1. STANDARD GRADE DEPRESSIONS & ANTICYCLONES
John Smith Invergordon Academy

2. CONTENTS Atmospheric pressure Low pressure (Depressions)
High Pressure (Anticyclones)
Weather Maps
Exit

3. 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

4. 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.

5. Outer edge of the atmosphere
ATMOSPHERIC PRESSURE
Outer edge of the atmosphere
The Earth’s atmosphere presses down on the surface of the Earth.
The Earth’s surface

6. 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.

7. 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.

8. LOW ATMOSPHERIC PRESSURE
Outer edge of the atmosphere
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.

9. 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.

10. HIGH ATMOSPHERIC PRESSURE
Outer edge of the atmosphere
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

11. HIGH AND LOW PRESSURE TOGETHER
Outer edge of the atmosphere
Warm air rising causes LOW pressure.
Cold air falling causes HIGH pressure.
WIND
The Earth’s surface
is warmed by the sun’s rays.

12. 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.

13. 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?
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14. Cold air will be shown in darker blue.
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.

15. 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

16. THE AIRMASSES MEET……
Cold Arctic air blows from the North East
Warm, moist Tropical air blows from the South West

17. ……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

18. WARM AND COLD SECTORS
Cold Arctic air
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)...
…this is called the COLD SECTOR.
Warm, moist Tropical air
…this is called the WARM SECTOR.

19. These systems usually move either East, or North East
DIRECTION OF MOVEMENT
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

20. 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.

21. A SIMPLE CROSS SECTION THROUGH A DEPRESSION
THE WARM SECTOR
(warm, moist air)
C O L D F R O N T
W A R M F R O N T
COLD SECTOR
(cold, dense air)
Dense cool air

22. CONDITIONS AT STATION X (1)
Warm, moist Tropical air
Cold Arctic air X
What conditions would be felt at X?
Cool
Cold
Warm

23. CONDITIONS AT STATION X (2)
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
What has passed over X?
Warm Front
Western Front
Cold Front
What conditions would be felt at X?
Cool
Cold
Warm X
From which way are the winds blowing at X?
South East
North West
South West

24. CONDITIONS AT STATION X (3)
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
What has passed over X?
Warm Front
Western Front
Cold Front X
What conditions would be felt at X?
Cool
Cold
Warm
From which way are the winds blowing at X?
North East
North West
South West

25. 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

26. 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

27. 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

28. CIRRUS CLOUDS

29. PRECIPITATION AT THE WARM FRONT
Clouds form lower down, and give prolonged rain
CIRRUS
These are CUMULUS
More moist air rises and cools
Dense cool air

30. CUMULUS CLOUDS

31. THE WARM FRONT - SUMMARY
CIRRUS are very high and wispy. They are usually the first clouds we see as the warm front approaches
CUMULUS clouds bring prolonged rain at the warm front
More moist air rises over the cooler air. As it does so it cools.
Dense cool air
Warm Front

32. AN APPROACHING WARM FRONT
CIRRUS
CUMULUS

33. 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.

34. 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)

35. 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)

36. CUMULONIMBUS CLOUDS

37. 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

38. THE WARM SECTOR Warm, moist air in the warm sector is rising.
Warm front
Dense cool air

39. 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
W A R M F R O N T
Dense cool air

40. CLOUD & PRECIPITATION – SUMMARY 1
CUMULONIMBUS
CIRRUS
CUMULUS
STRATUS
W A R M F R O N T
Dense cool air
HEAVY SHOWERS
LIGHT SHOWERS
PERSISTENT RAIN

41. A SATELLITE IMAGE OF A DEPRESSION

42. CROSS SECTION THROUGH A DEPRESSION
CLOUD
PRECIPITATION
TEMPERATURE
WIND DIRECTION
AIR PRESSURE
Cumulonimbus
Stratus
Cumulus
Cirrus
Heavy showers
Showers
Prolonged rain
Cool
Cold
Warm
N.W. or N
S.W.
E or N.E.
Rising
Falling
Low

43. HIGH PRESSURE SYSTEMS
High pressure systems are also known as anticyclones

44. HIGH ATMOSPHERIC PRESSURE
Outer edge of the atmosphere
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

45. 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.

46. 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.

47. HIGH PRESSURE IN WINTER
High pressure in winter gives us
clear skies
very low temperatures
calm, or light winds
no precipitation
frosty conditions

48. HIGH PRESSURE IN SUMMER
gives us
clear skies
high temperatures
calm, or light winds
no precipitation
© Microsoft Word clipart

49. WEATHER MAPS Weather symbols Low pressure Systems (Depressions)
High Pressure Systems (Anticyclones)
Main Menu

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

51. TYPICAL MEDIA WEATHER MAP
Look at the symbols.
What weather does Eastern England have?

52. 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

53. 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

54. 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.

55. LOW PRESSURE SYSTEMS
Low pressure systems are also known as depressions

56. 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.
996
992
988
984
LOW

57. A DEPRESSION, SHOWING THE WARM & COLD FRONTS

58. WINDS AROUND A DEPRESSION
Winds always blow from high pressure to low pressure
Winds blow inwards anticlockwise round a depression
LOW
984
988
992
996

59. When isobars are close together, we will get strong winds.
ISOBARS AND WIND SPEED
LOW
984
988
992
996
When isobars are close together, we will get strong winds.
LOW
984
988
992
996
The further apart the isobars are, the gentler the winds will be.

60. THE DEPRESSION WITH WINDS ADDED

61. TYPICAL SYNOPTIC CHART
You can recognise the isobars around the low pressure area.

62. SATELLITE IMAGE OF SAME DAY
HIGH
LOW

63. HIGH PRESSURE SYSTEMS
High pressure systems are also known as anticyclones

64. 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.
996
1000
1004
HIGH
1008

65. 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.
1004
HIGH
1008
996
1000

66. 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?

67. CHART EXERCISES

68. 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.

69. FORMATION OF AN OCCLUDED FRONT
The Cold Sector
(cold, dense air)
C O L D F R O N T
Warm sector has been lifted off the ground…
…so has the warm front
W A R M F R O N T
OCCLUDED FRONT

70. THE EFFECT OF AN OCCLUDED FRONT (1)
HEAVY SHOWERS
The Cold Sector
(cold, dense air)
C O L D F R O N T
CUMULONIMBUS
CIRRUS
CUMULUS
STRATUS
W A R M F R O N T
Dense cool air
LIGHT SHOWERS
PERSISTENT RAIN

71. 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.