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

Sailing Weather

Sailing Hazard

Sailing is one of the most weather-dependent sports. Unfortunately, wind is not just a useful source of power for sailing craft but also a hazard. Strong winds can capsize boats, either directly or in combination with the waves they may produce.

The wind is never steady. It always fluctuates between gusts of higher wind speed and lulls that may be so light as to be near-calm. However, sudden increases of wind on a larger scale can sometimes occur. These are called squalls and are often associated not only with strong, gusty wind but also with heavy rain.

The importance of any hazard varies with the skill level of the crew, the type of boat and the kind of sailing being undertaken. For example, a novice crew in a small boat may underestimate wind strength before setting off. This is an easy mistake to make, especially if they are launching from a relatively sheltered location. Sailors who are more experienced are unlikely to be caught out like this but are still vulnerable in other ways. Hard sailing, especially in colder conditions, can tire a crew very quickly. Exhaustion or exposure can creep up on them before they know what is happening. For example, a dinghy crew might be having great fun practising sailing across the wind, only to find when they feel they have had enough that they do not have the reserves of energy for a long struggle upwind or a tricky run downwind.

These days, yachtsmen do not have to rely on old folklore or gamble on good weather. All sailors should pay attention to weather forecasts. These are available through radio, television, the internet and other means of broadcasting.

The particular forecast that is most appropriate depends on the kind of sailing being planned. The shipping forecast broadcast on the radio is perhaps the most useful to offshore sailors. The terms used in it are precisely defined, and the information that is included in it on pressure changes and movements of weather systems is very useful to anyone with a deeper than average understanding of meteorology.

When yachtsmen study meteorology as part of a training course, either at sea or ashore, they are often asked to create a weather map from a recorded shipping forecast as an exercise. The Royal Yachting Association can provide forms called ‘Metmaps’ that make recording and interpreting the shipping forecast a lot easier. Completing one of these is a good exercise for anyone who wants to go into meteorology seriously. The ‘Metmap’ is a two-sided A4 form. On one side, a shipping bulletin broadcast by the BBC can be taken down. On the other, a simple up-to-date weather map can be drawn from the information contained in the bulletin.

The shipping forecast gives a lot of information about visibility at sea. This is because poor visibility can sometimes be a greater hazard than strong winds. The forecast will not only give a guide to overall visibility in terms of ‘good’, ‘moderate’ or ‘poor’ but will often indicate if visibility is poor for a specific reason, such as ‘visibility poor in showers’. This particular occurrence can give a sailor a real fright, as views of nearby vessels or navigation marks can be lost suddenly when showers occur.

Two possible hazards are not often mentioned in weather forecasts for sailors but usually are in forecasts for land areas:

old illustration of ship lighteningOne 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.

The other neglected hazard is exposure to sunlight and sunburn. Sailors are at particular risk for two reasons. First of all, yachtsmen can get an increased dose of sunlight because of reflections from the water. Secondly, they may not notice this because the wind will make them feel cool and unaware they are ‘cooking’. Many a sailor has returned to work on a Monday morning with a ‘yachtsman’s tan’ (from the neck up!). This might seem a nice problem to have, but all sailors should note the example of the America’s Cup crews, who often display extremely colourful suncream to protect against harmful solar radiation.

Despite the apparently long list of hazards described, sailing is, in fact, a very safe sport. It is also a sport where knowledge of meteorology can increase a participant’s enjoyment and even give a competitive advantage!

Acapulco in 1968, sailors competing in the Olympic Games had an unusual surprise from the weather. While not actually hazardous, it was certainly not pleasant. One day, following a sudden squall, the covers of the boats were covered with maggots, which had, presumably, been drawn up into clouds by a whirlwind or waterspout, only to fall out in a downpour of rain.

The year 1979 is famous to yachtsmen for the worst possible reason. In August of that year, during the Fastnet Race (from Cowes on the Isle of Wight to the Fastnet Rock [51°24’N 9°35’W] off south-west Ireland and back again to Plymouth), the fleet of yachts ran into severe storms and rough seas. Fifteen lives were lost. Despite ‘survival conditions’, many crews kept records of the severe conditions, based on their barometers and wind instruments. To the meteorologists who have analysed the Fastnet Storm and its structure, these records have proved invaluable. For a recent analysis of the storm, see the article by D.E.Pedgley in the August 1997 issue of Weather (Volume 52, pp.230-242).

Shipping forecasts are currently broadcast four times a day on BBC Radio 4 Long Wave. They are also available via the websites of the Met Office and the BBC Weather Centre. In shipping forecasts, the Beaufort Scale is used for describing wind strength. This scale originated in the days of sail but is now defined precisely in terms of the wind at a height of ten metres averaged over a ten-minute period.

Mountain Weather

MountainsEvery year people go and enjoy the wonders of the mountain scenery but for some the experience isn’t so pleasant as they venture onto the mountain unaware of the weather they may met. Every year, even in the UK people die on mountains as a consequence of the weather.


Some underestimate the mountain weather; some are unaware of the mountain weather and some leave decisions on the weather too late when on the mountainside.

Download mountain hazard Factfile

Clouds on the Mountains

big mountainsMountain 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.

clouds in the mountainsThe 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.

Wind chill on the mountains

wind in the mountainsThe 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!

What is a whiteout?

mountain whoteoutSnow 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

Clouds Wind CumulusLocal 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.

The main types of local winds are sea breezes and land breezes, Anabatic and katabatic winds, and Foehn winds.

You can discover different ways to measure wind speed on our Experiments and Demonstrations page, or on our Top 10 Ideas for Weather Fieldwork page.

Anabatic winds

Anabatic Wind DiagramThis 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 Winds

katabatic wind diagramKatabatic (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.

Katabatic wind speeds do not typically not exceed 3 or 4 m/s. However, where the ground is covered with snow or ice, katabatic winds can occur at any time of day or night with speeds often reaching 10 m/s, or even more if funnelling through narrow valleys occurs. Katabatic winds may lead to the formation of frost, mist and fog in valleys.

Sea Breeze

Sea breezes are the result of differential heating of the land and the sea. Sea breezes occur by day, when the land becomes warmer than the sea.

Warm air from the land cannot expand into the sea as the air is cooler and more dense, so it expands up into the atmosphere. Cumulus clouds tend to form as the warm air rises over the land to about 500-1500m.

The diagram below shows the whole sea breeze process.

Air in sea breezes is cool and moist compared to the air over the land. Sea breezes can move 70km inland in temperate climates by 9pm in the evening. Sea breezes can be noticed several kilometres out to sea. In the tropics they can be felt 20km from the land. Wind speeds from sea breezes can be about 4-8m/s but can be even stronger.

Sea Breeze Diagram

Land Breezes

Land breezes occur at night and in the early morning, when the land is cooler than the sea. This is because as the air cools in the night time (as there is less heating from the sun) it contracts. Pressure is higher over the land than the sea. This causes the air to flow from the land to the sea which is known as a land breeze. The circulation is completed by air rising and moving towards the land at 100-300m.

This is shown on the diagram below.

wind over land

Cumulus clouds from where there is rising air. Land sea breeze fronts tend to only affect a small area of 10-15km out to sea, in comparison to the much larger effect of sea breezes. Wind speeds are also lower at 2-3m/s.

 

 

 

As well as local winds and local weather phenomena, the following list includes seasonal winds with local names:

The Bise

A cold dry wind which blows from the north-east, north or north-west in the mountainous regions of southeastern France and western Switzerland in winter months. The bise is accompanied by heavy cloud.

The Bora

car overturnedThe 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 bora begins suddenly and without warning and the cold air typically descends to the coast so rapidly that it has little time to warm up.

The bora can reach speeds of more than 100 km/h and has been known to overturn vehicles and blow people off their feet.

The Chinook Wind

A warm, westerly wind found in western North America – Canada and the USA, when air from the Pacific blows over the Rocky Mountains and other upland areas. On January 15th, 1972, the Chinook caused the temperature in Lorna, Montana to rise from -48°C to 9°C in 24 hours! The fastest wind speed recorded during a Chinook was 107mph, in Alberta, Canada.

The Crachin

The name given to the drizzly weather with low stratus, mist or fog which occurs from time to time during the period January to April over the China Sea and in coastal areas between Shanghai and Cape Cambodia. It occurs when cool, moist air from the north encounters warm, moist air, and it is intensified by orographic lifting and/or by coastal convergence.

The Etesians/ Meltemi

The strong northerly winds which blow at times over the Aegean Sea and eastern parts of the Mediterranean Sea during the period May to October. The winds are known as meltemi in Turkey.

The Föhn (or Foehn)

foehnThe 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.

danger symbolThe 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.

An explanation of the Föhn effect in the UK from BBC weather.

The Gregale (or Grigale)/ Euroclydon/ Euraquilo

A notorious wind of the western Mediterranean which also blows across central parts of the Mediterranean Sea. It is a strong and cold wind from the north-east and occurs mainly in winter. It is most pronounced on the island of Malta, where it sometimes reaches hurricane force and endangers shipping.

The Bible tells us that the ship which carried St Paul and other prisoners across the Mediterranean en route to Rome was driven by a storm from Crete to Malta, where it was eventually wrecked.

The ship set sail from a bay called Fair Havens near Lasea on the south coast of Crete, having put into this bay to wait for favourable conditions after being delayed by contrary winds. As the bay was exposed to storms, it was not considered a safe place to remain at the time of year in question, which was either late September or early October. The harbour at Phoenix on the south-western coast of Crete was considered a safer place to spend the winter. Soon after the ship left Fair Havens, however, a severe gale began to blow. So violent was the storm that the sailors could do no more than drift with the wind. The Bible tells us that the ship reached Malta fourteen days later.

Paul experienced a gregale. As these winds normally last no more than four or five days, the storm he experienced appears to have been unusually persistent.

The Harmattan

A dry and comparatively cool wind which blows from the east or north-east on the coast of North Africa between Cabo Verde and the Gulf of Guinea during the dry season (November to March). It brings dust and sand from the Sahara Desert, often in sufficient quantity to form a thick haze which hinders navigation on rivers. Dust and sand are sometimes carried many hundreds of kilometres out to sea.

The Helm Wind

An easterly wind found in Cumbria, N. England, where the winds blow over Cross Fell and then descend into the Eden Valley. A bank of cloud forms over the hill tops, and the roaring of the wind can be heard as far away as Penrith.

Discovering Britain A walk across Great Dun Fell from the RGS, exploring the Helm wind and lots of other weather, as well as stories of sheep, socks, snow and spies and a relevant article from Weather: The Helm wind of Cross Fell .

The Kaus/Suahili

The name given to the south-easterly winds which prevail in winter (December to April) in the Persian Gulf. They are accompanied by gloomy weather, rain and squalls and are sometimes followed by very strong south-westerly winds called suahili.

The Khamsin

A hot, dry, dust-laden, southerly wind over Egypt, the Red Sea and eastern parts of the Mediterranean Sea ahead of eastward-moving depressions. It occurs during the period February to June, being most frequent in March and April. The Khamsin is a Sirocco wind, whose name comes from the Arabic word for ‘fifty’, which is approximately the length of time the wind blows for. In the nineteenth Century, the plague was worst in Egypt when the Khamsin was blowing.

The Kharif

A strong south-westerly wind which blows daily over the Gulf of Aden from about 22:00 hours until about noon the following day. It occurs in June, July and August (during the south-west monsoon) and frequently reaches gale force.

The Leste

A hot, dry, southerly wind which occurs in winter and spring between Madeira and Gibraltar and along the coast of North Africa ahead of an advancing depression.

The Levanter

A moist wind which blows from the east over the Strait of Gibraltar. It is frequently accompanied by haze or fog and may occur at any time of year, though it is most common in the period June to October. A feature is the occurrence of a ‘banner cloud’ extending a kilometre or more downwind from the summit of the Rock of Gibraltar.

The strength of the Levanter does not normally exceed Beaufort Force 5. When it is strong, however, complex and vigorous atmospheric eddies form in the lee of the Rock, causing difficult conditions for yachtsmen and the pilots of aircraft. The levanter can also cause persistently foggy weather on the coast of Spain.

The Levenche

A hot, dry, southerly wind which blows on the south-east coast of Spain ahead of an advancing depression. It is typically laden with sand and dust, and its approach is often heralded by a belt of brownish cloud on the southern horizon.

The Libeccio

A strong, squally, south-westerly wind which occurs over central parts of the Mediterranean Sea, most common in winter.

The Maestro

The name given to north-westerly winds over the Adriatic Sea, the Ionian Sea and coastal regions of Sardinia and Corsica.

The Marin

A strong south-easterly wind over the Golfe du Lion. It is usually accompanied by warm, cloudy weather with rain.

The Mistral

mistralThe 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.

It may blow continuously for a day or two and attain speeds of 100 km/h, causing considerable damage to crops and making driving conditions difficult in the Rhône Valley. It clears clouds and pollution out of the air. In the Rhone valley in France, trees lean to the South because of the force of the Mistral.

The Norther

This name for a wind is used in more than one place. In Chile, a Norther is a northerly gale with rain. It usually occurs in winter but occasionally occurs at other times of year. Typically, it can be identified by falling air pressure, a cloudy or overcast sky, good visibility and water levels below normal along the coast.

Over the Gulf of Mexico and western parts of the Caribbean Sea, Northers are strong, cool, northerly winds which blow mainly in winter. Over the Gulf of Mexico, they are sometimes humid and accompanied by precipitation, but over the Gulf of Tehuantepec they are dry winds.

Pampero

The name given to severe line squalls in Argentina and Uruguay, particularly in the Rio de la Plata area. They are associated with marked cold fronts and are usually accompanied by rain, thunder and lightning, a sharp drop in temperature and a sudden change of wind direction from northerly or northwesterly to southerly or south westerly. They are most likely to occur during the period June to September.

Roaring forties

Strong westerly winds found in the Southern Hemisphere between 40 and 50 degrees South. Sailing ships have always made use of them. The winds are much stronger in the Southern Hemisphere because there is very little land to slow them down – only Tasmania, New Zealand and the very bottom of South America.

The Santa Ana

A hot, dry, strong, blustery, föhn-type wind which blows from the north-east or east over southern California and carries with it large quantities of dust. It is most frequent in winter but may also occur in spring or autumn. It may get its name from the Santa Ana Mountains or the Santa Ana Canyon but other possibilities are that it derives from santanas, meaning ‘devil winds’, or the Spanish Satanás, meaning Satan. These winds can cause a great deal of damage. As they are hot and dry, they cause vegetation to dry out, so increasing the risk of wildfires; and once fires start the winds fan the flames and hasten the spread of the fires. In spring, Santa Ana winds can cause considerable damage to fruit trees.

Shamal

Strictly, the shamal is any north-westerly wind over the Persian Gulf and Gulf of Oman, but the term usually refers not to the normal prevailing winds but to the squally gale-force winds accompanied by rain and thunder which occur in winter.

The Scirocco/ Sirocco

A hot, dry wind southerly wind which blows from the Sahara in northern Africa into the southern Mediterranean. It picks up moisture as it crosses the Mediterranean and can reach Spain, France, Italy and Greece bringing Saharan dust and hot, windy, damp weather, often with fog or low stratus cloud. In spring, the Sirocco can bring gale force winds. There are many local names for the Sirocco, including chom, arifi, Simoom, Ghibli, Chili, Khamsin, Solano, Leveche, Marin and Jugo.

The Solano

An easterly or south-easterly wind, with rain, which occurs in the Strait of Gibraltar and over south-eastern coasts of Spain.

Southerly-buster

The name given to the violent squalls which are associated with well-defined active cold fronts over coastal regions of southern and south-eastern Australia. They are accompanied by lightning, thunder and gale-force winds and are similar to pamperos. They are most frequent in summer but may also occur in spring and autumn.

Sumatra

These are characteristically squally local winds which occur over the Malacca Strait several times a month during the period April to November. They are always accompanied by heavy rain from cumulonimbus clouds and are almost always accompanied by lightning and thunder. They are initiated by katabatic winds and therefore tend to occur at night.

Tehuantepecer

A violent squally wind from the north or north-east in the Gulf of Tehuantepec in winter. It originates in the Gulf of Mexico as a norther and blows across the Isthmus of Tehuantepec.

The Trade wind

Easterly winds in the Tropics (between the Tropic of Cancer and the Tropic of Capricorn). The Trade winds helped European explorers reach America.

Tramontana

A cold, dry, northerly or north-westerly wind over the coast of Catolina and a cold, dry northerly or north-easterly wind over the west coast of Italy and the north coast of Corsica. It is typically a strong wind but does not often reach gale force.

The Vendavales

Strong, squally, south-westerly winds off the east coast of Spain and in the Strait of Gibraltar. They occur mainly during the period September to March and are often accompanied by violent squalls, heavy rain and thunderstorms.

Williwaw

A sudden, cold, violent wind found in Alaska and Cape Horn which blows from the mountains to the sea.

Willy-Willy

A willy-willy is a dust storm over north-west Australia. More information.

The Zonda/Sondo

This term applies to two different phenomena. It usually refers to a dry and often dusty föhn wind that occurs over the eastern slopes of the Andes in central Argentina in winter months. It is also the name (or sondo) given to a hot, humid, northerly wind over the Pampas region of South America in advance of an eastward-moving depression and preceding a pampero occurrence.

Sunlight

Villa with pool in sunshineSunshine is essential for life on our planet. The existence of all plants, animals and life on land and sea depends on the sun. The movement of the atmosphere and the oceans are powered by the sun. Without sunlight plants would not grow and crops would not ripen. Sunlight in moderation is good for us; it helps us to maintain the balance of vitamins in our bodies and can help us to generate power through the use of solar panels. Many people do not like damp and cloudy weather and this is known as seasonal affective disorder. Black surfaces become hotter than white surfaces in sunlight, so buildings in sunny places tend to be painted white to keep them cooler, and people wear white clothes to keep cool.

Download Sun hazard Factfile

The UV index

Weather forecasters in the UK use the UV (ultra-violet index) to warn about the strength of the radiation from the sun. The index depends on two factors; the position of the sun in the sky and the amount of cloud cover. In the UK a scale of 1 to 10 is provided for the index, combined with risk categories, which basically tell you how harmful the sun is going to be to humans. The sun can burn our skin and hurt our eyes if we look at it directly.

LOW= sun will not prove harmful.
MEDIUM= sun is not dangerous but you should not expose bare skin to the skin for over 1-2 hours
HIGH= the sun is dangerous and you could burn in 30-60 minutes
VERY HIGH= the sun is very dangerous and you could burn in 20-30 minutes

In addition to all of these the sun can cause overheating and dehydration.

protect yourself from the sun graphic

Thunder and Lightning

What is Thunder?

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!

What is lightning?

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!

What should you do in a thunderstorm?

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.

Who discovered how to protect buildings from lightning?

Benjamin Franklin…in 1752 he flew a kite into a thunderstorm (don’t do this; he put his life at risk!) but luckily he survived and invented the lightning conductor. A lightning conductor is a metal rod or piece of wire which electrical discharges and led harmlessly to earth. They can now be seen on church towers and spires, skyscrapers and other tall building to protect them from damage.

How can a thunderstorm form?

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.

More information about thunderstorms.

Read about William Rankin, who survived falling through a thunderstorm.

Climate Change Teaching Resources

Climate negotiations resource for KS3/ 4 geography.

Resources to teach the climate of the last 2.6 million years.

Using Tree Rings to teach past climate change.

Past and future global and UK climate change resources from Weather and Climate: a Teacher’s Guide

A summary of the IPCC’s 1.5 Degree climate change report.

A climate change concept cartoon, produced in conjunction with Millgate House.

In Depth Climate and Climate Change information from the Met Office .

In Depth Carbon information from the Met Office.

Climate Change Schools Project Resources.

Climate change updates from the 2013/ 2014 IPCC report for geography teachers and science teachers with selected FAQs and downloadable figures.

Climate change updates from the 2013/ 2014 IPCC report for A level geography

Climate change Scheme of Work for year 8 geography, developed by Charlotte Woolliscroft at Lawrence Sheriff School:

Climate change teaching resources for GCSE science.

Climate/ climate change teaching resources for Scottish Curriculum for Excellence level 3/ KS3.

Weather Glossary

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