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STANDARD GRADE
DEPRESSIONS & ANTICYCLONES
John Smith Invergordon Academy
CONTENTS
Atmospheric pressure
Low pressure (Depressions)
High Pressure (Anticyclones)
Weather Maps
Exit
INTRODUCTION TO ATMOSPHERIC PRESSURE
There are two types of weather systems:
•
Low pressure systems
•
High pressure systems
These systems affect the weather we
receive from day to day.
They are caused by differences in
atmospheric pressure
WHAT DO WE MEAN BY AIR PRESSURE?
The earth’s atmosphere is made up of many gases, eg
oxygen
nitrogen
carbon dioxide
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.
ATMOSPHERIC PRESSURE
The Earth’s atmosphere
presses down on the surface
of the Earth.
The Earth’s surface
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.
LOW ATMOSPHERIC PRESSURE
The Earth’s atmosphere
presses down on the surface
of the Earth.
Warm air rises. This reduces
the weight of air pressing down
on the Earth’s surface.
The Earth’s surface
is warmed by the sun’s rays.
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.
HIGH ATMOSPHERIC PRESSURE
The Earth’s atmosphere
presses down on the surface of
the Earth.
Cold, dense air falls. This increases
the weight of the air pressing down
on the Earth’s surface.
The Earth’s surface
HIGH AND LOW PRESSURE TOGETHER
Warm air rising
causes LOW
pressure.
Cold air falling causes
HIGH pressure.
WIND
The Earth’s surface
is warmed by the sun’s rays.
PRESSURE SYSTEMS
Low pressure systems are also known as depressions.
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?
What happens in the warm sector?
What happens at the occluded front?
Main Menu
LOW PRESSURE SYSTEMS
Imagine an area out in the North Atlantic Ocean, to
the West of the UK where the air is fairly cool.
This cool air is represented by the light blue
background on the following slides.
Cold air will be shown in darker blue.
Warm air will be shown in red.
THE AIRMASSES OVER THE ATLANTIC
Imagine you are looking down on a large area of the
North Atlantic Ocean. The air is quite cool (light blue).
In this area, different types of air masses meet.
We will look at two of these.
1. Cold Arctic air blowing from the North East
2. Warm moist Tropical air blowing from the
South West
THE AIRMASSES MEET……
Cold Arctic air
blows from the
North East
Warm, moist Tropical
air blows from the
South West
……BUT THEY DO NOT MIX
The two sets of air do
not mix together
As they blow past each
other, friction causes
them to swirl round
WARM AND COLD SECTORS
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)….
…this is called the
COLD SECTOR.
Cold Arctic
air
Warm,
moist
Tropical air
There is also a wedge of
warm moist Tropical air.
(red triangle)...
…this is called the
WARM SECTOR.
DIRECTION OF MOVEMENT
These systems usually move
either East, or North East
Cold Arctic
Cold Arctic
Cold ArcticCold Arctic Cold Arctic
Cold Arctic
air
air
air
air
air
air
Warm, Warm,
moist moist
TropicalTropical
air
air
Warm,
moist
Tropical
air
Warm,
moist
Tropical
air
Warm,
moist
Tropical
air
Warm,
moist
Tropical
air
WARM AND COLD FRONTS
The line between the
warm sector and the
cold sector is called
the COLD FRONT.
Cold Arctic
air
Cool air
Warm,
moist
Tropical air
The line between the
cooler air and the warm
sector is called the
WARM FRONT.
A SIMPLE CROSS SECTION THROUGH A
DEPRESSION
COLD SECTOR
(cold, dense air)
THE WARM
SECTOR
(warm, moist air)
Dense cool air
CONDITIONS AT STATION X (1)
Cold Arctic
air
Warm,
moist
Tropical
air
X
What conditions would
be felt at X?
a)
Cool
b)
Cold
c)
Warm
CONDITIONS AT STATION X (2)
What has passed over X?
Cold Arctic
Cold Arctic
Cold Arctic
Cold Arctic
air
air
air air
Warm Front
b)
Western Front
c)
Cold Front
What conditions would be felt at X?
X
Warm,
Warm,
Warm,Warm, moist
moistmoist moist Tropical
Tropical
Tropical
Tropical
air
air
air air
a)
a)
Cool
b)
Cold
c)
Warm
From which way are the winds blowing
at X?
a)
South East
b)
North West
c)
South West
CONDITIONS AT STATION X (3)
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
Cold Arctic
Cold Arctic
Cold Arctic
Cold Arctic
air
air
air
air
Warm, Warm,
moist moist
TropicalTropical
From which way are the winds blowing
at
air
air
X?
a)
North East
b)
North West
c)
South West
X
Warm,
moist
Tropical
air
Warm,
moist
Tropical
air
WHAT HAVE YOU LEARNED SO FAR?
Low pressure systems are also called depressions
Cold Arctic winds blow from the North East
Warm moist Tropical winds blow from the South West
The wedge of cold air is called the cold sector
The wedge of warm air is called the warm sector
The leading edge of the cold sector is the cold front
The leading edge of the warm sector is the warm front
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 air
Water vapour begins to condense into water droplets
Water droplets begin to form clouds
The first – and highest – type of cloud to form
along the warm front is called Cirrus
AIR RISING ALONG THE WARM FRONT
(1)
These high-level
wispy clouds are
called CIRRUS
Water vapour condenses
and forms clouds
As the warm air
rises, it cools.
Warm air is forced to rise
over denser, cool air.
Dense cool air
CIRRUS CLOUDS
PRECIPITATION AT THE WARM FRONT
Clouds form lower
down, and give
prolonged rain
More moist air
rises and cools
CIRRUS
These are
CUMULUS
Dense
cool air
CUMULUS CLOUDS
THE WARM FRONT - SUMMARY
CUMULUS clouds
bring prolonged rain
at the warm front
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
Dense
cool air
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 Cold Sector
( cold dense air )
The Warm Sector
(warm, moist air)
PRECIPITATION AT THE COLD FRONT
Very tall clouds are
formed by the rapidly
rising air.
These are called
CUMULONIMBUS.
CUMULONIMBUS give very
heavy showers, sometimes
with thunder and lightning.
The Warm Sector
(warm, moist air)
CUMULONIMBUS CLOUDS
IN THE WARM SECTOR
Warm, moist winds blow from the South West
Air is forced to rise over cooler air
Condensation occurs, and forms Stratus clouds
The sky is very overcast
Showers are common
THE WARM SECTOR
Warm, moist air in the warm
sector is rising.
Dense cool air
CLOUD & PRECIPITATION IN THE WARM
SECTOR
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
Dense cool air
CLOUD & PRECIPITATION – SUMMARY 1
CUMULONIMBUS
CUMULUS CIRRUS
STRATUS
HEAVY SHOWERS
LIGHT SHOWERS
Dense cool
air
PERSISTENT RAIN
A SATELLITE IMAGE OF A DEPRESSION
CROSS SECTION THROUGH A DEPRESSION
CLOUD
Cumulonimbus
Stratus
PRECIPITATION Heavy showers
Showers
TEMPERATURE
WIND
DIRECTION
AIR PRESSURE
Cold
N.W. or N
Rising
Warm
S.W.
Low
Cumulus
Prolonged rain
Cool
E or N.E.
Falling
Cirrus
HIGH PRESSURE
SYSTEMS
High pressure systems are also known as anticyclones
HIGH ATMOSPHERIC PRESSURE
The Earth’s atmosphere
presses down on the surface of
the Earth.
Cold, dense air falls. This
increases the weight of the
air pressing down on the
Earth’s surface.
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.
There are no warm or cold fronts in high pressure areas.
This means that high pressure systems bring clear skies.
Winds are usually light, and blow out of the high
pressure area.
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
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
The shower symbol can be
combined with others….
Rain
shower
Hail
shower
Snow
shower
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
A gale force wind
This symbol shows that there was no
wind : calm
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
ISBOBARS 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.
996
992
988
984
LOW
A DEPRESSION, SHOWING THE WARM & COLD
FRONTS
WINDS AROUND A DEPRESSION
Winds always blow from high pressure to low
pressure
Winds blow inwards anticlockwise round a depression
996
992
988
984
LOW
ISBOBARS AND WIND SPEED
When isobars are
close together, we
will get strong
winds.
996
99
98
2
8984
LOW
996
992
988
984
LOW
The further apart
the isobars are,
the gentler the
winds will be.
THE DEPRESSION WITH WINDS ADDED
TYPICAL SYNOPTIC CHART
You can
recognise the
isobars around
the low pressure
area.
SATELLITE IMAGE OF SAME DAY
HIG
H
LOW
HIGH PRESSURE
SYSTEMS
High pressure systems are also known as
anticyclones
ISBOBARS 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.
996
1000
100
4
100
8
HIGH
WINDS AROUND A HIGH PRESSURE SYSTEM
Winds always blow from high pressure to low pressure
Winds blow outwards clockwise from high pressure
Winds blow gently, because the isobars are far apart.
996
1000
100
4
100
8
HIGH
A SYNOPTIC CHART
MARCH 14 2002
Scotland has
high pressure.
Look at the
large area with
no isobars.
What effects
would this have on
the weather in
Invergordon?
CHART EXERCISES
WHAT IS AN OCCLUDED FRONT?
You have already learned that the cold front
moves faster than the warm front.
The warm sector is squeezed upwards, so that is
right off the ground.
The result is an occluded front. It is what
happens when the cold front catches up with the
warm front.
FORMATION OF AN OCCLUDED FRONT
Warm sector has been lifted off the
ground…
…so has the warm front
The Cold Sector
(cold, dense air)
OCCLUDED FRONT
THE EFFECT OF AN OCCLUDED FRONT (1)
CUMULONIMBUS
CUMULUS
CIRRUS
The Cold
STRATUS
Sector
(cold, dense
air)
Dense cool
air
LIGHT SHOWERSHEAVY SHOWERS
PERSISTENT
RAIN
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.