An air mass is a large body of air with similar temperature and moisture properties that covers a large area. The weather over the British Isles is influenced by air masses that form over warm or cold land and sea areas. When different air masses meet, fronts are formed, such as warm fronts and cold fronts. As a depression passes over an area, it brings characteristic changes in precipitation, wind, cloud cover, temperature, and pressure.

1. HIGH WINTER HIGH SUMMER LOW WINTER LOW SUMMER PRESSURE RAIN TEMP

2. Depressions

3. AN AREA OF LOW PRESSURE

4. AIR WILL RISE

5. COOL CONDENSE CLOUDS COOLS CONDENSES CLOUDS

6. RAIN RAIN

7. Air Masses An air mass can be described as 'the air with similar properties of temperature and moisture which covers a large area'. The weather of the British Isles and surrounding countries is influenced by air masses. Air masses formed over :- a warm land are called Tropical Continental - Tc a warm sea are called Tropical Maritime - Tm a cold land are called Polar Continental - Pc a cold sea are called Polar or Arctic Maritime - Pm or Am When air masses meet, fronts are formed.

13. WEATHER ASSOCIATED WITH THE PASSAGE OF A DEPRESSION

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23. showers heavy rain, sometimes with hail, thunder or sleet rain turns to drizzle or stops continues, and sometimes heavy rainfall none at first, rain closer to front, sometimes snow on leading edge Precipitation winds are squally speeds increase, sometimes to gale force, sharp veer remain steady, backs slightly veers and becomes blustery with strong gusts speeds increase and direction backs Wind speed and direction clouds thin with some cumulus clouds thicken (sometimes with large cumulonimbus) cloud may thin and break cloud base is low and thick (nimbostratus) cloud base drops and thickens (cirrus and altostratus) Cloud cover remains cold sudden drop quite mild continues to rise quite cold, starts to rise Temperature continues to rise starts to rise steadies continues to fall starts to fall steadily Pressure Cold sector Passage of the cold front Warm sector Passage of the warm front Ahead of the warm front WEATHER ASSOCIATED WITH THE PASSAGE OF A CLASSIC DEPRESSION