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Air Masses Affecting the British Isles

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Air Masses Affecting the British Isles

  2. 2. The British Isles experiences varied weather, which is brought about by winds which drive air masses from their source region. At their source they have certain characteristics, e.g. wet or dry, hot or cold. On their journey (path) air masses are modified. When these air masses reach British Isles they influence local weather conditions.
  3. 3. The table below summarises the typical Air mass characteristics typical characteristics of the six main air masses which Tropical Continental (Tc) Polar Continental (Pc) Tropical Maritime (Tm) Polar Maritim e (Pm) Arctic Maritim e (Am) Returning Polar Maritime (rPm) Summer Winter Long Sea Track Short Sea Track Expose d Sheltere d Temp Very warm or hot Average Cold Very cold Near sea temp Warm Rather cold Cold (colder than Pm) Warm (warmer than Pm) Humidity Relatively dry Rather moist Moist in lowest layers Very dry Very moist Moist Moist Fairly moist (not as moist as Pm) Fairly moist (not as moist as Pm) Change of Lapse Rate Little change Cooled from below Heated from below Little change Cooled from below Warmed in summer Heated from below Heated from below Heated from below Stability Generally stable Stable Unstabl e Stable Stable Stable aloft Unstable Unstable Unstable Weather Clear, occasional thundery showers Clear Rain or snow shower s Clear Low cloud, drizzle Broken cloud, dry Variable cloud, showers Showers (mainly coastal) Showers (mainly coastal) Visibility Moderate or poor Moderate or poor Good Moderate or poor Often poor with coastal Moderat e Good Very good Very good affect the British Isles.