The UK experiences a great variety of weather and distinct differences in temperature and precipitation are found in the North, South, East and West of the country. These variations are a direct result of the air masses that affect the UK.
When air remains in one area for a period of time, it assumes the characteristics of the surface over which it has been present, known as a source region. The temperature of an air mass is determined by its source region (for example, arctic, tropical) whilst its moisture content is dependent on if it has formed over land or sea. Every air mass is different and depends on:
- The climatic conditions at time of formation in source region.
- The path it then follows (does it travel over land or sea?).
- The season.
- The atmospheric characteristics.
Five major air masses affect the UK, and which is more dominant at a particular time is closely related to seasons.
When two air masses meet, a front is formed due to the differences in temperature and density. The Polar front is the most common type.
Are high-pressure areas of descending or stable air. They originate in the upper atmosphere where conditions are dry. Winds are gentle and blow clockwise, skies clear, and temperatures high in summer months. In the UK they can act to 'block out' other weather systems, such as depressions bringing fine, settled weather for several days.
|Winter anticyclone:||Clear skies, low temperatures (little Insolation) fog and frost can be common. Some snowfall on the east coast.|
|Summer anticyclone:||Clear skies, little rain, high temperatures (much insolation) fog and temperature inversions. If heating is intense enough thunderstorms can result.|
These are one of the most common weather systems to affect the UK. They last for 2-3 days, cross the UK from west to east. They are areas of low pressure, bringing strong winds, cloud and rain.
Stages in a depression
The information in the table below corresponds to the three subsequent diagrams:
|1||Embryo||Starts as 'wave' on the Polar front, where moist, warm Tm air meets drier, colder more stable Pm air. The warm, moist air is forced aloft, by the colder denser air, creating low pressure.|
|2||Mature||Pressure falls further and the distinctive cold and warm sector and warm front are clearly recognisable. Winds become stronger, clouds develop, and precipitation occurs. Rainfall is steady at the warm front where warm air rises slowly over cold air. It is heaviest at the cold front due to the rapid rising of the air.|
|3||Decay||The faster moving cold front has caught up with the warm front reducing the warm sector and creating an occluded front. Condensation is reduced as smaller amounts of air are forced aloft.|
Weather associated with the passage of a depression
The weather conditions associated with a depression are strongly linked to the fronts and are outlined below. You can click your mouse on the picture and move it left and right to see different parts of the diagram:
Note: Scale - height can be 10-12km; ground area can be up to 2000km; vertical scale is much exaggerated.
|Precipitation and clouds:||Temperature:||Wind (speed and strength):||Visibility:|
|Post cold front:||Showers, cumulus clouds clearing.||Low 3-4 degrees.||Steady increase to force 6 north-westerly.||Good.|
|With passing of cold front:||Heavy showers, thick, tall cumulonimbus clouds.||Falls further.||Strong, up to force 8, north-westerly.||Limited.|
|Warm sector:||Little rain patchy stratus clouds.||Rise, mild 12 degrees.||Decrease to force 3 westerly.||Limited.|
|Arrival of warm front:||Continuous rain, nimbostratus clouds.||Slight fall.||South westerly, force 3/4.||Worsens.|
|Approach of warm front:||Light drizzle, altostratus clouds.||6 degrees.||Southerly, strong winds.||Good/fair.|
|Far in advance of depression:||No rain, cirrus and cirrostratus clouds.||6 degrees.||Calm, NE direction prevalent.||Good.|