella yet! Time to get out the umbrella, continuous heavy rain. A steady breeze, nothing too uncomfortable. Heavy rain! Is that thunder I hear? A few showers.
 Slight increase in wind speed. The wind is much stronger now – struggling to hold onto my umbrella.  Struggling to stay on my feet, as I’m battered by the strong winds. Time to go home I think!

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
lower on your left than on your right.

– 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.

Beaufort scale

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?

Drawing of isobars

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?

Drawing of isotherms

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?Weather map

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.

Weather map

Weather map January

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.Map of the UK showing wind direction

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 – Teachers

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).

Diagram red sky

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:

Diagram of weather system that has not 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).

Red sky and weather systems

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:

Weather system that has not 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.Diagram for drawing on

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.Second diagram for drawing on

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


 


 


 


 


 

Weather Systems

Fronts

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.

 

Fig 1: A warm front in diagrammatic form
Fig 2: Cross-section through a warm front, with associated cloud, temperature and weather changes
Fig 2: Cross-section through a warm front, with associated cloud, temperature and weather changes

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.

Fig 3: A cold front in diagrammatic form
Fig 3: A cold front in diagrammatic form
Fig 4: Cross-section through a cold front, with associated cloud, temperature and weather changes
Fig 4: Cross-section through a cold front, with associated cloud, temperature and weather changes

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.

Fig 5: An occluded front in diagrammatic form
Fig 5: An occluded front in diagrammatic form

 

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

Fig 6: The point of occlusion
Fig 6: The point of occlusion

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

Frontal Depressions

Introduction
Development of a depression
Weather associated with a classic depression

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.
Fig 7: Synoptic chart, 7 Sep 2005, 1200 GMT
Fig 7: Synoptic chart, 7 Sep 2005, 1200 GMT
Fig 8: Infrared satellite image, 7 Sep 2005, 1200 GMT
Fig 8: Infrared satellite image, 7 Sep 2005, 1200 GMT

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
Fig 9: Synoptic chart, 8 Sep 2005, 1200 GMT
Fig 9: Synoptic chart, 8 Sep 2005, 1200 GMT
Fig 10: Infrared satellite image, 8 Sep 2005, 1200 GMT
Fig 10: Infrared satellite image, 8 Sep 2005, 1200 GMT

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
Fig 11: Synoptic chart, 9 Sep 2005, 1200 GMT
Fig 11: Synoptic chart, 9 Sep 2005, 1200 GMT
Fig 12: Infrared satellite image, 9 Sep 2005, 1200 GMT
Fig 12: Infrared satellite image, 9 Sep 2005, 1200 GMT

Stage 4 – Death

Fig 13: Animation of the life cycle of a depression, 7 Sep 2005, 0000 GMT to 9 Sep 2005, 1800 GMT
Fig 13: Animation of the life cycle of a depression, 7 Sep 2005, 0000 GMT to 9 Sep 2005, 1800 GMT

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.

Figure 14: Cross-section through a classic depression
Figure 14: Cross-section through a classic 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
Pressurestarts to fall steadilycontinues to fallsteadiesstarts to risecontinues to rise
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.

Other types of depression (or low)

Polar lows
Thundery lows
Lee lows

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.

Fig 15: Synoptic chart, 25 Dec 2004, 0000 GMT
Fig 15: Synoptic chart, 25 Dec 2004, 0000 GMT
Fig 16: Infrared satellite image
Fig 16: Infrared satellite image, 25 Dec 2004, 0000 GMT

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.

Fig 17: Synoptic chart, 14 Jul 2003, 1800 GMT
Fig 17: Synoptic chart, 14 Jul 2003, 1800 GMT

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.

Fig 18: Infrared satellite image, 14 Jul 2003, 1400 GMT
Fig 18: Infrared satellite image, 14 Jul 2003, 1400 GMT
Fig 19: Infrared satellite image, 14 Jul 2003, 2200 GMT
Fig 19: Infrared satellite image, 14 Jul 2003, 2200 GMT

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.

Fig 20: Synoptic chart, 7 Oct 2003, 1200 GMT
Fig 20: Synoptic chart, 7 Oct 2003, 1200 GMT
Fig 21: Infrared satellite image, 7 Oct 2003, 1200 GMT
Fig 21: Infrared satellite image, 7 Oct 2003, 1200 GMT

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.
Fig 22: Synoptic chart, 11 Apr 2005, 1200 GMT
Fig 22: Synoptic chart, 11 Apr 2005, 1200 GMT
Fig 23: Visible satellite image, 11 Apr 2005, 1200 GMT
Fig 23: Visible satellite image, 11 Apr 2005, 1200 GMT

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.