Download Warm air - D6 Admin system

MID LATTITUDE
CYCLONES
•Cold & warm fronts
•Stages
•Weather changes
HIGH PRESSURE
LOW PRESSURE
NAMES
Anticyclone
Cyclones / depressions
CHARACTERISTICS
High pressure
Descending air
Anticlockwise wind in
SH
Lower pressure
Ascending air
Clockwise wind in SH
SIZE
Large air mass – can be
more than 1000km in
diameter
Small, intense tropical
cyclones 2 or 3 km in
diameter or to large midlatitude cyclones more than
1 000km
WEATHER
CONDITIONS
Dry, warmer weather
Air temperatures
increase by 1°C for
every 100m it sinks
Wetter, cooler weather
Moist air tends to decrease
temperature by 0.5° for
every 100m it rises
HIGH AND LOW PRESSURES
GLOBAL AIR CIRCULATION PATTERN
Polar high
Western winds
Tropical
easterlies
Tropical
easterlies
Western
winds
Polar high
Winds deflect to the RIGHT in NH and to the LEFT in SH
Mid-latitude Cyclone
Temperate / Mid-latitude Cyclone
The different types of Air Masses and
fronts.
Areas where Temperate Cyclones form.
Conditions necessary for their formation.
The stages of development.
Cross sectional diagrams and weather
conditions.
Characteristics of Temperate Cyclones.
Reading and interpreting Satellite Images.
Scoop It
MID LATTITUDE CYCLONES: CONCEPTS
WARM FRONT
COLD FRONT
What happens when air masses meet?
The junction between two different air masses is called a
front. A front is associated with a change in the weather.
Cold air
follows a
cold front.
Cold air pushes under the
warm air, producing
strong winds and heavy
rain.
Warm air
follows a
warm front.
Warm air rises over
cold air, usually
producing clouds and
rain.
Depressions (low pressure systems) form
when a cold air mass meets a warm air mass.
Cold Front
• Rapidly moving cold air runs into slowly
moving warm air, and the cold air slides under
and pushes the warm air upwards
• As the warm air rises, it cools, forming clouds
and releasing heavy rain or snow
• Quickly moving, can cause abrupt weather
changes
Cold Front
1. Cold air advances into the warm air, forcing the warm air the rise.
2. Cold air is heavier than warm air, thus the warm overruns the cold air.
3. Intense precipitation over shorter period of time compared to warm front.
Warm Front
• A warm air mass collides and with slowly
moving cold air mass, and the warm air moves
over the cold air
• Can cause showers, light rain, and fog for
several days
Warm Front
1. warm air advances, pushing the cold air to retreat.
2. Cold air is heavier thus more difficult for the warm air to displace.
3. Light-to-moderate rain over large area for an extended period.
Cold Front
and Squall Line
This is a cold front
seen from space. Due
to the rapid lifting of
warm air by the
advancing cold air, a
sharp line of
cumulonimbus
clouds can occur,
called the squall line.
Occluded Front
• A warm air mass is caught between two cooler
air masses
• The warm air mass is pushed upwards by the
two denser cooler air masses
• Weather may turn cloudy and rainy or snowy
Occluded Front
Cold Front Occlusion
Cold Front Occlusion
MID-LATITUDE CYCLONE AND HURRICANE TRACKS
Where do Mid-latitude Cyclones occur?
CONDITIONS NECESSARY FOR THE DEVELOPMENT OF MIDLATITUDE CYCLONES
Two High Pressure systems are in
contact at the polar front.
The warm, subtropical high
pressure contains a warm,
moist maritime air mass.
The polar high pressure contains a cold, dry air mass.
The air flow converges from opposite directions on the
two sides of the polar front.
Disturbances on the polar front result in an unstable
situation, creating a local low pressure into which air will
flow and around which air will circulate.
MIDLATITUDE CYCLONE:
DEVELOPMENT
1 INITIAL STAGE
Warm subtropical air (westerlies)
meets cool sub-polar.
Different temperatures and moisture
content – forms a front.
Air masses flow parallel to each other in
opposite directions.
2 DEVELOPMENT STAGE
Front is disturbed when frictional
drag occurs between different air
masses, because of uneven surfaces.
Front has a ‘wavy’ appearance.
A decrease in pressure occurs and a
tongue of warm less dense air is
uplifted by the cold dense air
Result: a low pressure centre forming.
Over 24 hours the ‘wavy’ front continues to
develop.
Air begins to rotate in a clockwise direction
(LP in SH)
Frontal characteristics start to form:
• Cold front leads the cold air in the
cold sector. Cold air moves under
the warm air ahead of it and uplift
occurs .
Cross section of a Mature Cyclone
• Warm front leads the warm sector.
Warm air rises above the cooler air
ahead of it and uplift occurs.
• Both fronts are associated with uplift,
cooling, condensation, cloud
formation and rain.
• At cold front – rapid uplift - tall
cumulonimbus clouds.
• At warm front - more gentle uplift –
broad band of stratus and
nimbostratus clouds.
4 OCCLUSION / OCCLUDED STAGE
The warm sector begins to narrow until
the cold front ‘catches’ the warm front.
The warm air of the warm sector is
isolated from the ground.
COLD FRONT OCCLUSION
WARM FRONT OCCLUSION
MIDLATITUDE CYCLONE: DEVELOPMENT
1 INITIAL STAGE
2 DEVELOPMENT STAGE
3 MATURE STAGE
4 OCCLUSION
FAMILY OF CYCLONES
source: http://sageography.myschoolstuff.co.za
WEATHER CONDITIONS ASSOCIATED WITH
MID-LATITUDE CYCLONES
Two movements in a cyclone:
• Air is rotating clockwise around the centre of low pressure.
• The cyclone is being moved west to east by the westerly winds
The result: different fronts and sectors will move over you as the
cyclone passes.
 Temperature increases
Pressure decreases
Humidity increases
• Ahead of the warm front, the wind
direction will be northwest.
• As the warm front arrives, moist, unstable air rises
along the front forming nimbostratus clouds.
Warm, moist air rises because it is less dense than
cold, dry air. This often causes gentle rainfall.
• After the warm front has passed
over, the pressure continues to
drop.
• This occurs because the warm
sector has arrived The air here is
warm, less dense. The skies
become covered with
stratocumulus and altostratus
clouds. Wind direction in the
warm sector is north westerly (SH)
• Temperature decreases
• Pressure at its lowest but increases as front passes over
• Humidity decreases because cold air contains less water vapour.
After the cold front has passed a change in wind
direction occurs, In the SH winds change from north westerly in
the warm sector, to south westerly in the cold sector. This is called
backing. (NH – veering)
The cold sector is therefore dominated by south westerly
winds in the SH. The air is less humid.
Passage of a Temperate / Mid-latitude Cyclone
Source: http://www.boardworks.co.uk/
Depressions – the changing weather!
Source: http://www.boardworks.co.uk/
Passage of a depression
Source: http://www.boardworks.co.uk/
Cross section through a cold and warm front
WEATHER CHANGES COLD FRONT
Cold front
NW Wind
SW Wind
Warm front
source: http://sageography.myschoolstuff.co.za
WEATHER CHANGES: COLD FRONT
Movement of cyclone
•SW-wind
•Lower temp
•Denser cloud cover
•Precipitation
•Higher Pressure
Cold
Sector
source: http://sageography.myschoolstuff.co.za
•NW-wind
•Higher temp
•Few/no clouds
•Little /no precipitation
•Lower Pressure
Warm
Sector
LP
Cold
Sector
MID-LATITUDE CYCLONES IN SA
Form to the south and west of SA.
They are moved along by the westerly winds.
SA is only ever affected by the cold front.
They have their greatest impact on the Western Cape, but
under certain conditions, can bring cold and rainy weather
into the interior.
They form all the time, but affects SA during the winter
months –because of the shift of the ITCZ. During the winter
the ITCZ shifts north of the equator.
If there are no interruptions, several mid-latitude cyclones may
follow each other – zonal flow – the result: cloudy, rainy
conditions over longer periods
The path of the mid-latitude cyclone can be influenced by the
presence of the subtropical anticyclones:
West of SA – South Atlantic anticyclone
East of SA – South Indian anticyclone
Sometimes the South Atlantic anticyclone ridges in behind a midlatitude cyclone.
The anticlockwise movement around the anticyclone results in cold
winds blowing into the cold front.
These winds intensify the front.