standard grade depressions & anticyclones john smith invergordon academy
TRANSCRIPT
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STANDARD GRADEDEPRESSIONS & ANTICYCLONES
John Smith Invergordon Academy
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CONTENTSAtmospheric pressure
Low pressure (Depressions)High Pressure (Anticyclones)Weather Maps
Exit
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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
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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
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The Earth’s surface
The Earth’s atmosphere presses down on the surface of the Earth.
ATMOSPHERIC PRESSURE
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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?
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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
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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.
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Cold Arctic air blows from the North East
Warm, moist Tropical air blows from the South West
THE AIRMASSES MEET……
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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
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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
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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
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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
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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)
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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)
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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)
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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)
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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
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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
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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)
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CIRRUS CLOUDS
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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
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CUMULUS CLOUDS
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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
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AN APPROACHING WARM FRONT
CIRRUS
CUMULUS
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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.
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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 )
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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
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CUMULONIMBUS CLOUDS
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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
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Dense cool air
THE WARM SECTOR
Warm front
Warm, moist air in the warm sector is rising.
Cold Sectorcold dense air
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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
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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
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A SATELLITE IMAGE OF A DEPRESSION
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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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HIGH PRESSURE SYSTEMS
High pressure systems are also known as anticyclones
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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.
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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.
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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.
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HIGH PRESSURE IN WINTER
High pressure in winter gives us
clear skies
very low temperatures
calm, or light winds
no precipitation
frosty conditions
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HIGH PRESSURE IN SUMMER
High pressure in summer gives us
clear skies
high temperatures
calm, or light winds
no precipitation© Microsoft Word clipart
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WEATHER MAPS
Weather symbols
Low pressure Systems (Depressions)
High Pressure Systems (Anticyclones)
Main Menu
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HOW IS WEATHER SHOWN ON A WEATHER MAP? (1)
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TYPICAL MEDIA WEATHER MAP
Look at the symbols.
What weather does Eastern England have?
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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
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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
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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.
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LOW PRESSURE SYSTEMS
Low pressure systems are also known as depressions
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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
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A DEPRESSION, SHOWING THE WARM & COLD FRONTS
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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
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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
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THE DEPRESSION WITH WINDS ADDED
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TYPICAL SYNOPTIC CHART
You can recognise the isobars around the low pressure area.
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SATELLITE IMAGE OF SAME DAY
HIGH
LOW
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HIGH PRESSURE SYSTEMS
High pressure systems are also known as anticyclones
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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
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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.
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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
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CHART EXERCISES
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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.
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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
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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
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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.