Depressions!Depressions!(Cyclones)(Cyclones)

Depressions – Cyclones - LowsDepressions – Cyclones - Lows Depressions over the British Isles occur Depressions over the British Isles occur

frequently, given the location of the frequently, given the location of the Polar Polar Front Jet StreamFront Jet Stream and meeting point of and meeting point of different different air massesair masses..

Depressions give rise to Depressions give rise to unsettledunsettled, wet and , wet and windy weather.windy weather.

They tend to be short lived features, They tend to be short lived features, moved over the UK by wind within a few moved over the UK by wind within a few hours. However, they may travel in hours. However, they may travel in ‘‘familiesfamilies’’ which gives rise to a longer which gives rise to a longer period of unsettled weather.period of unsettled weather.

Depressions are characterised by:Depressions are characterised by: Low pressure reading in the middle.Low pressure reading in the middle. Close isobars = strong wind.Close isobars = strong wind. Air moving toward the centre of the low.Air moving toward the centre of the low. Air spins anticlockwise around the low.Air spins anticlockwise around the low. They normal travel W -> E over the UK, They normal travel W -> E over the UK,

due to the prevailing wind direction.due to the prevailing wind direction. Low pressure systems form commonly Low pressure systems form commonly

over Iceland – over Iceland – ‘‘Icelandic LowIcelandic Low’’..

Frontal Rainfall ClipFrontal Rainfall Clip

Surface Pressure ChartSurface Pressure Chart

These diagrams show the wind moving in towards the low pressure centre. Notice the air then lifts, creating cloud and rain.

Warm wind

Cold wind

Warm air from the south meets the colder air from the North

Polar Front

Early Stage

Warm wind

Cold wind

Warm air rises over the cold air – creating the warm front

The cold air undercuts the warm air, creating a steeper cold front.

Open Stage

Warm wind

Cold wind

The cold front travels faster, eventually catching up the warm front – forming an occlusion.

Occlusion – the cold front over rides the warm front.

Occluded Stage

Cold wind

The occlusion is fully closed – therefore dissolved.

Dissolved Stage

Depressions

Occluded fronts are produced when a fast Occluded fronts are produced when a fast moving cold front catches and overtakes a moving cold front catches and overtakes a slower moving warm frontslower moving warm front

Occluded fronts

Below is an example of a weather map showing a warm Below is an example of a weather map showing a warm and a cold front for the British Isles.and a cold front for the British Isles.

Describe what the weather will be like in East Anglia as the warm front and cold front pass overhead

1. 9am

2. 11am (warm front overhead)

3. 3pm (cold front overhead)

Weather forecastingWeather forecasting

ForecastForecast