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.

Air Masses and Fronts

Air masses and fronts 

1. Introduction
1.1 Definition
1.2 Source of an air mass
1.3 Air-mass modification

2. Air-mass types
2.1 Tropical continental
2.2 Polar continental
2.3 Tropical maritime
2.4 Polar maritime
2.5 Arctic maritime
2.6 Returning polar maritime

3. Between the air masses
3.1 Historical introduction
3.2 Fronts
3.3 Models of mid-latitude depressions
3.4 The passage of a mature depression across the United Kingdom
4. Questions

1. Introduction

1.1 Definition

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

An air mass is separated from an adjacent body of air by a transition that may be more sharply defined. This transition zone or boundary is called a front. An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.

1.2 Source of an air mass

The temperature of an air mass will depend largely on its point of origin, and its subsequent journey over the land or sea. This might lead to warming or cooling by the prolonged contact with a warm or cool surface. The processes that warm or cool the air mass take place only slowly, for example it may take a week or more for an air mass to warm up by 10 °C right through the troposphere. For this to take place, an air mass must lie virtually in a stagnant state over the influencing region. Hence, those parts of the Earth’s surface where air masses can stagnate and gradually attain the properties of the underlying surface are called source regions.

The main source regions are the high pressure belts in the subtropics, which produce tropical air masses, and around the poles, that are the source of polar air masses.

1.3 Air-mass modification

As we have seen, it is in the source regions that the air mass acquires distinctive properties that are the characteristics of the underlying surface. The air mass may be cool or warm, or dry or moist. The stability of the air within the mass can also be deducted. Tropical air is unstable because it is heated from below, while polar air is stable because it is cooled from below.

As an air mass moves away from its source region towards the British Isles, the air is further modified due to variations in the type or nature of the surface over which it passes. Two processes act independently, or together, to modify an air mass.

An air mass that has a maritime track, i.e. a track predominantly over the sea, will increase its moisture content, particularly in its lower layers. This happens through evaporation of water from the sea surface. An air mass with a long land or continental track will remain dry.

 

Fig 2: Modification of air mass by land and ocean surfaces

 

A cold air mass flowing away from its source region over a warmer surface will be warmed from below making the air more unstable in the lowest layers. A warm air mass moving over a cooler surface is cooled from below and becomes stable in the lowest layers.

 

Fig 3: Modification of air mass due to surface temperature

 

If we look at the temperature profiles of the previous example, the effects of warming and cooling on the respective air masses are very different.

 

 

 

 

 

 

Fig 4: Modified vertical temperature profiles (—– line) typical of: a) tropical air cooled from below and b) polar air heated from below on its way to the British Isles. Note that where the air is heated from below the effect is spread to a greater depth of the atmosphere.

2.Air mass types

There are four main types of air mass.

  • Tropical continental (Tc)
  • Tropical maritime (Tm)
  • Polar continental (Pc)
  • Polar maritime (Pm)

And two further sub-divisions.

  • Arctic maritime (Am)
  • Returning polar maritime (rPm)

Each of these air-mass types has its own distinctive combination of properties in terms of:

  • temperature;
  • moisture content (relative humidity);
  • change of lapse rate;
  • stability;
  • weather;
  • visibility.

2.1 Tropical continental

Tropical continental air affects the British Isles predominantly during the summer. This air mass affects Britain when pressure is high over northern or eastern Europe with surface winds between east and south drawing hot air from North Africa (see Figure 5). This air is initially unstable but very dry. It acquires some moisture in its passage across the Mediterranean Sea but it is still usually too dry to produce any significant amounts of precipitation. South-westerly winds aloft can sometimes inject sufficient moisture to produce high-based showers or thunderstorms.

Fig 5: Tropical continental air mass

Our highest temperatures usually occur under the influence of tropical continental air (over 30 °C by day and around 15-20 °C at night). Visibility is usually moderate or poor due to the air picking up pollutants during its passage over Europe and from sand particles blown into the air from Saharan dust storms. Occasionally, the Saharan dust is washed out in showers producing coloured rain and leaving cars covered in a thin layer of orange dust.

Tropical continental air may affect the country from March to October, although it is most common in June, July and August. During winter months, tropical continental air is more difficult to identify, but may reach the UK on south or south-easterly winds ahead of slow-moving Atlantic fronts. When high pressure prevails during the winter, strong cooling near the surface makes the air stable, rather than cold and moist. Low cloud and poor visibility may be very persistent under such conditions.

See table 1 for a summary

2.2 Polar continental

This air mass has a source region over the Eurasian land mass north of 50° N and east of 25° E. It can only be considered a winter phenomenon, as in summer months the land mass becomes very warm, and there are no high-pressure cells develop to ‘push’ the air over the country.

Polar continental air affects the British Isles when pressure is high over Scandinavia with surface winds from an easterly direction (see Figure 6). The characteristics of the air depend on the length of the sea track during its passage from Europe to the British Isles. The air is inherently very cold and dry and it can reach southern Britain after a short sea track over the English Channel, producing weather characterised by clear skies and severe frost. If there is a longer sea track over the North Sea, the air becoming unstable and moisture is added, giving rise to showers of rain or snow. This can be a hazard, especially near the east coast, when the air mass passes over the warmer seas before reaching the colder land surface.

Fig 6: Polar continental air mass

The UK’s lowest temperatures usually occur under this air mass, with temperatures falling below minus 10 °C at night, and sometimes remaining below freezing all day.

Polar continental air only reaches Britain between November and April; at other times of the year, the source region is neither cold nor snow covered and winds from north-eastern Europe bring a more tropical continental form of air.

See table 1 for a summary

2.3 Tropical maritime

The source region for this air mass is over the waters of the Atlantic Ocean between the Azores and Bermuda. The air usually reaches the British Isles on south-westerly winds, commonly in the warm sector of a depression or around the periphery of an anticyclone located over central Europe (see Figure 7).

It is warm and moist in its lower layers and during the passage over the cooler waters the air becomes stable and close to saturation. The air mass is modified quickly during its passage across the British Isles and its characteristics vary considerable from place to place depending on exposure to the moist south-westerly flow.

Fig 7: Tropical maritime air mass

In the western parts of the British Isles, the tropical maritime air is stable and saturated in its lowest layers. As a result, the weather is characterised by much low cloud, drizzle and hill fog. During the winter months the air may reach eastern Britain with very similar characteristics providing the ground is cold, but for much of the year insolation is sufficient to warm the air appreciably.

Visibility is more difficult to quantify. When tropical maritime air reaches our western shores it is near to saturation and any uplift produces low cloud and fog which can reduce visibility to near zero. Following a short land track however, and providing the air has dried out appreciably, visibility can become excellent because the air is inherently clear and aerosol free. Following a lengthy land track, haze particles increase in number and are trapped by the stable air reducing visibility to the moderate category.

See table 1 for a summary

2.4 Polar maritime

Polar maritime air flows towards the British Isles on north-westerly winds from the Arctic regions of Greenland and northern Canada. A typical synoptic situation for such an airflow would have a low pressure centred near Iceland (see Figure 8).

This air starts very cold and dry and during its long journey across the comparatively warm waters of the North Atlantic its temperature rises rapidly. The air temperature rises rapidly, allowing it to become unstable to a great depth and causing the moisture content to rise significantly.

Fig 8: Polar maritime air mass

Polar maritime air is perhaps the most familiar air mass. The instability produces showers over the sea, and in the exposed west and north of the British Isles these showers are frequent. In winter months when convection is most vigorous over the sea, hail and thunder are common in these exposed hilly areas. In eastern Britain, where moisture input and surface heating are reduced, showers are less frequent except when troughs of low pressure pass. In the summer months, land temperatures are higher than sea temperatures and the heaviest showers occur over eastern England.

Large variations in shower activity can happen on a diurnal basis as well as on a day-to-day timescale, largely due to subtle changes in stability and moisture content of the air.

See table 1 for a summary

2.5 Arctic maritime

Arctic maritime air is a more direct form of Polar maritime. Its source region lies over the Arctic Ocean close to the North Pole.

The synoptic pattern that favours an outbreak of Arctic maritime air is one where high pressure lies to the west of Ireland with low pressure over eastern Europe and southern Scandinavia (see Figure 9). Its characteristics are similar to polar maritime air, but because of the shorter sea track the air is colder and has a lower humidity.

Fig 9: Arctic maritime air mass

Between October and May, the air is cold enough to produce hail showers or snow, and these are most frequent over Scotland and along the coasts exposed to northerly winds. Polar low-pressure systems forming in this air mass can sometimes lead to widespread and heavy snowfall, but otherwise inland areas remain free of cloud in the winter months. In northern Scotland, arctic maritime is usually the coldest air mass, but over the rest of Britain, this air mass is not as cold as polar continental.

Arctic air is uncommon during the summer, but when it does occur it may bring heavy showers or thunderstorms and unseasonably low temperatures.

See table 1 for a summary

2.6 Returning polar maritime

Returning polar maritime is another variation of polar maritime, but this time, with a much longer sea track which takes the air first southwards over the North Atlantic, then north-eastwards across the British Isles. A typical synoptic pattern associated with this air mass would be a slow moving low pressure in mid-Atlantic with a deep trough extending south (see Figure 10).

During its passage south, the air becomes unstable but on moving north-east it passes over cooler water making it stable in its lower layer whilst remaining unstable aloft.

Fig 10: Returning polar maritime air mass

 

 
Tropical
continental
(Tc)
Polar
continental
(Pc)
 
Summer
Winter
Long sea track
Short sea
track
Temp
Very warm or hot
Average
Cold
Very cold
Humidity
Relatively dry
Rather moist
Moist in lowest layers
Very dry
Change of lapse rate
Little change
Cooled from below
Heated from below
Little change
Stability
Generally stable
Stable
Unstable
Stable
Weather
Clear, occasional thundery showers
Clear
Rain or snow showers
Clear
Visibility
Moderate or poor
Moderate of poor
Good
Moderate or poor
 
Tropical
maritime
(Tm)
Polar
Maritime
(Pm)
Arctic
Maritime
(Am)
Returning
Polar
Maritime
(rPm)
 
Exposed
Sheltered
 
 
 
Temp
Near sea
temperature
Warm
Rather cold
Cold
(colder
than Pm)
Warm
(warmer
than Pm)
Humidity
Very moist
Moist
Moist
Fairly moist (not as
moist as Pm)
Fairly moist (not as
moist as Pm)
Change of lapse rate
Cooled from below
Warmed in summer
Heated from below
Heated from below
Heated from below
Stability
Stable
Stable aloft
Unstable
Unstable
Unstable
Weather
Low cloud, drizzle
Broken cloud, dry
Variable cloud, showers
Showers (mainly coastal)
Showers (mainly coastal)
Visibility
Often poor with coastal fog
Moderate
Good
Very good
Very good

Table 1. Typical characteristics of the six major air masses which affect the British Isles.

3. Between the air masses

3.1 Historical introduction

We have looked at the individual types of air masses in some detail, but what is just as, or perhaps, even more important for weather forecasting is what happens in the immediate region where the air masses meet. As we have seen, the air masses have quite different properties so when they meet, perhaps one is cold dry and dry and the other is relatively very warm and moist. These differences produce a reaction in a zone known as a front.

The pioneer in the study of frontal development was Vilhelm Bjerknes, a Norwegian scientist, who analysed their formation around the time of the Great War. The war cut Norway off from outside weather information, so a geophysical institute was founded by Bjerknes in Bergen. He also persuaded the Norwegian government to install a dense network of surface observing stations to provide data for his meteorological studies.

It was known at this time that areas of organised rain were often related to confluence lines in the surface wind field. In 1919, at the age of 22, Vilhelm’s son, Jacob, published an eight-page paper, which introduced the concept of warm, cold and occluded fronts that correctly explained their relationship to extra-tropical depressions.

The term front was introduced as an analogy to the recent war, with air masses, rather than nations, coming together with fronts between air masses compared with the fronts where opposing armies came into contact. By 1926, in collaboration with others at the institute (know collectively as the Bergen School), Bjerknes described the structure and life cycle of frontal depressions.

In recent years, satellites, radar and numerical modelling techniques have provided additional information that has shown that the Norwegian concepts are very simplistic. They do, however, provide a helpful starting point for analysing and forecasting the weather in temperate latitudes.

3.2 Fronts

Fig 11: Vertical cross section through a warm sector of a depression

3.3 Models of mid-latitude depressions

The Bergen School, led by Bjerknes, devised a simple model that shows how depressions, or low pressure systems, develop in mid-latitudes as warm and cold air masses meet. Their model has the following stages.

Origin and infancy – a warm air mass, such as tropical maritime or tropical continental meets a cooler air mass, such as polar maritime or polar continental.

Fig 12: Origin

Maturity – the warm air rises and spirals up in an anticlockwise manner over the sinking cold air. A distinctive warm sector exists between the warm and cold fronts.

Fig 13: Maturity

Occlusion – the warm sector disappears, as the cold front quickly advances. Its faster movement is because the cold front is the leading edge of cold, denser air, pushing up the warmer lighter air. It is harder for the warmer, lighter air at the warm front to cause the cooler, denser air to sink. Hence, the warm front advances at 20 to 30 miles per hour, whilst the cold front can move forward more quickly at 40 to 50 miles per hour.

Fig 14: Occlusion

Death – the frontal system dies as the warm air has completely risen and cooled, and is now underlain by the cold air. The differences in temperature have therefore been equalled out, and the occluded front disappears.

Frontal systems tend to occur in ‘families‘, which migrate in an easterly direction across the Atlantic. Sometimes as many as four or five mature depressions may make their way across the United Kingdom, before a ridge of high pressure builds up to prevent any more from advancing over the country. The origin stage tends to occur over the mid-Atlantic, with the mature stage occurring over the United Kingdom.

The death stage usually occurs over the European mainland and Scandinavia. The depressions follow the zigzag path of the fast jet streams in the upper troposphere. The jet streams may blow at 120 miles per hour in the upper troposphere, but the weather systems below it will usually move more slowly, often at about 40 miles per hour.

Britain’s changeable and damp climate is largely the result of the frequent movement of the rain-bearing fronts across the country. The regularity of their passage, and the standard sequence of changes they produce, allow quite accurate forecasts to be made.

3.4 The passage of a mature depression across the United Kingdom

 

Fig 15: The passage of a mature depression

The passage of a mature depression across the United Kingdom will produce the following sequence of weather changes.

Ahead of the depression in the cold sector

High cirrus clouds may occur in long feather-like streaks. Some cirrostratus may also occur up to 600 miles ahead of the surface position of the warm front. As the front approaches, temperatures start to rise, and barometric pressure falls steadily.

The warm front passes over

Drizzle and then rain will usually start to fall from altostratus and nimbostratus clouds. The amount of cloud will increase and the cloud base will fall. Continuous rain will persist as pressure carries on falling.

In the warm sector

Pressure stabilises and the amount of cloud falls as the clouds start to thin out. The precipitation also stops, and the weather is generally fine, with a little stratus or stratocumulus. As the cold front approaches, pressures slightly rise and temperatures start to fall slightly.

The cold front passes over

Large, towering cumulonimbus clouds develop as the cold front passes over. This produces heavy downpours of rain and fierce squalls, sometimes with hail and thunder. Pressures rise steadily and air temperatures start to drop as the cold front passes over.

Behind the cold front

There is an end to the heavy rain as the cumulonimbus clouds move away. Barometric pressure continues to rise in a steady fashion. A few showers may occur from some small cumulus clouds, but it is generally fine and cool behind the cold front.

4. Questions

1. Make a simple definition of the following terms.
(a) Air mass.
(b) Front.

2. Which of the following statements is true?
(a) Tropical air is stable because it is heated from below.
(b) Tropical air is unstable because it is heated from below.
(c) Polar air is stable because it is cooled from below.
(d) Polar air is stable because it is heated from below.
(e) Polar air is unstable because it is cooled from below.

3. Explain the thermal differences which will occur when:
(a) an air mass has a maritime track;
(b) a cold air mass moves over a warmer surface.

4. What are the four main types of air mass that affect the UK?

5. Which of the following are the other two subdivisions of air masses which affect the UK?
(a) Arctic maritime.
(b) Returning polar continental.
(c) Returning polar maritime.
(d) Arctic continental.

6. Explain how initially dry tropical continental air may acquire enough moisture to produce precipitation.

7. Where is the source region for polar maritime air, and what synoptic situation would allow it to flow over the United Kingdom?

8. Outline the weather features that the UK might experience with returning polar maritime air.

9. Explain why arctic maritime air is likely to lead to good visibility over much of the UK.

10. Which air mass is most associated with thundery showers?

11. Which group of meteorologists first produced a model of frontal development. Was it the:
(a) Bristol School,
(b) Brighton School,
(c) Bergen School, or
(d) Berlin School?

12. Why was the term ‘front’ used as an analogy to describe the leading edge of an air mass?

13. Which air mass would move over the UK if pressure was high over Scandinavia and there are surface winds from the east?

14. Which air mass is most likely to produce daytime temperatures, during the summer, in excess of 30 °C?

15. Which air mass is most likely to lead to temperatures at night falling below -10 °C?

16. Outline the features and cloud types associated with the following.
(a) Cold front
(b) Warm front
(c) Occluded front

17. Why does the cold front move faster than the warm front?

More information on air masses

A worksheet on air masses

Web page reproduced with the kind permission of the Met Office

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

Weather Charts

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

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

Part A – Isobars, pressure and wind

Part B – Identifying pressure systems and fronts

Part C – Plotted weather charts

Teachers’ notes

Teachers’ Notes

Resources required

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

Prior knowledge required

A basic background of weather and climate.

Teaching activities

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

Weather charts

Interpreting Weather Maps

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

Exercises

Three worksheets with exercises are provided to consolidate learning.

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

Suggestions for homework

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

 

Part A – Isobars, pressure and wind

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

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

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

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

What to do next

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

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

Part B – Identifying pressure systems and fronts

Anticyclones

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

Depressions

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

depressions

Troughs

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

Ridges

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

 

Col

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

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

Cold fronts and warm fronts

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

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

Occluded fronts

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

Warm Sector

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

What to do next

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

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

 Part C – Plotted weather charts

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

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

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

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

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

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

Cloud cover

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

Temperature

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

Pressure

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

Weather

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

What to do next

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

Then you can complete extension exercise 3 .

Worksheet 1 – Reading pressure values from a surface pressure chart

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

Worksheet 2 – Surface chart analysis

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

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

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

Worksheet 3 – Station circle plots

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

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

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

Worksheet 1 – extension exercise

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

 

Worksheet 2 – extension exercise

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

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


 

Worksheet 3 – extension exercise

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

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

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

Location A
Location B

 
Location C

 
Location D

 
Location E

 
Location F

 
Location G

 
Location H

 

Web page reproduced with the kind permission of the Met Office

In Depth – Understanding Weather

Understanding weather

Air masses

Atmosphere

Clouds

El Niño and La Niña

High pressure or anticyclone

Low pressure or depression

Temperature differences

Weather fronts

Weather systems

Wind

Air masses

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

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

Atmosphere

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

Clouds

A classification of clouds was introduced by Luke Howard (1772-1864) who used Latin words to describe their characteristics.

  • Cirrus – a tuft or filament (e.g. of hair)
  • Cumulus – a heap or pile
  • Stratus – a layer
  • Nimbus – rain bearing

There are now ten basic cloud types with names based on combinations of these words (the word ‘alto’, meaning high but now used to denote medium-level cloud, is also used).

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

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

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

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

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

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

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

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

blue sky with clouds

 

El Niño and La Niña

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

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

El Niño and La Niña

High pressure or anticyclone

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

Low pressure or depression

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

Temperature differences

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

Factors affecting temperature:

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

Weather fronts

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

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

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

Cold front

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

On a synoptic chart a cold front appears blue

cold front symbol

 

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

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

Warm front

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

On a synoptic chart a warm front appears red

warm front symbol

 

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

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

Occluded front

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

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

On a synoptic chart an occluded front appears purple

occluded front symbol

 

Weather systems

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

Wind

windsock

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

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

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

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

windsock

Web page reproduced with the kind permission of the Met Office

Anticyclones, Depressions and Fronts

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


 

Length 1 lesson

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

Resources Required
None.

Prior Knowledge Required
A basic knowledge of weather and climate

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

Weather Systems

Student Charts

Weather Systems

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

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

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

PART A – ANTICYCLONES AND DEPRESSIONS

High Pressure Systems

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

Winter Anticyclones

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

Summer Anticyclones

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

Low Pressure Systems

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

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

  • PART B – FRONTS

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

Warm Fronts

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

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

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

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

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

anticyclones_depressions_fronts

 

 

 

 

 

 

 

 

Cold Fronts

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

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

anticyclones_depressions_fronts_3

 

 

 

 

 

 

 

 

 

 

 

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

anticyclones depressions fronts

 

 

 

 

 

 

Occlusions

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

The following diagrams depict the formation of an occlusion

anticyclones_depressions_fronts_5

anticyclones_depressions_fronts_6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

PART C – LIFE CYCLE OF A DEPRESSION

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

Origin and Infancy

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

anticyclones depressions

Maturity

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

anticyclones depressions fronts

Occlusion

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

anticyclones depressions fronts

Death

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

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

PART D – DEPRESSION CROSS SECTION AND WEATHER SEQUENCE

 

Cross-section through a Classic Depression

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

anticyclones_depressions

table

What to do next

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

 

Web page reproduced with the kind permission of the Met Office

BACK TO TOP