depressions

TEMP RAIN

PRESSURE

LOW SUMMER

LOW WINTER

HIGH SUMMER

HIGH WINTER

Depressions

AN AREA OF LOW PRESSURE

AIR WILL RISE

COOLCONDENSE

CLOUDS

COOLSCONDENSES

CLOUDS

RAINRAIN

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. 

What is a front? 

Daily variations in the weather are caused by the influence of different air masses. 

The front is the point where these different air masses meet. 

Fronts can either be Warm, Cold, or Occluded.   

WEATHER ASSOCIATED WITH THE PASSAGE OF A

DEPRESSION

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WEATHER ASSOCIATED WITH THE PASSAGE OF A CLASSIC DEPRESSION

  Ahead of the warm frontPassage of the warm front

Warm sectorPassage of the cold front

Cold sector

Pressure starts to fall steadily continues to fall steadies starts to rise continues to rise

Temperature quite cold, starts to rise continues to rise quite mild sudden drop remains cold

Cloud cover cloud base drops and thickens (cirrus and altostratus)

cloud base is low and thick (nimbostratus)

cloud may thin and break clouds thicken (sometimes with large cumulonimbus)

clouds thin with some cumulus

Wind speed and direction

speeds increase and direction backs

veers and becomes blustery with strong gusts

remain steady, backs slightly

speeds increase, sometimes to gale force, sharp veer

winds are squally

Precipitation none at first, rain closer to front, sometimes snow on leading edge

continues, and sometimes heavy rainfall

rain turns to drizzle or stops heavy rain, sometimes with hail, thunder or sleet

showers