Just to help them visualise what happens

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