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Photo of Cumulus clouds, angle of the shot has been taken from the ground.
Cumulus clouds

Air masses introduction

Before we look into high and low pressure weather systems in further detail, let’s take a step back and think about the properties and characteristics of the air reaching the UK.

Let’s start with the concept of air masses and consider the properties of the air that will impact the weather.

An air mass is a large body of air with relatively uniform characteristics (temperature and humidity). Air masses are classified according to their source region and track.

There are six air masses (Figure 1) which can affect the weather in the UK – Polar Maritime is the most common, but we can also experience Polar Continental, Tropical Maritime, Tropical Continental, Arctic Maritime and Returning Polar Maritime air.

The source regions tend to be semi-permanent anticyclones (associated with the sinking regions of the global atmospheric circulation) in the sub-tropics and polar regions (‘tropical’ or ‘polar’ air). Air masses acquire their characteristics by contact with the underlying surface in the source region.

The UK sometimes also get Arctic air, which has travelled straight south from the Arctic. Returning polar air is polar air which changed direction over the Atlantic, hitting the UK from the west or even south of west, but still polar in nature.

A map showing the directions the 6 air masses that hit the UK . Artic Maritime air comes from the north, Polar Continental air comes from the northeast, Tropical Continental air comes from the south, Tropical Maritime air comes from the southwest, Polar Maritime comes from the northwest with the returning Polar Maritime air hitting the UK from the west.

Figure 1: The 6 air masses which can affect the weather in the UK

Southward moving air is warmed from below as it passes over warmer land and water and becomes more unstable, eventually rising and producing convective cloud – eg puffy cumulus clouds. When you look at these clouds you can sometimes watch the air rising and the cloud bubbling up. In contrast, northward flowing air is cooled from below and becomes more stable.

Air travelling over the sea is moistened and we refer to this as ‘maritime’ air, whereas the moisture in air with a continental track hardly changes and so this is known as ‘continental’ air.

Looking at Figure 1, it’s easy to think that the North East of the UK always experiences Polar Continental air, whilst the South West always experiences Tropical Maritime air etc, but this is not the case. Usually, the whole country experiences the same air mass at the same time.

You may like to watch the following clip from the BBC programme, The Great British Weather, on YouTube, to find out more and for a great introduction to air masses.

In the next Step, we’ll think about what’s happening to the air from the four major air masses as it approaches the UK.

Don’t forget to mark this Step as complete before you move on.

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This article is from the free online course:

Come Rain or Shine: Understanding the Weather

University of Reading