Air masses, clouds and the frontal depression

An air mass is a huge body of air whose temperature and humidity change only slightly from one side to the other. It takes them from the region where it forms, its source region, and carries them with it.

Air masses are named by where they come from: maritime if they form over the sea, continental if over land; and arctic, polar or tropical by how far north or south.

On the way they change. Over the sea they pick up moisture. Over a warmer surface they are warmed from below and become unstable; over a cooler one, stable. That comes back in topic 2.

The six that reach the British Isles

The Met Office names six air masses that reach the British Isles. Each arrow on the map shows where one arrives from.

Arctic maritime (Am) comes from the North Pole and the Arctic Ocean and arrives from the N. Colder and less moist than polar maritime; heavy showers, in winter of snow and hail; very good visibility.

Polar maritime (Pm) comes from northern Canada and Greenland and arrives from the NW. The most common air mass over the British Isles: showers at any time of year, in winter with hail and thunder in the west and north.