WeatherWebb

Making the weather more accessible

The UK’s air masses

on February 26, 2015

The UK’s weather is often a discussed, mainly because it is so changeable. In one day you can see sunshine, rain, snow, thunder and strong winds. The reason the British weather changes so much is for several reasons, such as; we’re an island, we’re often under the path of the jet stream, we’re affected by 6 different air masses.

For my diploma, I recently produced the map below to illustrate the different air masses that affect the UK and the different weather conditions they can bring. In general, the polar and arctic air masses are most frequent in the winter months and the tropical in the summer months. However, they can be seen at almost any point in the year. The most frequent air masses to influence the UK are the polar maritime and tropical maritime; we are frequently in a south-west to north-westerly flow.

airmasses

The maritime air masses are often very moist as they come from the seas and oceans and therefore collect moisture as they travel towards the UK. Continental air masses come from continental Europe and therefore are often drier in comparison to maritime air masses as they don’t have as much of an opportunity to collect moisture from the seas.

When it comes to winter, there are 3 main air masses that are associated with snow; the arctic maritime, polar maritime and polar continental. In 2012, I wrote an article on the “Perfect conditions for snow” for the MeteoGroup website WeatherCast. Below you can see the three maps I produced to illustrate what areas of the UK are likely to see the heaviest snow depending on which air mass is affecting the UK.

So how does an air mass change? When watching the weather on TV you will see or hear the forecaster discuss a front. Frontal systems are associated with low pressure systems, which develop, in general, to the north of the jet stream (as discussed in my previous blog; Pressure, highs and lows). Behind each front, there will be a different air mass; behind a cold front you may see a arctic maritime air mass or behind a warm front you may see a tropical maritime air mass. This leads to variations in the British weather, and it is often why it can start mild and wet in the morning, and then turn cold and dry in the afternoon. Additionally, high pressure systems can also influence the air mass that affects the UK. For example, if high pressure is situated over Germany, the winds travel in a clockwise direction, therefore warm air from France and Spain will move into the UK. This would be the tropical continental air mass.

Next time you’re looking at a synoptic map or weather forecast, see if you can determine which air mass is going to affect the UK in the coming days.


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