Meteorology - Air Masses
The type of air mass that is affecting the UK has characteristics and behaviours determined, primarily, from its point of origin and how it got to the UK. A knowledge of these factors, along with some basic physics, enables us to make a general forecast for the type of weather we should be experiencing.
To ease our job, the six types of air mass that affect the UK are named as shown in the diagram below:
Pm - Polar Maritime. This air comes from the Cold north and is gradually warmed as it travels south over warmer seas. As it warms it collects more moisture, and being warmed from below wants to rise - it is said to be 'unstable'. When triggered it will rise and thus this air mass brings squally, showery conditions with excellent visibility outside showers.
Tm - Tropical Maritime. Warm, moist, air moving over cooling sea. Stable, poor visibility, precipitation will be rain/drizzle and longer lasting.
Tc - Tropical Continental. Warm, even hot, air. Dry and stable. Hazy, with layered cloud, if any, Local heating may trigger a thunderstorm if conditions are right.
Pc - Polar Continental. In summer, warm air heated by its passage over land and made more stable by passage over the (relatively) cold North Sea. Dry, hot, hazy and stable. Longer sea track may lead to fog in East.. In winter, very cold, stable air. Longer sea track over the relatively warm sea may bring showers, often of snow, to the East.
Am - Arctic Maritime. Very Cold air moving South over a warmer sea. Unstable, good visibility, showers, mainly of snow in winter, to the North of the UK.
RPm - Returning Polar Maritime. As Pm air, although stabilised in the lower layers to resemble Tm. Upper layers remain unstable so if triggered, heavy showers/towering clouds are likely to develop quickly. Poor visibility and drizzle, possibly developing into much heavier showers.