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STANDARD GRADE DEPRESSIONS & ANTICYCLONES John Smith Invergordon Academy.

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Presentation on theme: "STANDARD GRADE DEPRESSIONS & ANTICYCLONES John Smith Invergordon Academy."— Presentation transcript:

1 STANDARD GRADE DEPRESSIONS & ANTICYCLONES John Smith Invergordon Academy

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

3 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

4 WHAT DO WE MEAN BY AIR PRESSURE? The earths 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 nitrogencarbon dioxide

5 The Earths surface The Earths atmosphere presses down on the surface of the Earth. ATMOSPHERIC PRESSURE

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 Earths surface. Rising air reduces the weight of air pressing down on the Earths surface. This means that air pressure is low.

8 The Earths surface is warmed by the suns rays. The Earths 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 Earths surface.

9 WHY DOES ATMOSPHERIC PRESSURE CHANGE? 2 When air is cold, high up in the atmosphere it falls towards the earths surface. Falling air increases the weight of air pressing down on the Earths surface. This means that air pressure is high.

10 The Earths surface The Earths 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 Earths surface.

11 The Earths surface is warmed by the suns rays. HIGH AND LOW PRESSURE TOGETHER Warm air rising causes LOW pressure. Cold air falling causes HIGH pressure. WIND

12 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.

13 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?

14 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

15 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.

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

17 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

18 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

19 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

20 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

21 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)

22 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)

23 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)

24 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)

25 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

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 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)

28 CIRRUS CLOUDS

29 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

30 CUMULUS CLOUDS

31 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

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 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

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 Showers are common The sky is very overcast

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

39 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

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

41 A SATELLITE IMAGE OF A DEPRESSION

42 CLOUD PRECIPITATION TEMPERATURE WIND DIRECTION AIR PRESSURE Cumulonimbus StratusCumulusCirrus Heavy showers ShowersProlonged rain ColdWarm N.W. or NS.W.E or N.E. CROSS SECTION THROUGH A DEPRESSION Falling Low Rising Cool

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

44 The Earths surface The Earths 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 Earths surface.

45 WHAT CAUSES HIGH PRESSURE? When air high in the atmosphere is cold, it falls towards the earths surface. Falling air increases the weight of air pressing down on the Earths 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. 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.

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 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 FogThunderDrizzleRainSnowHailShower 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 that there was no wind : calm This symbol shows A gale force wind

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. LOW 984 988 992 996

57 A DEPRESSION, SHOWING THE WARM & COLD FRONTS

58 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

59 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 98 8 99 2 996

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 HIG H 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. 100 4 HIGH 100 8 996 1000

65 WINDS AROUND A HIGH PRESSURE SYSTEM Winds blow outwards clockwise from high pressure Winds always blow from high pressure to low pressure 100 4 HIGH 100 8 996 1000 Winds blow gently, because the isobars are far apart.

66 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

67 CHART EXERCISES

68 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.

69 FORMATION OF AN OCCLUDED FRONT Warm sector has been lifted off the ground… …so has the warm front The 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

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

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 Earths surface.


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