What are air masses?
Air masses are large volumes of air with a similar temperature and water vapour content, which can cover large areas and travel long distances. The classification of air masses is based upon the region they form over:
Arctic/Polar - form at high latitudes (so are cooler)
Tropical - form at low latitudes (so are warmer)
Maritime - form over oceans (have a higher water vapour content)
Continental - form over land (low water vapour content)
Arctic/Polar - form at high latitudes (so are cooler)
Tropical - form at low latitudes (so are warmer)
Maritime - form over oceans (have a higher water vapour content)
Continental - form over land (low water vapour content)
The UK is affected by 5 air masses; these air masses are dominant at different times of the year and bring different types of weather. This is one factor explaining why the UK weather is so variable.
1) Arctic Maritime: this air mass originates from over the Arctic Ocean and is particularly cold and wet which results in cold and rainy weather . It is uncommon during summer, however when it does occur, it may bring heavy showers or thunderstorms at unreasonably low temperatures. 2) Polar Maritime: this air mass reaches the British Isles on a north westerly air stream after forming over the North Atlantic Ocean. It has similar characteristics to the Arctic Maritime, cold and dry air that brings cold weather and rain. Over its long journey across the North Atlantic, the air mass temperatures can increase which can make it quite unstable. It is characterised by frequent showers around the year; in particular during the winter months where instability is most evident, hail and thunderstorms can be common over western and northern Britain, whilst in summer heavy showers can occur over Eastern England. |
3) Polar Continental: these air masses are only recognised in winter (November-April), where it forms over Eastern Europe and Russia and consists of cold, dry air that results in cold, dry weather. However, the weather brought is dependent upon the the track of the air masses, because if the air picks up moisture from the North Sea, it can bring snow.
4)Tropical Continental: these air masses form over Africa which provide a source of warm, dry air that causes hot, dry weather. It is most common during the summer months (June, July and August), however has been known to occur at other times of the year.
5) Tropical Maritime: air masses that approach the British Isles in a south-westerly direction after forming over the Atlantic Ocean as a relatively unstable mass over its source region. However, the air masses become more stable over its passage towards the British Isles. These air masses consist of warm, wet air, and hence bring warm weather and rain, which in the winter months can raise the temperatures considerably.
Air masses do play a key role in explaining the complex and variable climate of the UK, however, there are many factors such as its location that also have an influence. For instance, low pressure systems dominate the UK and create depressions, which are known to result in unsettled conditions and bring rain throughout the whole year.
4)Tropical Continental: these air masses form over Africa which provide a source of warm, dry air that causes hot, dry weather. It is most common during the summer months (June, July and August), however has been known to occur at other times of the year.
5) Tropical Maritime: air masses that approach the British Isles in a south-westerly direction after forming over the Atlantic Ocean as a relatively unstable mass over its source region. However, the air masses become more stable over its passage towards the British Isles. These air masses consist of warm, wet air, and hence bring warm weather and rain, which in the winter months can raise the temperatures considerably.
Air masses do play a key role in explaining the complex and variable climate of the UK, however, there are many factors such as its location that also have an influence. For instance, low pressure systems dominate the UK and create depressions, which are known to result in unsettled conditions and bring rain throughout the whole year.
UK Weather Systems
Low pressure systems tend to dominate in the UK; they create depressions, which generally form over the Atlantic Ocean and move east over the UK. Depressions usually form where the Tropical Maritime and Polar Maritime air masses meet, and the warm air rises above the cold air (as it is less dense), meaning the atmospheric pressure is reduced.
The UK also experiences anticyclones (areas of high pressure), which can result in more settled weather conditions. Some anticyclones can be described as 'blocking' as blocking anticyclones sit over the UK and remain there for many days, so depressions that would normally travel across the UK are forced around the upper edge of the anticyclone. |
Climate variations across UK
The UK climate is temperate , with variations coinciding with different seasons; cool, wet winters and warm, wet summers. However, it is important to recognise that the climate across the UK is not uniform and temperature, wind and precipitation exhibit variations across the country.
Temperature
It is warmer in the south, as the southern UK receives more insolation, and more hours of daylight on an average day. See diagram on right. Precipitation The western UK experiences higher precipitation levels. This is because the south-westerly winds bring warm, wet air from the Atlantic Ocean (Tropical Maritime), which is forced upwards over the land, the air the cools and condenses to then form rain. This is also known as orographic rainfall. Wind Winds are strongest in the west , as the south westerly winds cross the ocean. This ocean is flat, unlike land that presents many obstacles such as hills, therefore there is little across oceans to reduce wind speeds. |
Despite these trends in variations across the UK that have been recognised over many years, it is important to note that these are averages, and years such as 2012, demonstrate that these trends are not always followed. This is becoming increasingly apparent, with climate change.