1. STANDARD GRADE DEPRESSIONS John Smith Invergordon Academy

2. 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?

3. 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.

4. 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

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

6. ……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

7. 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.

8. 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

9. 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.

10. 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

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

12. 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

13. 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

14. 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

15. 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

16. 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

17. CIRRUS CLOUDS

18. 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

19. CUMULUS CLOUDS

20. 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

21. AN APPROACHING WARM FRONT
CIRRUS
CUMULUS

22. 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.

23. 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)

24. 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)

25. CUMULONIMBUS CLOUDS

26. 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

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

28. 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

29. 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

30. A SATELLITE IMAGE OF A DEPRESSION

31. 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