This document contains summaries made from " FOCUS geography " textbook. This document summarizes Mid - latitude cyclones; the development of Mid-latitude cyclones; cold fronts; warm fronts; Occluded fronts; Tropical cyclones; formation of tropical cyclones; damages of tropical cyclones; weather c...
- Low – pressure cell which develop in the mid – latitudes
and travels from west to east.
General characteristics of mid - latitude
cyclones are due to pressure, wind patterns
and the air masses involved in their formation.
- Effect of pressure and wind pattern on mid - latitude cyclones:
Pressure is low at the centre and increases outwards.
Pressure lower than 1000 hPa is usually regarded as
a low pressure.
Winds blow around and into low pressure cell in a
clockwise direction in the southern hemisphere.
Winds blow around and into low pressure cell in
anticlockwise direction in the Northern
hemisphere.
Winds do not blow straight along the pressure gradient
but are deflected by Coriolis force.
- The effect of air masses on the mid - latitude cyclones:
Mid latitude cyclones develop between 30 ֯ and 60 ֯ north and south of
equator, this is where the cold polar air mass meets warm subtropical air mass
= polar front.
When air masses meet = warm Subtropical air rises and cold polar air
wedges in underneath.
As warm air rises, it cools and condensation occurs, which produce clouds and
rain {main weather characteristics of mid - latitude}.
Mid – latitude cyclones cover large areas, and weather brought by cyclones can
last up to 5 days or even weeks if more than one cyclone occurs after the other.
KEYWORDS:
Coriolis force: force that causes the deflection of moving objects due to
the Earth’s rotation on its own axis. {greatest at poles, least at equator}.
Pressure belts: bands of high or low pressure that surround Earth at
certain latitudes.
Air mass: a huge mass of air, extending for hundreds of kilometers.
Polar front: A zone where the cold polar air mass and warm tropical air
mass meet.
Jet stream: a band of very strong westerly winds high in the atmosphere.
Cold front: the border zone between a cold air mass and a warm air mass.
Warm front: the border zone between a warm air mass and a cold air
, World distribution of mid – latitude cyclones:
- Mid – latitude cyclones move from west
to east in both the northern and
southern hemispheres.
- They occur mainly over the oceans and
coastal areas [ do not spread over large
areas of land]
- They affect regions between 30 ֯ and 60 ֯
north/south of equator.
Mid – latitude cyclone in south Africa:
Warm fronts are bent southwards away from south Africa.
Cold fronts only reach south Africa in winter where all pressure belts and
wind systems move slightly north. [ Southwestern cape receives winter
rainfall because of this.]
The rain is brought by cold fronts passing across the region from west to
east.
Conditions necessary for the formation of mid – latitude cyclones:
- A bend forms in the polar front, which is
due to a disturbance in winds high in the
atmosphere in the Jetstream or it can be
caused by a mountain range or shape of
the coastline.
- This change in condition of the polar
front causes isobars to form more of a
circular pattern, and a cell of low
pressure begins to form.
- Winds then deflect and blow into low
pressure cell, which is necessary for the
beginning of a mid – latitude cyclone.
KEYWORDS:
Warm sector: the area on the ground affected by warm air in a mid – latitude
cyclone.
Cold sector: the area on ground affected by cold air in a mid – latitude cyclone.
Backing: a change in direction of winds in an anticlockwise direction.
Veering: a change in direction of winds in a clockwise direction.
, Development of mid – latitude cyclone:
- Would take to 2 to 5 days for these stages to pass a given point on the ground, the area affected
could cover several thousand KM.
Stages of development of mid latitude
cyclone:
1. Initial stage.
2. Mature stage.
3. Partially occluded stage.
4. Occluded stage.
5. Degeneration stage.
Initial stage:
- First stage of development.
- Bend forms in the polar front.
- Low pressure cell begins to form, winds start to deflect and blow
into low pressure cell.
- Weather features = strengthening winds & clouds appearing
(because of warm air starting to rise, cool down and water vapour
is condensing).
Mature stage:
- Bend in polar front deepens.
- Pressure gradient becomes
steeper.
- Winds blow more strongly.
- Air blows in clockwise direction
around and into the low-pressure
cell.
- Polar front now forms warm and
cold fronts.
- Fronts divide the area into
Partially occluded stage:
- Air in warm sector is warmer and lighter than air in cold
sector, which is cold and dense.
- Warm sector air rises and cold sector air wedges in beneath
it.
[causes warm air sector to narrow as the cold front gradually
overtakes the warm front.]
Occluded stage:
- Mainly cold air on ground.
- Almost all warm air has risen above ground level.
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