The Climate of the British Isles
The UK experiences a great variety of weather conditions. There are also distinct differences and variations between the North, South, East and West regions of the country. These differences are a direct result of the air masses that affect the UK.
Air Masses
An air masses is a large volume of air which has similar temperature and humidity characteristics. Generally, this occurs when air is stationary for a period of time over one area, and it assumes the characteristics of the surface over which it is present, known as its source region. Air masses are classified according to the source region they originated from:
- Arctic (A): very cold air
- Polar (P): cold air
- Tropical (T): warm air
- Maritime (m): moist air
- Continental (c): dry air
There are five major air masses which affect the British Isles. These are dominant at different times of the year and bring different types of weather. When an air mass moves towards the British Isles, its characteristics will be modified. If it moves from a colder area, it will warm up at its base and become unstable, rising and causing precipitation. If it cools down as it moves towards the UK, it will become more stable and not rise, yet it may increase its humidity which may cause condensation.
An air masses is a large volume of air which has similar temperature and humidity characteristics. Generally, this occurs when air is stationary for a period of time over one area, and it assumes the characteristics of the surface over which it is present, known as its source region. Air masses are classified according to the source region they originated from:
- Arctic (A): very cold air
- Polar (P): cold air
- Tropical (T): warm air
- Maritime (m): moist air
- Continental (c): dry air
There are five major air masses which affect the British Isles. These are dominant at different times of the year and bring different types of weather. When an air mass moves towards the British Isles, its characteristics will be modified. If it moves from a colder area, it will warm up at its base and become unstable, rising and causing precipitation. If it cools down as it moves towards the UK, it will become more stable and not rise, yet it may increase its humidity which may cause condensation.
Arctic Maritime (Am)
Source region: Arctic Ocean
Stability: Unstable
Direction of approach: North
Winter weather: Very cold conditions, strong northerly winds with frequent wintry showers
Summer weather: Rare in summer, but when does occur, brings heavy showers or thunderstorms at low temperatures
Source region: Arctic Ocean
Stability: Unstable
Direction of approach: North
Winter weather: Very cold conditions, strong northerly winds with frequent wintry showers
Summer weather: Rare in summer, but when does occur, brings heavy showers or thunderstorms at low temperatures
Polar Maritime (Pm)
Source region: Northern Atlantic Ocean
Stability: Unstable
Direction of approach: Northwest
Winter weather: Pm air is warmed as it crosses the Atlantic. This results in instability, causing rain showers which are most frequent in the west. Average temperatures, clear skies at night which can cause frost and fog.
Summer weather: Cool northwesterly winds. Some rain showers in the west, heavy convectional showers over Eastern England, maybe with hail and thunder due to instability.
This is the most common air mass affecting the British Isles.
Source region: Northern Atlantic Ocean
Stability: Unstable
Direction of approach: Northwest
Winter weather: Pm air is warmed as it crosses the Atlantic. This results in instability, causing rain showers which are most frequent in the west. Average temperatures, clear skies at night which can cause frost and fog.
Summer weather: Cool northwesterly winds. Some rain showers in the west, heavy convectional showers over Eastern England, maybe with hail and thunder due to instability.
This is the most common air mass affecting the British Isles.
Polar Continental (Pc)
Source region: Eastern Europe and Siberia
Stability: Unstable in winter, stable in summer
Direction of approach: East
Winter weather: Very cold in source region but warms as it moves west over the North Sea (in winter the North Sea temperatures range from 6-10 degrees) and this causes instability. Wintry showers occur, especially in Eastern Britain and around North Sea coasts. Low daytime temperatures everywhere
Summer weather: Pc originates in a warm continent in summer and is cooled as it progresses west. The cool North Sea stabilises Pc. Inland Britain Pc gives clear skies, long hours of sunshine and temperatures of 25 degrees or more
Source region: Eastern Europe and Siberia
Stability: Unstable in winter, stable in summer
Direction of approach: East
Winter weather: Very cold in source region but warms as it moves west over the North Sea (in winter the North Sea temperatures range from 6-10 degrees) and this causes instability. Wintry showers occur, especially in Eastern Britain and around North Sea coasts. Low daytime temperatures everywhere
Summer weather: Pc originates in a warm continent in summer and is cooled as it progresses west. The cool North Sea stabilises Pc. Inland Britain Pc gives clear skies, long hours of sunshine and temperatures of 25 degrees or more
Tropical Maritime (Tm)
Source region: Azores
Stability: Stable
Direction of approach: South west
Winter weather: TM cools and becomes more humid as it moves northeast across the Atlantic Ocean. It brings overcast but mild conditions to the UK. There may be some drizzle. Temperatures may reach 15 degrees in southern England
Summer weather: Dry, sunny conditions, temperatures in the mid-20s
Source region: Azores
Stability: Stable
Direction of approach: South west
Winter weather: TM cools and becomes more humid as it moves northeast across the Atlantic Ocean. It brings overcast but mild conditions to the UK. There may be some drizzle. Temperatures may reach 15 degrees in southern England
Summer weather: Dry, sunny conditions, temperatures in the mid-20s
Tropical Continental (Tc)
Source region: North Africa
Stability: Stable
Direction of approach: South
Winter weather: Does not affect weather in the British Isles in winter
Summer weather: A dry, hot air mass. Its low humidity results in clear skies, maximum sunshine and heatwave conditions (30 degrees +). Can be hazy due to dust and smoke from industrial areas in Europe.
Source region: North Africa
Stability: Stable
Direction of approach: South
Winter weather: Does not affect weather in the British Isles in winter
Summer weather: A dry, hot air mass. Its low humidity results in clear skies, maximum sunshine and heatwave conditions (30 degrees +). Can be hazy due to dust and smoke from industrial areas in Europe.
UK Weather Systems
Air masses are a key factor in the variable climate of the UK, yet there are also other important factors to consider, such as the weather systems. The UK is affected by two weather systems, anticyclones and depressions, and dominated by the latter. These bring rain and unsettled conditions through the year.
Air masses are a key factor in the variable climate of the UK, yet there are also other important factors to consider, such as the weather systems. The UK is affected by two weather systems, anticyclones and depressions, and dominated by the latter. These bring rain and unsettled conditions through the year.
Depressions
Depressions are mid-latitude low pressure systems that are predominant in the UK, affecting the country throughout the year. A depression often occurs as a polar front boundary between cold and warm air, for example Tm and Pm.
origin, nature, weather associated
Depressions are mid-latitude low pressure systems that are predominant in the UK, affecting the country throughout the year. A depression often occurs as a polar front boundary between cold and warm air, for example Tm and Pm.
origin, nature, weather associated
Anticyclones
Anticyclones are areas of high pressure of descending or stable air, and they result in more stable weather conditions. Winds are gentle and blow clockwise, skies are clear and temperatures are high in summer months. In the UK they can act to 'block out' other weather systems such as depressions.
If an anticyclone occurs in summer, it will result in much of Europe experiencing a heatwave. It will be fine, sunny and hot over the UK. Land and sea breezes may occur.
If an anticyclone occurs in winter, the continental air will result in very cold weather over most of central and northern Europe. It will be dry and fine over the UK. Clear skies at night will mean frosts and clear mornings.
Anticyclones are areas of high pressure of descending or stable air, and they result in more stable weather conditions. Winds are gentle and blow clockwise, skies are clear and temperatures are high in summer months. In the UK they can act to 'block out' other weather systems such as depressions.
If an anticyclone occurs in summer, it will result in much of Europe experiencing a heatwave. It will be fine, sunny and hot over the UK. Land and sea breezes may occur.
If an anticyclone occurs in winter, the continental air will result in very cold weather over most of central and northern Europe. It will be dry and fine over the UK. Clear skies at night will mean frosts and clear mornings.
Climate Variations across the UK
The British Isles have a temperate rainy climate. Precipitation occurs all year round, and for areas at sea level the warmest month above 10 degrees and the coldest is above freezing. However, the climate across the UK is not uniform, and factors such as temperature, precipitation and wind vary across the country.
The British Isles have a temperate rainy climate. Precipitation occurs all year round, and for areas at sea level the warmest month above 10 degrees and the coldest is above freezing. However, the climate across the UK is not uniform, and factors such as temperature, precipitation and wind vary across the country.
Temperature
It is warmer in the south of the UK than the north because of the impact latitude has on temperature, and the South receiving more hours of daylight and more insolation than the North. Temperatures also tend to be warmer in the west than the east, as the west is warmed by the Gulf Stream and the tropical maritime air masses, whilst the east gets a cold sea current from the Atlantic Ocean.
It is warmer in the south of the UK than the north because of the impact latitude has on temperature, and the South receiving more hours of daylight and more insolation than the North. Temperatures also tend to be warmer in the west than the east, as the west is warmed by the Gulf Stream and the tropical maritime air masses, whilst the east gets a cold sea current from the Atlantic Ocean.
Precipitation
It is wetter in the West and the uplands of Britain. This is because the prevailing (dominant) south-westerly winds bring warm, wet air from the Atlantic Ocean (Tropical Maritime air mass). As this air mass is forced upwards over the land, it cools and condenses into rain
It is wetter in the West and the uplands of Britain. This is because the prevailing (dominant) south-westerly winds bring warm, wet air from the Atlantic Ocean (Tropical Maritime air mass). As this air mass is forced upwards over the land, it cools and condenses into rain
Wind
Winds are strongest in the west of the UK, as it is exposed to the winds from across the wide flat expanse of the Atlantic Ocean.
Winds are strongest in the west of the UK, as it is exposed to the winds from across the wide flat expanse of the Atlantic Ocean.
Never a dull weather day in the British Isles!