STANDARD GRADE
DEPRESSIONS
John Smith Invergordon Academy
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?
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