Our variable weather in the British Isles is caused by different air masses that originate from large semi-permanent areas of high pressure in different regions. As the air masses move from these areas, they bring characteristics of their source region, such as warm tropical air from the Azores, but also change as they travel, which can lead to stable or unstable conditions. The source of the air mass, whether cool polar or warm tropical, affects temperatures and moisture levels, but changes to the air mass also impact weather through stable or unstable conditions.

2. Our variable weather is caused by the fact that the B.Isles experiences different air masses. Air masses usually have their ‘source’ in large, semi-permanent areas of high-pressure, that develop over a relatively large, uniform surface. As the air mass moves out from these different areas of high pressure - it brings to Britain some of the characteristics of that area (e.g. tropical maritime air), but undergoes some changes on the way that can bring either stable or unstable conditions… For example, during the winter, a large area of high pressure develops over the Asian continent because the land mass cools down. The cooling causes air to …..

3. Here are the main air masses that can affect the British Isles… Look at the sources of these air masses – why are they areas of high pressure? (At least for some of the year)

4. The source of the air will affect whether it is cool/cold or warm/mild - and whether it is relatively dry or contains a lot of vapour. However – the way in which it changes has an important effect on our weather…

5. Individual cumulus clouds NW SE Polar maritime air moving south from Canadian Arctic / Greenland Lower layers warm up Upper layers remain cool ‘ Unstable conditions!

6. Tropical maritime air moving north from the Azores or even the Caribbean Lower layers cool down Upper layers remain warm Stable conditions! Fog in coastal areas! SW NE Vapour condenses

7. However this warm stable air may be forced to rise over hills – until it reaches its dew point – creating ‘hill fog’ This warm air can hold a lot of vapour –because it is warm and has passed over water. It has the potential to become unstable if initially forced to rise. There could be rain in mountain areas

8. Individual cumulus clouds N S Arctic maritime air moving south Lower layers warm up Upper layers remain cool ‘ Unstable conditions!        Precipitation from the clouds remains as snow because this air is so cold

9. N S Polar continental air from Russia or Scandinavia – in the winter Lower layers warm up Upper layers remain cool Unstable conditions!        Precipitation from the clouds remains as snow because this air is so cold But there may be some warming in winter if it has crossed the North Sea over a longer track

10. Tropical continental air moving north from North Africa Lower layers cool down Upper layers remain warm Stable conditions! S N Stable, dry conditions - heatwaves But … in clear skies – intense heating of ground can lead to convection