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Depressions are areas of low pressure:

• they move from west to east in the northern hemisphere

Depressions are characterised by fronts:

• warm front - warm air rises over cold air bringing steady, continuous rain
• cold front - cold air forces warm air up bringing heavy rain showers
• an occluded frontcould form - cold front catches up with warm front bringing sudden downpours

Anticyclones are areas of high pressure:

• skies are clear and the weather is usually dry
• winds are gentle
• summer - hots day but cold nights
• winter - very cold

In this video, revise the characteristics of weather associated with depressions and anticyclones.

A depression is a low pressure area which moves from west to east in the northern hemisphere.

There are usually steep changes in pressure which cause high winds.

Depressions often feature fronts where different air masses meet.

Low pressure systems can be identified from a synoptic chart due to:

• warm fronts - the red line with semicircles
• cold fronts - the blue line with triangles
• possible occluded fronts - the purple line with alternating semicircles and triangles
• tightly packed isobars
• isobars showing pressure decreasing towards the centre
• pressure will tend to be under 1000 mb

Warm front

• In a low pressure system the warm front is the first to pass over.
• This occurs when warm air meets cold air and the warm air rises above it.
• Warm fronts bring steady continuous rain.

Cold front

• The next front to pass over is the cold front.
• This occurs when cold air meets warm air.
• The cold air pushes the warm air upwards.
• Cold fronts bring heavy rain showers.

Occluded front

• When the cold front catches up with the warm front the result is an occluded front.
• Occluded fronts bring sudden downpours of heavy rain.

Where isobars are close together the wind is greatest. This is because of a rapid change in air pressure.

• wind - winds blow anticlockwise in a depression and wind blows along the isobars. You can work out the wind direction by following the isobars in an anticlockwise direction.
• wet - where warm air meets cold air, the warm air is pushed upwards where it cools, condenses and precipitates (usually as rain). A front is a band of cloud and clouds bring rain.
• temperature - in general, the warm sector behind the warm front brings warmer temperatures and the cold sector behind the cold front brings cooler temperatures.

As a depression passes over the following changes occur.

(read the graphic from right (east) to left (west)

The table below explains the changes as a depression passes over.

(read the table from right to left)

• An anticyclone is a high pressure area which brings long periods of settled weather.
• An area of heavy cool air sinks and it warms up as it does so.
• As it warms it is able to hold more moisture.
• This means clouds do not form.

High pressure systems can be identified from a synoptic chart due to:

• widely spaced isobars
• isobars showing pressure decreasing from a high pressure centre
• pressure above 1008mb would tend to suggest an anticyclone
• no fronts/clouds

Isobars are spread far apart bringing gentle winds. This is caused by a very gradual change in air pressure. Because of the gentle winds these systems can remain in place for several days.

• wind - winds blow clockwise in high pressure and wind blows along the isobars. Wind blows gently when isobars are widely spaced.
• temperature - clear skies mean that summer temperatures are high, while winter temperatures are low

In the UK in summer, anticyclones bring warm, dry weather and can cause heat waves.

• lack of cloud means there is no cloud or precipitation
• during the day, cloudless conditions result in sunshine which warms the ground and air above it
• lack of cloud at night means heat is lost quickly and temperatures drop
• cooling at ground level can cause condensation causing dew and mist
• lack of wind means anticyclones can stay in position for long periods
• heat can build up over several days causing a heatwave
• eventually a layer of hot air can build up - this can rise and bring thunderstorms

In the UK in winter, anticyclones bring cold, dry weather and can cause frost and fog.

• lack of cloud means there is no cloud or precipitation
• during the short days, cloudless conditions result in sunshine but it is weak and has a limited warming effect
• lack of cloud during long nights means any heat is lost quickly and temperatures drop significantly
• very cold air at ground level can hold little moisture causing heavy dew, front and fog
• lack of wind and weak sunlight means frost and fog can be slow to clear

Depressions are areas of low pressure.

They move from west to east.

Depressions are characterised by fronts:

• warm front - warm air rises over cold air bringing steady, continuous rain
• cold front - cold air forces warm air up bringing heavy rain showers
• sometimes an occluded front forms - cold front catches up with warm front bringing sudden downpours

Anticyclones are areas of high pressure

• skies are clear and the weather is usually dry
• winds are gentle
• summer - hot during the day but cold at night
• winter - very cold