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:
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.
The following diagram shows a cross section through a warm front, with associated cloud, temperature and weather changes.
Cold Fronts
A cold front indicates that cold air is advancing and pushing underneath warmer air at the surface. This occurs because the cold air is ‘heavier’ or denser than the warm air. Therefore cold fronts occur where cooler air is replacing warmer air at the surface. The passage of weather associated with a cold front is much shorter lived than that with a warm front. As there is often a lot of cloud in the warmer air ahead of the cold front, there is often little indication of the approaching cold front.
As the front passes temperatures fall and there is often a short spell of very heavy rain, sometimes with inbedded thunderstorms and cumulonimbus clouds. Behind the front the weather is much brighter with broken clouds but occasional showers. Winds veer with the passage of the cold front and are often strong and gusty, especially near showers.
Pressure rises throughout the approach and passage of the cold front.
The following diagram shows the formation of a cold front in diagrammatic form.
The following diagram shows a cross section through a cold front, with associated cloud, temperature and weather changes.
Occlusions
In a mature depression the warm front normally precedes the cold front. Cold fronts generally travel much quicker than warm fronts, and eventually it will catch up with the warm front.
Where the two fronts meet, warm air is lifted from the surface and an occlusion is formed. An occlusion can be thought of as having similar characteristics to both warm and cold fronts.
The weather ahead of an occlusion is similar to that ahead of a warm front, whilst the weather behind is similar to that behind a cold front.
The following diagrams depict the formation of an occlusion.
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.
Maturity
The warm air rises up over the colder air which is sinking. A warm sector develops between the warm and cold fronts. The mature stage of a depression often occurs over the UK.
Occlusion
The cold front travels at around 40 to 50 miles per hour, compared to the warm front which travels at only 20 to 30 miles per hour. Therefore the cold front eventually catches up with the warm front. When this occurs an occlusion is formed.
Death
Eventually the frontal system dies as all the warm air has been pushed up from the surface and all that remains is cold air. The occlusion dies out as temperatures are similar on both sides. This stage normally occurs over Europe or Scandinavia.
What to do next
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.
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
One of these is lightning, though the possibility of its occurrence may be indicated indirectly in a forecast or station report as ‘thundery showers’. In fact, lightning is not such a risk to sailors as it might at first appear. Boats are surrounded by a very good conductor of electricity – water – and unless the boat suffers a direct hit, which is unlikely, the current is dissipated much more quickly than on land.
Mountain tops are often hidden by clouds, which can result in people getting lost. But why does this happen? Clouds are formed by the condensation of water vapour in rising air. Air is very moist (holds lots of water) when it is near the sea or when there is persistent rain. In the UK the cloud base (i.e. the bottom of the clouds) is often below 1000m, so low in fact that much of the time the clouds cover the mountain tops. Cloud cover on the mountains is particularly common in the west of the UK where the moist air blows in across the Atlantic. Clouds can be supercooled i.e. water remains as a liquid even when the temperature is below freezing (0°C)! these droplets of water freeze when they hit solid objects such as fences and even people. The soft ice can build up into a think layer, known as rime which can cause as much difficulty for walkers as lying snow.
The rain and snow which falls over mountains tends to come from nimbostratus clouds, and occasionally cumulonimbus clouds, tend to be heavier and longer lasting than over nearby low lying areas. This is because the air is forced to rise over the mountains, causing the air to cool as it rises condensing the water from a gas to liquid; forming more clouds. However strong winds at the mountain top can blown the heavier rain over the mountain top, so the heaviest rain will not necessarily be at the highest point. Waterproof clothing and footwear on mountains is essential as there can be heavy rain, driving winds and mountain streams can become deep quickly. Deep depressions coming in from across the West coast of the UK can bring heavy rain and strong winds to the UK. Snow combined with wind can be life-threatening on a mountain top.
The higher up the mountain we climb, the colder and windier it usually gets so that the wind chill factor increases. Warm clothing on the top of mountains is definitely needed. On the Munros (the Scottish mountains with tops above 3000m) it can be 10°C cooler at the top of the mountain in comparison to the valley bottom below. In fact air can cool by 6°C in every 1000m and sometimes as much as 10°C. But why does the air get cooler and windier? It is windy high up in the atmosphere as the effect of gravity is reduced and cooler because air temperatures decrease as you get closer to the poles. Therefore gale force winds are stronger and more common at the top of mountains than at sea level. Winds also get faster around mountains as they do around tall buildings in our towns and cities. Over the Himalayas winds of 150km/hr (approximately 42 metres per second) are not uncommon!
Snow combined with poor visibility in cloud also causes problems because shadows disappear. Navigation becomes almost impossible and can lead to blundering into dangerous places ‘ even over the edges of cliffs. This is known as a ‘whiteout’. If there is already much lying snow and a risk of the cloud base descending onto the hills, many climbers often consider it wise to abandon the trip.
Local winds occur on a small spatial scale, their horizontal dimensions typically several tens to a few hundreds of kilometres. They also tend to be short-lived lasting typically several hours to a day. There are many such winds around the world, some of them cold, some warm, some wet, some dry. There are many hazards associated with the winds.
This wind is caused by thermal (heat) processes. Anabatic (upslope) winds occur over slopes which are heated by the sun. Air which is in contact with slopes that are warmed expands upward and cool and sinks over neighbouring valleys (see diagram). Anabatic winds are usually slow, at only 1-2m/s and are rarely important expect near coasts where they can increase the strength of sea breezes.
Katabatic (downslope) winds occur over slopes which are cooled. Katabatic winds occur where air in contact with sloping ground is colder than air at the same level away from the hillside over the valley (see diagram). Katabatic winds are nocturnal phenomena in most parts of the world (i.e. they tend to happen at night) as there is surface cooling, especially when there is little cloud and due to lack of heating by the sun.
The bora is a strong, cold and gusty north-easterly wind which descends to the Adriatic Sea from the Dinaric Alps, the mountains behind the Dalmatian coast (the coast of Croatia). It is a winter phenomenon that develops when a slow-moving depression is centred over the Plain of Hungary and western Balkans so that winds are blowing from the east towards the Dinaric Alps. These mountains form a barrier which trap the cold air to the east of them whilst the Adriatic coast remains comparatively mild. Gradually, though, the depth of the cold air increases until the air flows over passes and through valleys to reach the Adriatic Sea.
The Föhn is a warm, dry, gusty wind which occurs over the lower slopes on the lee side (the side which is not directly exposed to wind and weather) of a mountain barrier. It is a result of forcing stable air over a mountain barrier. The onset of a Föhn is generally sudden. For example, the temperature may rise more than 10°C in five minutes and the wind strength increase from almost calm to gale force just as quickly. Föhn winds occur quite often in the Alps (where the name Föhn originated) and in the Rockies (where the name chinook is used). They also occur in the Moray Firth and over eastern parts of New Zealand’s South Island. In addition, they occur over eastern Sri Lanka during the south-west monsoon.
The danger of a Föhn where there are steep snow-covered slopes is that avalanches may result from the sudden warming and blustery conditions. In Föhn conditions, relative humidity may fall to less than 30%, causing vegetation and wooden buildings to dry out. This is a long-standing problem in Switzerland, where so many fire disasters have occurred during Föhn conditions that fire-watching is obligatory when a Föhn is blowing.
The mistral is also a strong and often violent wind. It blows from the north or north-west down the Rhône Valley of southern France and across the Rhône Delta to the Golfe du Lion and sometimes beyond. Though strongest and most frequent in winter, it may blow at any time of year and develops when stable air is forced through the Rhône Valley. It occurs when a depression is centred over north-west Italy and the Ligurian Sea and a ridge of high pressure extends north-eastward across the Bay of Biscay.
Thunder is the loud noise which follows a flash of lightning. Lightning can be seen before thunder is heard as light travels faster than sound. The speed of sound in air is just over 300m/s. This means that if you count the seconds between seeing the lightning and hearing the thunder, and divide by three, you can work out how many kilometres away the storm is (for example, if you start counting when you see the lightning and get to 9, then the storm is about 3km away). The noise of thunder is caused by the rapid expansion of heating the air. You can normally hear thunder up to 6 miles (10km) away from the lightning flash. The sound can last quite a few seconds!
Lightning can be seen virtually instantaneously as light travels very fast (about 300,000,000 m/s!). Lightning can be seen up to 50 miles away! lightning. Lightning is produced by discharges of electricity from cloud to cloud or from cloud to ground. A large positive charge builds up in the upper part of a thunder cloud and a negative charge builds up near the base of the cloud. When the potential difference between the charged areas becomes large enough, electrical energy is discharged and a flash of lightning occurs. Huge quantities of electricity are discharged in lightning flashes and temperatures of over 30,000°C or more can be reached!
In a thunderstorm you should not stand under a tree! Lightning tends to strike the highest point around and everything near this can be a target for the lightning too. Very few people survive being hit by lightning. To increase your safety in a thunderstorm you should avoid high ground, water, open spaces such as parks and golf courses, staying in a tent or shed, being within 30m of wire fences or using your umbrella. You should make yourself as small as possible – curling up in a ball is good. It is however safe to stay in the car…do you know why?! It is because the car acts as what is known as a Faraday cage, protecting you from the electric field generated by the storm.
For thunderstorms to occur, cumulonimbus clouds are required. These are heavy, dense, towering clouds with tops shaped like anvils or vast plumes, where the speed of air rising through the cloud can reach 20m/s. Pilots tend to fly around these clouds if they can. They can fly around them as often they are only 10-12km in width. In cumulonimbus clouds weather such as heavy rain, lightning, hail, turbulence and strong winds can occur.
