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weather [fronts]

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Wind Direction Arrows

What determines the weather in the UK?
In general, the weather is strongly influenced by the wind direction, so information about the wind provides an indication of the type of weather likely to be experienced. Northerly winds tend to bring relatively cold air from polar regions to the British Isles.
  
  The position of the jet stream over the UK determines the type of weather we experience. If the polar front jet is situated significantly to the south of the UK we will experience colder than average weather. If the polar front jet is situated to the north of the UK we will experience warmer than average weather.

The position of the jet stream over the UK determines the type of weather we experience.
If the polar front jet is situated significantly to the south of the UK we will experience colder than average weather.
If the polar front jet is situated to the north of the UK we will experience warmer than average weather.
If the polar front jet is situated over the UK we will experience wetter and windier than average weather.
If the polar front jet has a large amplification then cold air will travel further south than average and warm air will travel further north than average.
The direction and angle of the jet stream arriving at the UK will determine what source of air (i.e. cold, dry, warm, wet, from maritime or continental sources) the UK experiences.
  
 Location
The UK lies in the 'battleground' between warm tropical air to the south and cold polar air to the north. As two distinctly different types of air battle for control over the mid-latitudes, the UK experiences contrasting and changeable weather.

The warm North Atlantic Drift significantly impacts the UK's climate. It carries warm water from the South Atlantic to the western shores of the UK. The prevailing south-westerly winds then spread these warmer conditions, giving the western parts of the country mild winters.

This diagram shows how the origin and journey of the air mass can affect the climate of the UK. For example, the polar maritime air mass originates in the poles and travels over the Arctic sea, therefore it brings cold and wet weather. The tropical continental air mass originates in the tropics and travels over land bringing hot and dry weather to the UK.
  
  Why is the weather in the UK so unpredictable?
Tropical Maritime air masses are formed when hot air from the tropics moves over water. This air mass is hot and moist, and it can bring cloudy skies and rain or thunderstorms to the UK. The UK's weather is constantly changing as these air masses interact with each other.

Air masses

Air masses are areas of air that bring distinctive weather features to the country. 
 An air mass is a body or 'mass' of air that have very little change temperature and humidity within them. 
 That is to say the air making up the mass is very uniform in temperature and humidity.

An air mass is separated from another body of air by a weather front. 
 An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.

High pressure (or anticyclone)

In an anticyclone (also referred to as a 'high') the winds tend to be light and blow in a clockwise direction. 
 Also the air is descending, which stops the formation of clouds. 
 The light winds and clear skies can lead to overnight fog or frost. 
 If an anticyclone persists over northern Europe in winter, then much of the British Isles can be affected by very cold east winds from Siberia. 
 However, in summer an anticyclone in the vicinity of the British Isles often brings fine, warm weather.

Low pressure (or depression) 
In a depression (also referred to as a 'low'), air is rising. 
 As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. 
 Consequently, the weather in a depression is often cloudy, wet and windy (with winds blowing in an anticlockwise direction around the depression). 
 There are usually frontal systems associated with depressions.

Weather fronts

A weather front is simply the boundary between two air masses.

There are three different types of weather front

1. Cold front 
This is the boundary between warm air and cold air and is where cold air is replacing warm air at a point on the Earth's surface.

On a weather chart) a cold front is a blue line with blue triangles on it.

The presence of a cold front means cold air is moving in and pushing underneath warmer air. 
 This is because the cold air is 'heavier' or denser, than the warmer air so sinks below it. 
 So cold air is replacing warm air at the surface. 
 The triangles on the front point to the direction that the front is moving.

You can tell when a cold front has passed by a rise of pressure, a fall of temperature and dew point, and a change in wind direction. 
 Rain occurs with most cold fronts and may extend some 100 to 200 km ahead of or behind the front. 
 Some cold fronts give only a shower at the front, while others give no precipitation. 
 Thunder may occur at a cold front.

2. Warm front 
This is the boundary between cold air and warm air and is when warm air replaces cold air at a point on the Earth's surface.

On a synoptic chart a warm front is a red line with red semi-circles on it.

The presence of a warm front means warm air is moving in and rising up over cold air. 
 This is because the warm air is 'lighter' or less dense, than the colder air. 
 So warm air is replacing cold air at the surface. 
 The semi-circles on the front point to which direction the front is moving.

As a warm front approaches, temperature and dew-point within the cold air gradually rise and pressure falls more quickly. Precipitation usually occurs in a wide belt about 400 km just ahead of the front. 
 When the front has passed, the will be a steadying of the barometer (pressure will stop falling), a jump in temperature and dew point, a change of wind direction, and precipitation will stop, or mostly stop.

3. Occluded front 
These are more complex than cold or warm fronts. 
 An occlusion is formed when a cold front catches up with a warm front. 
 When a cold front catches up with a warm front, the warm air in the warm sector is forced up from the surface.

On a synoptic chart an occluded front is a purple line with purple semi-circles and triangles.

Not all occlusions follow the same rules, as some will be more similar to warm fronts and some will be more like cold fronts, so some will cause it to feel warmer once it has passed, others cooler, some will lower the dew point, others will cause it to increase. 
 All occlusions will cause a rise in pressure, a change in wind direction and precipitation will slowly clear when they have passed.

information over single pictures [template]

cold and warm front animation underline the important words
cold front/warm front/picture underline the important words	cold front/picture underline the important words	cold front animation underline the important words
cold front animation underline the important words	occluded front animation underline the important words	warm front weather chart underline the important words
simple front animation underline the important words	front/radar/animation underline the important words	warm front picture underline the important words
pressure/ fronts underline the important words	fronts/ clouds underline the important words	this simply shows how a warm front can quickly move across the UK bringing warmer temperatures. warm and cold front animation underline the important words
cold front/weather/picture underline the important words
jet stream/high and low pressure areas/picture underline the important words
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