Anticyclones

This image shows an anticyclone over Spain on 31st January 1999. Notice how the sky is clear - this is a defining feature of an anticyclone.

Anticyclones are areas of high pressure. They bring settled weather. If an anticyclone is sitting over the UK in the summer, it's an ideal opportunity to head for the beach!

In an anticyclone, air descends and pressure increases. This brings very light winds (which blow outwards from the centre in a clockwise motion), clear skies and dry conditions (because the air is descending and warming up, there are no clouds and no rain).

Summer Anticyclones

Summer anticyclones bring hot and sunny weather due to there being clear skies (because of the descending air). The days are long and the sun high in the sky, which gives plenty of opportunity for the land to be heated up. These conditions could therefore lead to convectional rainfall occurring.

Winter anticyclones

Winter anticyclones bring clear days too. However the sun is low in the sky and the days are short, meaning you get cool, crisp days. Clear skies on a winter’s night will allow frost to form. The land quickly loses heat during the night, as there is no cloud cover to act as insulation. The rapidly cooling ground cools and condenses any moisture in the air above it, forming droplets of ice when the temperature falls below freezing. This is frost.
Fogs are also caused by clear winter nights. The ground loses heat. This cools the air above it causing moisture to condensate around dust particles in the layer of air closest to the ground surface. This is fog.

Thanks to YouTube user BlackDollarBills for this video about the features of summer and winter anticyclones:



This video will help you to remember the key features of anticylcones.

You should now be able to complete the worksheet about the differences between depressions and anticyclones. Click on the image to be taken to the task.

The Great Gale of the Millennium - Lothar and Martin - 25-28 December 1999

We studied the Great Gale as an alternative case study of the effects of a low pressure system.



Again, you need to be able to recall specific detail about the storms. Most exam questions focus on effects on both people and environment - it would be a good idea to organise your notes under these two headings. The PowerPoint we used in the lesson is reproduced below - you should use it to add detail to your notes.

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Depression case study - The Great Storm of October 1987

On Paper 1/2, each question ends with a case study. This is worth 5 marks on the Foundation Tier paper and 8 marks on the Higher Tier paper. To score well, you need to include specific detail in your answer.



Questions about low pressure weather systems are common in the exam. You should use the case study handout sheets and the YouTube clips to add to your notes and then have a go at writing an answer to one of the following questions. In the exam, you will have about 8-10 minutes to write your case study.

A weather event (June 2001)

(i) Name a weather event you have studied
(ii) Describe how the weather event affected different groups of people and the environment
(iii) Explain why the weather event took place

A weather event caused by a high or low pressure weather system (June 2003)

(i) Name a location where you have studied a weather event
(ii) State whether the weather event was caused by a high or low pressure system
(iii) Describe how the weather event affected people and the environment
(iv) Explain what caused the weather event (use diagrams if you wish)

A weather event caused by low pressure (June 2005)

(i) Name and locate a weather event that has been caused by a low pressure system
(ii) Describe the weather event
(iii) Explain how the weather event affected different groups of people and/or organisations













These clips are from broadcasts made for the 20th anniversary of the storm, in 2007:

Low pressure systems - depressions

You need to know:

- what a depression is
- how it forms
- what it looks like on a satellite image and a synoptic chart
- the type of weather it brings

REMEMBER: a depression is a low-pressure system (if you are feeling depressed, you are sometimes said to be feeling 'low').

A depression forms when cold polar air meets warm tropical air at a front. The less dense warm air starts to rise over the colder air, and this starts to lower the air pressure. Depressions are common over the UK as we are on the boundary between the warm and cold air masses. They move from west to east across the UK, bringing rainy weather with them. The air in a depression blows anti-clockwise and upwards.

The boundary between the two masses is called a front. There are 2 fronts in a typical depression:

1. the warm front - this passes first, and has the warm air behind it (i.e. it is the front of the warm air). It is where the advancing warm air is forced to rise over the cold air

2. the cold front - this follows, and has the cold air behind it (i.e. it is the front of the cold air). This is where the advancing cold air undercuts the warm air in front of it.

On a weather map, the warm front is shown as red semi-circles and the cold front as blue triangles. To remember this, think about the colour on bathtaps (cold=blue, red=hot) and the shape of a lady's chest in hot/cold weather! The isobars are closely spaced, indicating strong winds, and the numbers decrease to a low in the centre. An occluded front is shown by alternating blue triangles and red semicircles.

The sequence of a depression

We used a mnemonic to remember this:

Chesterton Army Went Fighting With Sticks, Coleridge Fled Cowardly Away

which stands for:

Cold Air, Warm Front, Warm Sector, Cold Front, Cold Air

This mnemonic can also be useful when remembering the pattern of weather that a depression brings. This is shown in the PowerPoint below.

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In class, we used Mr Cassidy's fantastic Postman Pat movie and PowerPoint to help us with the depressions topic. You can download the movie and presentation from here. It's a huge download (over 50mb) so may take a while - but it's definitely worth it!

We also used the interactive files from the NGFL in class. You can work through them here.

We didn't make out own movie about the passage of a depression, but if we had it would probably have turned out something like this one from Teale Green school.

What factors affect temperature?

You should be able to explain a variety of factors that affect temperature on a global scale, including:

- latitude (distance from the equator)
- altitude (height)
- continentality (distance from the sea)
- prevailing winds and ocean currents

Latitude

Latitude is the main factor affecting global climate - the further you go from the equator, the cooler it gets (all other factors remaining constant). This is because the earth in curved which means that the sun's energy is more concentrated at the equator. This, and the thinner atmosphere at the equator, means that the earth gets hotter here.

Altitude

Temperatures decrease by 1 degree celsius for every 100 metres increase in altitude. Mountainous areas are therefore cooler.

Continentality (distance from the sea)

The sea is cooler than the land in summer, but warmer in winter. This is because it takes the sea a long time to heat up, but it is slower to cool down than land.

Prevailing winds and ocean currents

The prevailing wind is the direction that the wind blows from most often. Winds take on the characteristics of their source region. Our climate in the UK would be much colder if the prevailing wind came from the north (the Arctic).

Ocean currents can be warm or cold, and they affect the temperature of coastal areas. The warm North Atlantic Drift keeps the British Isles warm in winter.

For a really clear summary of the main factors affecting global temperatures, download and read Robert Scott's essay. Robert wrote this essay when he was in Year 8!

The British Climate

The British climate is temperate - this means that it does not experience extremes of temperature or rainfall. The climate can be divided into four sections.
In winter, the west is kept warm by the Gulf Stream whilst the east is cooled by air masses from continental Europe. In summer, temperatures are influenced much more by latitude.

These images show surface temperatures for January and July.





The four sectors are as shown here:

Types of rainfall

By the end of this lesson you should have been able to name and explain the three types of rainfall. You should also have drawn an annotated diagram to show why rain falls in each of the three cases.

We get rain because of rising air. As air rises, it cools. As cool air can't hold as much water vapour as warm air, the water vapour condenses and turns into water droplets. These fall to earth as precipitation.

In class, we used an animation to show why convectional, relief and frontal rainfall occur. You can see the animation here.

Convectional Rainfall

This is very common in areas where ground is heated by the hot sun, such as the Tropics. We do get some of this type of rainfall in Cambridge in the summer - in fact, its the reason why last year's school sports day was cancelled!

The sun heats up the ground, which heats the air above it. This then rises and cools, and the water vapour condenses into water droplets, forming clouds. The droplets falls as rain under the influence of gravity.

This video shows convectional rainfall in Costa Rica.



Relief Rainfall

With relief rainfall, it is the presence of hills or mountains that leads to the warm aid rising. Its particularly common on the west coast of the UK, since the prevailing winds come from the west and have to rise to pass over hills and mountains in areas such as the Lake District and Snowdonia.

The winds pick up moisture from the sea as they pass over it, and this makes the air moist. As the air rises to pass over the higher land, it cools and the water vapour condenses, forming clouds. The droplets then fall as rain, sleet, hail, or snow, depending on the atmospheric conditions.

On the leeward side of the mountain (the side not facing the wind), it is much drier. This area is said to be in the 'rainshadow'. There is little rain here as the air is descending and warming up.

This video clip shows relief rainfall on New Zealand's south island.



Frontal Rainfall

Frontal rainfall in the UK is associated with depressions. It happens when two air masses (a warm one and a cold one) meet. The warm air is pushed upwards over the cold air, and, as it rises, it cools down. The water vapour condenses to form clouds, which give rain.

This video shows frontal rainfall.



Test Yourself!

Download this PowerPoint, then move the objects into the correct place on each slide and custom animate to help you explain the rainfall processes.

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Note for teachers:
I'm not sure who wrote this activity - but I think it may be someone from Manchester. Please let me know if it's you so that I can add an acknowledgement.