Air masses affecting the British Isles

Overview

What an air mass is and how it is named, why it changes character as it travels, and the source region, track and weather of each type that reaches the British Isles.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an air mass?

   Model answer

   An air mass is a very large body of air, often thousands of kilometres across, in which temperature and humidity are broadly uniform at any given height. It acquires those properties by resting for days or weeks over a source region with a uniform surface, such as an ocean, a desert or an ice sheet, and it carries them with it when it moves away.

2. 2

   How are air masses classified and named?

   Model answer

   They are named for the temperature of their source region and for whether that source is land or sea. Temperature gives arctic, polar or tropical; surface gives maritime, meaning it formed over ocean and is therefore moist, or continental, meaning it formed over land and is therefore dry. Combining the two gives names such as tropical maritime, warm and moist, or polar continental, cold and dry.

3. 3

   Why does an air mass change its character as it travels away from its source region?

   Model answer

   It is modified by the surface it crosses. If it moves over a surface warmer than itself it is heated from below, which makes the lowest air buoyant and the mass unstable, so it develops cumulus cloud and showers and its visibility improves. If it moves over a surface colder than itself it is cooled from below, which makes the lowest air dense and the mass stable, so it develops flat stratus, drizzle, mist or fog and its visibility worsens. It also gains moisture over sea and loses it over land.

4. 4

   Describe the source region, track and weather of tropical maritime air over the British Isles.

   Model answer

   It forms over the warm subtropical Atlantic around the Azores and reaches the islands on a south-westerly track, so it is warm and very moist. Because it crosses progressively cooler water on its way north it is cooled from below and becomes stable. It brings mild, damp, overcast weather with low stratus cloud, drizzle, hill fog and poor visibility, and advection fog can form over cold coastal seas. In winter it gives unseasonably mild, muggy conditions that melt lying snow; in summer it gives warm, humid, dull weather. It is the air of the warm sector of a depression.

5. 5

   Describe the source region, track and weather of polar maritime air over the British Isles.

   Model answer

   It forms over the cold northern Atlantic near Greenland and Iceland and reaches the islands from the north-west. Crossing progressively warmer water on the way south, it is heated from below and becomes unstable and moist. It brings cool, bright, showery weather with well-developed cumulus and cumulonimbus, heavy showers of rain or, in winter, hail and sleet, clear intervals between them and excellent visibility. It is the commonest air mass over the British Isles and is the air found behind the cold front of a depression.

6. 6

   Describe the source region, track and weather of arctic maritime air over the British Isles.

   Model answer

   It forms over the frozen Arctic Ocean and Greenland and arrives from due north. It is very cold and, having crossed some open sea, picks up moisture and becomes unstable in its lowest layers. It brings the coldest conditions of the year to Scotland and Northern Ireland, with snow and hail showers on north-facing coasts and hills, severe wind chill, and bright clear intervals with very good visibility between the showers. It is chiefly a winter and early spring visitor.

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