What is occlusion, and why does it happen?

Occlusion is the stage at which the cold front catches up with the warm front and lifts the warm sector completely off the ground. It happens because the cold front travels roughly one and a half times faster than the warm front, since the dense cold air behind it is undercutting the warm air rather than having to climb over a wedge of it. The catching-up begins at the centre of the depression and works outwards along the fronts, and the combined boundary is called an occluded front, with the warm air now trapped aloft.

Distinguish between a cold occlusion and a warm occlusion.

The distinction depends on which of the two cold air masses is colder. In a cold occlusion the air behind the cold front is colder than the air ahead of the warm front, so it undercuts both and pushes the older cold air upwards as well as the warm air; this gives cold-front-type weather with cumulonimbus and heavy showery rain. In a warm occlusion the air behind the cold front is less cold than the air ahead of the warm front, so it rides up over it; this gives warm-front-type weather with layered cloud and steadier, more prolonged rain. Warm occlusions are the more common over the British Isles in winter, because the Atlantic air arriving behind the cold front is milder than the cold continental air ahead; cold occlusions are more typical of summer, when that relationship is reversed.

Describe the final stage in the life cycle of a depression.

In the dissipating or decaying stage the whole warm sector has been lifted clear of the ground, so warm and cold air no longer lie side by side at the surface and the temperature contrast that powered the system has gone. Without that energy source the uplift weakens, air begins to subside into the centre, pressure rises, the isobars open out, winds slacken and the cloud thins and breaks. The remaining occluded front gradually becomes weaker and the system fills and fades away over a day or two.

CCEA A-Level Geography 3910

Processes That Shape Fluvial Environments

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Human Interaction in Fluvial Environments

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Topic overview The global distribution of biomes The tundra biome: climate and soils The temperate grassland biome: climate and soils Climate change and tundra ecosystems

Small-Scale Ecosystems

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The Processes That Shape Our Weather and Climate

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Weather in the British Isles

Topic overview How precipitation forms Air masses affecting the British Isles The polar front and the birth of a depression The life cycle of a depression, and its fronts The sequence of weather as a depression passes Anticyclones in summer and winter Isobars, wind, and the front symbols on a synoptic chart Station circles, recognising systems, and satellite images

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Topic overview The Pacific in a normal year: the Walker circulation El Niño events and their global consequences La Niña events and their global consequences How tropical cyclones form The structure of a tropical cyclone The impacts of a tropical cyclone on people and property Managing tropical cyclones: prediction, preparation and response

Population Data

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Population Change

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Population and Resources

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Coastal Management and SustainabilityCoastal

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Current Global Climate Change: Human Causes and ImpactsClimate

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Managing Global Climate ChangeClimate

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Cultural Groups, Social Inequality and LandscapeCultural

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The Geographies of CyberspaceCultural

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Urban Design, Planning and Management for SustainabilityPlanning

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Traffic and TransportPlanning

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Defining EthnicityEthnic

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Ethnic ConflictEthnic

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Challenges and Management of Mass TourismTourism

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Decision Making in Geography

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Weather in the British Isles

The life cycle of a depression, and its fronts

3 things to know
Unit AS 1

Overview

First stage in the life cycle of a depression, mature stage of a depression and cold occlusion and a warm occlusion.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is a depression, and what are its typical size, central pressure and speed of movement?

Process

Describe the first stage in the life cycle of a depression.

Process

Describe the mature stage of a depression.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a depression, and what are its typical size, central pressure and speed of movement?
Model answer
A depression is an area of low atmospheric pressure in the mid-latitudes, formed on the polar front, in which air converges and rises. It is typically 1000 to 2000 kilometres across, has a central pressure usually between about 950 and 1000 millibars, and travels from west to east at around 30 to 50 kilometres per hour, so it takes a day or two to cross the British Isles. The deeper the central pressure relative to its surroundings, the steeper the pressure gradient and the stronger the winds.
2
Describe the first stage in the life cycle of a depression.
Model answer
In the embryo or incipient stage, warm tropical maritime air and cold polar maritime air are moving in opposite directions alongside each other on the polar front. A small kink or wave develops where the warm air bulges polewards, encouraged by divergence in the jet stream above. Pressure begins to fall at the tip of the wave, a small area of low pressure appears, and the front separates into a warm front ahead of the warm air and a cold front behind it. Cloud and light precipitation begin to develop along the fronts.
3
Describe the mature stage of a depression.
Model answer
The wave amplifies into an open, well-developed low. Central pressure falls further, the isobars close into a series of concentric rings, and winds strengthen and blow anticlockwise and inwards around the centre in the Northern Hemisphere. The warm front runs ahead of the centre and the cold front behind it, and between them lies the warm sector, a wedge of mild moist tropical maritime air at the surface. Warm air rises along both fronts and over the top of the system, producing an extensive band of cloud and precipitation, and this is when the depression brings its heaviest rain and strongest winds.
4
What is occlusion, and why does it happen?
Model answer
Occlusion is the stage at which the cold front catches up with the warm front and lifts the warm sector completely off the ground. It happens because the cold front travels roughly one and a half times faster than the warm front, since the dense cold air behind it is undercutting the warm air rather than having to climb over a wedge of it. The catching-up begins at the centre of the depression and works outwards along the fronts, and the combined boundary is called an occluded front, with the warm air now trapped aloft.
5
Distinguish between a cold occlusion and a warm occlusion.
Model answer
The distinction depends on which of the two cold air masses is colder. In a cold occlusion the air behind the cold front is colder than the air ahead of the warm front, so it undercuts both and pushes the older cold air upwards as well as the warm air; this gives cold-front-type weather with cumulonimbus and heavy showery rain. In a warm occlusion the air behind the cold front is less cold than the air ahead of the warm front, so it rides up over it; this gives warm-front-type weather with layered cloud and steadier, more prolonged rain. Warm occlusions are the more common over the British Isles in winter, because the Atlantic air arriving behind the cold front is milder than the cold continental air ahead; cold occlusions are more typical of summer, when that relationship is reversed.
6
Describe the final stage in the life cycle of a depression.
Model answer
In the dissipating or decaying stage the whole warm sector has been lifted clear of the ground, so warm and cold air no longer lie side by side at the surface and the temperature contrast that powered the system has gone. Without that energy source the uplift weakens, air begins to subside into the centre, pressure rises, the isobars open out, winds slacken and the cloud thins and breaks. The remaining occluded front gradually becomes weaker and the system fills and fades away over a day or two.

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