STANDARD GRADEDEPRESSIONS & ANTICYCLONES

John Smith Invergordon Academy

CONTENTSAtmospheric pressure

Low pressure (Depressions)High Pressure (Anticyclones)Weather Maps

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There are two types of weather systems:

These systems affect the weather we receive from day to day.

INTRODUCTION TO ATMOSPHERIC PRESSURE

They are caused by differences in atmospheric pressure

• Low pressure systems

• High pressure systems

WHAT DO WE MEAN BY AIR PRESSURE?

The earth’s atmosphere is made up of many gases, eg

Atmospheric pressure is the weight of these gases pressing down on the surface of the earth.

If we could take a column of air covering 1 square centimetre, from sea level to the outer edge of the atmosphere, it would weigh 1 kilogram.

oxygen

nitrogen carbon dioxide

The Earth’s surface

The Earth’s atmosphere presses down on the surface of the Earth.

ATMOSPHERIC PRESSURE

CHANGES IN ATMOSPHERIC PRESSURE

Atmospheric pressure is the weight of gases pressing down on the surface of the earth.Atmospheric pressure is measured in millibars (mb). Average atmospheric pressure is 1000mb.

Sometimes atmospheric pressure is higher than average. We call this high pressure.

Sometimes atmospheric pressure is lower than average. We call this low pressure.

WHY DOES ATMOSPHERIC PRESSURE CHANGE? 1

If air is heated it rises away from the Earth’s surface.

Rising air reduces the weight of air pressing down on the Earth’s surface.

This means that air pressure is low.

The Earth’s surfaceis warmed by the sun’s

rays.

The Earth’s atmosphere presses down on the surface of the Earth.

LOW ATMOSPHERIC PRESSURE

Warm air rises. This reduces the weight of air pressing down on the Earth’s surface.

WHY DOES ATMOSPHERIC PRESSURE CHANGE? 2

When air is cold, high up in the atmosphere it falls towards the earth’s surface.

Falling air increases the weight of air pressing down on the Earth’s surface.

This means that air pressure is high.

The Earth’s surface

The Earth’s atmosphere presses down on the surface of the Earth.

HIGH ATMOSPHERIC PRESSURE

Cold, dense air falls. This increases the weight of the air pressing down on the Earth’s surface.

The Earth’s surfaceis warmed by the sun’s

rays.

HIGH AND LOW PRESSURE TOGETHER

Warm air rising causes LOW pressure.

Cold air falling causes HIGH pressure.WIND

Low pressure systems are also known as depressions.

PRESSURE SYSTEMS

High pressure systems are also known as anticyclones.

The aim of this presentation is to help you understand how these pressure systems affect our weather.

Click on the buttons on the next page to carry on.

LOW PRESSURE SYSTEMS

How depressions form

What happens at the warm front?

What happens at the cold front?

Main Menu

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

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical 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

Warm, moist Tropical 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)

Warm, moist Tropical air

Cold Arctic air X

What conditions would be felt at X?

a) Cool

b) Cold

c) Warm

CONDITIONS AT STATION X (1)

Warm, moist Tropical air

Cold Arctic air

What has passed over X?

a) Warm Front

b) Western Front

c) Cold Front

What conditions would be felt at X?

a) Cool

b) Cold

c) Warm

From which way are the winds blowing at X?

a) South East

b) North West

c) South West

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

X

CONDITIONS AT STATION X (2)

What has passed over X?

a) Warm Front

b) Western Front

c) Cold Front

Warm, moist Tropical air

Cold Arctic air

From which way are the winds blowing at X?

a) North East

b) North West

c) South West

What conditions would be felt at X?

a) Cool

b) Cold

c) Warm Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

Warm, moist Tropical air

Cold Arctic air

X

CONDITIONS AT STATION X (3)

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

Cold Sectorcold dense air

CLOUD & PRECIPITATION – SUMMARY 1

Dense cool air

STRATUS

CIRRUSCUMULUS

CUMULONIMBUS

W A R M F R O N T

PERSISTENT RAINLIGHT SHOWERSHEAVY SHOWERS

Cold Sectorcold dense air

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

HIGH PRESSURE SYSTEMS

High pressure systems are also known as anticyclones

The Earth’s surface

The Earth’s atmosphere presses down on the surface of the Earth.

HIGH ATMOSPHERIC PRESSURE

Cold, dense air falls. This increases the weight of the air pressing down on the Earth’s surface.

WHAT CAUSES HIGH PRESSURE?

When air high in the atmosphere is cold, it falls towards the earth’s surface.

Falling air increases the weight of air pressing down on the Earth’s surface.

This means that air pressure is high.

WHAT WEATHER DOES HIGH PRESSURE BRING?

As the cold air falls through the atmosphere, it becomes slightly warmer.

Because warm air can hold more moisture than cold air, no condensation or clouds occur.

This means that high pressure systems bring clear skies.

Winds are usually light, and blow out of the high pressure area.

There are no warm or cold fronts in high pressure areas.

HIGH PRESSURE IN WINTER

High pressure in winter gives us

clear skies

very low temperatures

calm, or light winds

no precipitation

frosty conditions

HIGH PRESSURE IN SUMMER

High pressure in summer gives us

clear skies

high temperatures

calm, or light winds

no precipitation© Microsoft Word clipart

WEATHER MAPS

Weather symbols

Low pressure Systems (Depressions)

High Pressure Systems (Anticyclones)

Main Menu

HOW IS WEATHER SHOWN ON A WEATHER MAP? (1)

TYPICAL MEDIA WEATHER MAP

Look at the symbols.

What weather does Eastern England have?

HOW IS WEATHER SHOWN ON A WEATHER MAP? (2)

Present weather

Mist Fog Thunder

Drizzle

Rain Snow Hail Shower

The shower symbol can be combined with others….

Rain shower

Hail shower

Snow shower

HOW IS WEATHER SHOWN ON A WEATHER MAP? (3)

Wind direction and wind speed

Wind direction is shown by a line leading into the station circle.

This symbol shows that the wind blew into the station circle from the east

This symbol shows that there was no wind : calm

This symbol showsA gale force wind

FRONTS ON WEATHER MAPS

Warm fronts are shown by the following symbol.

Cold fronts are shown by the following symbol.

There is a third type of front that we will study later – the OCCLUDED front. They are shown by the following symbol.

LOW PRESSURE SYSTEMS

Low pressure systems are also known as depressions

ISOBARS AROUND A DEPRESSION (1)

Isobars are lines on a weather map that join places that have the same atmospheric pressure.

The pressure indicated by the isobars decreases as you move towards the centre of a depression.

LOW

984

988

992

996

A DEPRESSION, SHOWING THE WARM & COLD FRONTS

WINDS AROUND A DEPRESSION

Winds blow inwards anticlockwise round a depression

Winds always blow from high pressure to low pressure

LOW

984

988

992

996

ISOBARS AND WIND SPEED

When isobars are close together, we will get strong winds.

The further apart the isobars are, the gentler the winds will be.

LOW

984

988

992

996

LOW

984

988

992

996

THE DEPRESSION WITH WINDS ADDED

TYPICAL SYNOPTIC CHART

You can recognise the isobars around the low pressure area.

SATELLITE IMAGE OF SAME DAY

HIGH

LOW

HIGH PRESSURE SYSTEMS

High pressure systems are also known as anticyclones

ISOBARS AROUND A HIGH PRESSURE AREA

The isobars around an area of high pressure are usually quite far apart

The pressure indicated by the isobars increases as you move towards the centre of a high pressure area.

1004

HIGH

1008

9961000

WINDS AROUND A HIGH PRESSURE SYSTEM

Winds blow outwards clockwise from high pressure

Winds always blow from high pressure to low pressure

1004

HIGH

1008

9961000

Winds blow gently, because the isobars are far apart.

A SYNOPTIC CHART

Scotland has high pressure.

Look at the large area with no isobars. What effects would this have on the weather in Invergordon?

MARCH 14 2002

CHART EXERCISES

WHAT IS AN OCCLUDED FRONT?

You have already learned that the cold front moves faster than the warm front.

The result is an occluded front. It is what happens when the cold front catches up with the warm front.

The warm sector is squeezed upwards, so that is right off the ground.

FORMATION OF AN OCCLUDED FRONT

Warm sector has been lifted off the ground…

…so has the warm frontThe Cold Sector

(cold, dense air)

C O L D F R O N T

W A R M F R O

N T

OCCLUDED FRONT

THE EFFECT OF AN OCCLUDED FRONT (1)

Dense cool air

STRATUS

CIRRUSCUMULUS

W A R M F R O N

T

PERSISTENT RAIN

LIGHT SHOWERSHEAVY SHOWERS

The Cold Sector

(cold, dense air)

C O L D F R O N

T

CUMULONIMBUS

THE EFFECT OF AN OCCLUDED FRONT (2)

The approaching warm front looks normal – cirrus and then cumulus clouds, which bring rain.The warm front does not reach the ground, it has been replaced by a section of the cold front.There are no warm southerly winds at ground level. Instead, cold northerly winds are felt behind the cold front.The warm sector, now pushed high up, cools. This is the end of the low pressure system, because there is no more warm air rising away from the Earth’s surface.