This report is written by Peter Dray, a regular contributor to the website.

Why is it important to study air masses?

Air masses determine a huge amount about weather; temperature, rainfall, cloud cover and so on. Examining air masses and their edges, or fronts, is how weather predictions are made. In Britain, it is more difficult to predict the weather than in other areas as positions of air masses are always moving.

Why is this?

In simple terms, the latitudes below 40° N receives much more radiation than the higher latitudes. If there were no wind streams and ocean currents this would mean that below 40° N the earth would get hotter and hotter, and above it would get colder and colder. As a result, the cold and warm air masses mix to distribute heat. This happens roughly over Britain, which is about 40° N.

So Britain is below all this cocktail mixing of air masses?

If you put it that way, yes. This makes Britain’s weather volatile. In fact, only two-thirds of weather in a particular place in Britain is the same as the previous day. This is one of the lowest probabilities in the world.

How many types of air masses are there?

There are five - Polar maritime (abbreviated Pm), Tropical maritime (abbreviated Tm), Arctic maritime (Am), Polar continental (Pc) and Tropical continental (Tc). The names are worked out by combining the latitude from which they originate, and the surface over which they develop.

And each of these air masses has different characteristics?

Yes, that’s right. The most common across Britain is Tm which is associated with south-westerly winds. In the winter it brings quite mild weather. This means that it is very rare for there to be a frosty evening. It can also bring stratus cloud, fog and wet weather. In the summer weather is warm but not hot; it can be thundery.

And the other air masses are associated with different winds?

Yes. Pm comes from the north-west, Am from the north, Pc from Siberia in the east and Tc from the south, originating in the Sahara desert.

OK.  So, what are fronts?

They are the edges of the air masses, for instance when warm air forces cold air back, there is a so-called ‘warm front’. Fronts have weather types associated with them. For instance, when Tm and Pm meet (at a polar front) there is often heavy rainfall.