1. 1 You may have heard this before somewhere… Most of the world has climate But in Britain they have …. Weather!

2. 2 This is because … We are on the edge of several air masses All of which are having arguments with each other And who wins changes all the time.

3. 3 The High’s and Low’s bring us different weather!

4. 4 We are going to watch a video (I hope!) I want you to notice the weather. We are then going to talk about what causes it. So you have seen (??) What was the weather like in the video? This due to a pressure system called an anticyclone We are going to see what an anticyclone is like

5. 5 Weather on Friday at Midday Does anyone know what these lines are called? What do they measure? Why do you think the word HIGH is written in the middle over the UK? If you were in the UK on Friday what was your weather like? Notice that compared with the low pressure, the lines are widely spaced

6. 6 Here are the isobars again. What does the other map show? Where the high is, what is the wind like? Which direction is the wind flowing around the high? Look at the spacing of the isobars around the high and the low. What do you notice?

7. 7 What do you notice about the cloud? The cloud to the NW of the UK is a thick layer of cloud. Do you know what that is called? The cloud over southern France and South Spain is little lumps. Do you know what that is called?

8. 8 So an anticyclone has … High pressure Hardly any wind at the centre Very little cloud Very light winds around the outside going clockwise on this occasion In fact in the northern hemisphere ALL anticyclones have winds rotating in a clockwise direction. How do you think they rotate in the southern hemisphere? The rotation is due to the fact that earth is spinning known as the Coriolis Effect

9. 9 So why does high pressure usually give clear skies? The air is a high pressure, so it sinks towards the ground. As the air descends, the molecules become compressed, the pressure increases and it warms. When air is warming, any moisture in the atmosphere is evaporated so no clouds can form. The sky is clear. Anticyclones can be very large, typically at least 3,000 km wide. Once they become established, they can give several days (or even longer) of settled weather.

10. 10 Here is the diagram that on the wiki moves! There is one last animation on the wiki to remind you

11. 11 But weather forecasters will get caught out by high-pressure conditions. Usually they will produce clear skies, but occasionally it's a little different. The sinking air warms up, but there is a possibility that a temperature inversion could occur, where there is warmer air above cooler air (usually it is the other way round in the atmosphere). The inversion happens because the clear skies mean the Earth's surface loses a lot of heat - there is no insulating layer of cloud. A cool surface means cool air temperatures near the surface. This cool air is dense and hard to shift, so the warmer descending air sits on top. If this air is moist, the water vapour can condense into a dreary layer of cloud which hangs around for days, since the winds are too light to disperse it.

12. 12 This is what happened for some of us last week There was an inversion layer that kept the cloud hanging about For others the problem was early morning mist that took a long time to clear As there is no cloud, once the sun has gone down, heat is radiated out from the Earth into space. Falling temperatures allow any water vapour in the air to condense into low lying mist. It takes the sun a while in the morning to ‘burn it off’ – in other words warm up the air enough to evaporate the water droplets into water vapour

13. 13 Another issue with Anticylones Because the air is pressing down and there is little wind. In cities, such as London, and other places too, the air gets a hazy look, both in Summer and in winter. This is because any pollution produced is stuck at a low level and builds up over time. In the past when everyone burnt coal on their fires, winter fogs could last for days. These were called smogs ( smoke + fog) and lead to many deaths from pneumonia, pleurisy and bronchitis.

14. 14 Choked streets: For years London was synonymous with thick fogs - indeed, the city's pea-soupers were immortalised in the works of Charles Dickens. But the smoke- laden fog that shrouded London from 5-9 December 1952 was the worst single incident of air pollution in the UK. Conductors walked in front of buses holding torches or flares in order to see street signs - and so drivers could see them. Coal deliveries: Although soon coal deliveries were to become a thing of the past, change came too late to save the thousands who fell prey to the trapped toxins. The smog of 1952 was thought to have killed at least 4,000 people - particularly the elderly and infirm - and the death rate remained above average up until Christmas

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16. 16 Next week we are going to look at an altogether more complex weather system As part of that, you need to be familiar with various cloud types – so a small direction!

17. 17 This method of cloud classification was proposed by Luke Howard (1803) who named the clouds based on their form: Cirrus - curl Stratus - layer Cumulus - heap Nimbus - rain and on their height:

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19. 19 Cirrus & cirrostratus The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 6,000 meters, Cirrus clouds are composed of ice crystals that originate from the freezing of supercooled water droplets. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation.

20. 20 Altocumulus Middle layer clouds Altocumulus may appear as parallel bands or rounded masses. Typically a portion of an altocumulus cloud is shaded, a characteristic which makes them distinguishable from the high-level clouds. They are often come before a cold front which usually carries rain with it. Or altocumulus clouds on a warm and humid summer morning is commonly followed by thunderstorms later in the day.

21. 21 Alto stratus Alto is middle level Stratus is layered Not much rain should come with this one

22. 22 Nimbo- stratus Nimbostratus are dark, low-level clouds at a height of 2,000 meters Nimbo meaning rain and stratus meaning layer gives continuous not-very-heavy rain However, when temperatures are cold enough, these clouds may also contain ice particles and snow.

23. 23 Stratocumulus Stratus is a layer and cumulus means heap So the clouds form a layer of heaps, and can give quite heavy showers, but they do not usally last too long They vary in colour from dark grey to light grey and may appear as rounded masses, rolls, etc., with breaks of clear sky in between.

24. 24 Fog Fog is the lowest cloud of all!

25. 25 Fair weather Cumulus These are know as fair weather cumulus ( heaps) small white like cotton wool balls They have flat bottoms which are quite low down and a smallish heap of white cloud above

26. 26 You may sometimes see one of these cumulonimbus The bottom is flat and black, often with the rain already falling out of it in huge drops. Quite often there is thunder and lightening The ‘heap’ part reaches high into the sky – often to the level of the cirrus clouds

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