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Discovering Air Masses |
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The weather affecting the British Isles has much to do with the source and track of air masses.
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The properties of the air affecting the British Isles depend on the source of the air and its track before it reaches us. This means that it is convenient to categorise the "air masses" according to their origin and track. To a large extent the characteristics of the weather affecting the British Isles is determined by the wind direction. ...air which has travelled over the sea is moist whilst that from over the land is relatively dry...
The idea that northerly winds are cold and southerly winds are warm (at least in the Northern Hemisphere) is quite common. Similarly, it would be reasonable to accept that air which has travelled over the sea is moist whilst that from over the land is relatively dry. These simple concepts help in the understanding of air masses.
In polar and sub-tropical regions there are large semi permanent anticyclones (high-pressure areas). The air resides in these systems for a long time and is gradually influenced by the underlying surface - air at the poles is cooled and air in the tropics is warmed. The result is a large body of air with little horizontal variation in temperature and moisture content. Sometimes there is a large outflow of air from the anticyclones, and these "air masses" may approach the British Isles (usually "polar" air from the north and "tropical" air from the south). However, on their journey they are modified by contact with the underlying surface. Air that travels over the sea ("maritime" air) is moistened, whereas there is little change in moisture content of air which travels over the land ("continental" air). The wind can blow over the UK from all directions...
The British Isles are interestingly placed with sea to the west and land to the east and, being at mid latitudes, with warm climates to the south and cooler ones to the north. The wind can blow over the UK from all directions so we experience a range of air masses with different sources and tracks and this gives UK weather its variability.
Air masses affecting the British Isles can be broadly categorised in terms of their source and their track. This leads to four possible types:
To these must be added another air mass, "returning polar maritime", which consists of polar air which has moved southwards over the sea, and then turns northwards and approaches the British Isles from the south. The air mass only gives an indication of the type of weather which may occur.
As an example, consider air that has been trapped in an anticyclone over the Sahara in June. Slowly the air heats up and dries. After a while the air moves out of the anticyclone and may head for the British Isles. On its way a little moisture may be added over the Mediterranean, but the journey over Spain and France has little affect on its properties. The air then arrives here as a hot, dry air mass - a tropical continental air mass.
In reality, the type of air mass affecting the British Isles only gives an indication of the type of weather, which may occur. The actual weather depends upon the detailed history of the air, the speed of movement, and the surface over which it flows. The boundary between polar maritime and tropical maritime air masses is referred to as a "front". Usually fronts have bands of cloud and rain associated with them. In the winter it is common for the British Isles to be affected by a sequence of fronts and this gives rise to changeable weather.
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