STANDARD GRADE DEPRESSIONS John Smith Invergordon Academy

STANDARD GRADEDEPRESSIONS

John Smith Invergordon Academy

LOW PRESSURE SYSTEMS

How depressions form

What happens at the warm front?

What happens at the cold front?

What happens in the warm sector?

What happens at the occluded front?

Imagine an area out in the North Atlantic Ocean, to the West of the UK where the air is fairly cool.

Cold air will be shown in darker blue.

Warm air will be shown in red.

This cool air is represented by the light blue background on the following slides.

LOW PRESSURE SYSTEMS

1. Cold Arctic air blowing from the North East

2. Warm moist Tropical air blowing from the South West

Imagine you are looking down on a large area of the North Atlantic Ocean. The air is quite cool (light blue).

THE AIRMASSES OVER THE ATLANTIC

In this area, different types of air masses meet. We will look at two of these.

Cold Arctic air blows from the North East

Warm, moist Tropical air blows from the South West

THE AIRMASSES MEET……

The two sets of air do not mix together

As they blow past each other, friction causes them to swirl round

……BUT THEY DO NOT MIX

This means that we now have a cool area (shown by the light blue background)….…with a wedge of very cold Arctic air (shown by the darker blue triangle)….

There is also a wedge of warm moist Tropical air.

(red triangle)...

Warm, moist Tropical air

Cold Arctic air

…this is called the COLD SECTOR.

…this is called the WARM SECTOR.

WARM AND COLD SECTORS

These systems usually move either East, or North East


Cold Arctic air


Cold Arctic air


Cold Arctic air


Cold Arctic air


Cold Arctic air


Cold Arctic air

DIRECTION OF MOVEMENT

The line between the cooler air and the warm sector is called the WARM FRONT.

The line between the warm sector and the cold sector is called the COLD FRONT.

Cool air


Cold Arctic air

WARM AND COLD FRONTS

Dense cool air

A SIMPLE CROSS SECTION THROUGH A DEPRESSION

COLD SECTOR

(cold, dense air) W A R M F R O N T

C O L D F

R O N T

THE WARM SECTOR

(warm, moist air)


Cold Arctic air X

What conditions would be felt at X?

a) Cool

b) Cold

c) Warm

CONDITIONS AT STATION X (1)


Cold Arctic air

What has passed over X?

a) Warm Front

b) Western Front

c) Cold Front


a) Cool

b) Cold

c) Warm

From which way are the winds blowing at X?

a) South East

b) North West

c) South West


Cold Arctic air


Cold Arctic air


Cold Arctic air

X


What has passed over X?

a) Warm Front

b) Western Front

c) Cold Front


Cold Arctic air

From which way are the winds blowing at X?

a) North East

b) North West

c) South West


a) Cool

b) Cold

c) Warm Warm, moist Tropical air

Cold Arctic air


Cold Arctic air


Cold Arctic air

X


WHAT HAVE YOU LEARNED SO FAR?

Warm moist Tropical winds blow from the South West

Cold Arctic winds blow from the North East

The wedge of warm air is called the warm sector

The wedge of cold air is called the cold sector

The leading edge of the warm sector is the warm front

The leading edge of the cold sector is the cold front

Low pressure systems are also called depressions

WHAT HAPPENS AT THE WARM FRONT?

Warm air begins to rise over the cooler air

As this air rises, it begins to cool

Cool air can hold less water vapour than warm airWater vapour begins to condense into water dropletsWater droplets begin to form clouds

The first – and highest – type of cloud to form along the warm front is called Cirrus

As the warm air rises, it cools.

Dense cool air

Warm air is forced to rise over denser, cool air.

Water vapour condenses and forms clouds

These high-level wispy clouds are called CIRRUS

AIR RISING ALONG THE WARM FRONT (1)

CIRRUS CLOUDS

Dense cool air

More moist air rises and cools

Clouds form lower down, and give prolonged rain

CIRRUS

PRECIPITATION AT THE WARM FRONT

These are CUMULUS

CUMULUS CLOUDS

Dense cool air

More moist air rises over the cooler air. As it does so it cools.

CIRRUS are very high and wispy. They are usually the first clouds we see as the warm front approaches

THE WARM FRONT - SUMMARY

CUMULUS clouds bring prolonged rain at the warm front

Warm

Front

AN APPROACHING WARM FRONT

CIRRUS

CUMULUS

WHAT HAPPENS AT THE COLD FRONT?

The cold Arctic air moves faster than the warm air.

The cold Arctic air is denser than the warm air.

The cold Arctic air undercuts the warm air, forcing it up. Water vapour begins to condense into water droplets

Water droplets begin to form very tall clouds

The clouds along the cold front are called Cumulonimbus.

THE COLD FRONT

The cold front rapidly undercuts the air in the warm sector, making it rise very quickly.

The Warm Sector(warm, moist air)

The Cold Sector( cold dense air )

Very tall clouds are formed by the rapidly rising air.

These are called CUMULONIMBUS.

The Warm Sector(warm, moist air)

CUMULONIMBUS give very heavy showers, sometimes with thunder and lightning.

PRECIPITATION AT THE COLD FRONT

CUMULONIMBUS CLOUDS

IN THE WARM SECTOR

Warm, moist winds blow from the South WestAir is forced to rise over cooler air

Condensation occurs, and forms Stratus clouds

Showers are common

The sky is very overcast

Dense cool air

THE WARM SECTOR

Warm front

Warm, moist air in the warm sector is rising.

Cold Sectorcold dense air

Dense cool air

CLOUD & PRECIPITATION IN THE WARM SECTOR

W A R M F R O N T

Warm, moist air in the warm sector is rising.

The clouds formed here are mostly Stratus

Stratus are flat layer clouds. They give showers.

STRATUS


CLOUD & PRECIPITATION – SUMMARY 1

Dense cool air

STRATUS

CIRRUSCUMULUS

CUMULONIMBUS

W A R M F R O N T

PERSISTENT RAINLIGHT SHOWERSHEAVY SHOWERS


A SATELLITE IMAGE OF A DEPRESSION

CLOUD

PRECIPITATION

TEMPERATURE

WIND DIRECTION

AIR PRESSURE

Cumulonimbus

Stratus Cumulus Cirrus

Heavy showers

Showers Prolonged rain

Cold Warm

N.W. or N S.W. E or N.E.

CROSS SECTION THROUGH A DEPRESSION

FallingLowRising

Cool

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STANDARD GRADE DEPRESSIONS John Smith Invergordon Academy