What is a MLC? low pressure travelling system (west to east)
throughout the year but mostly affect SA during winter when the South Atlantic and
When do MLCs form? South Indian HP shift equatorward and the Thermal Low over the interior changes to
the Kalahari High.
They form when warm, subtropical air masses (subtropical westerly winds) meet with
How do MLCs form?
cold, polar air masses (polar easterly winds) along the polar front (60° line of latitude)
West to East
Where do MLCs move to and how do they they move in an easterly direction (from west to east) driven along by the upper
move there? atmospheric jet streams* which are located near the tropopause. (the major jet streams
are westerly winds)
temperate cyclones
extratropical cyclones
What are some other names for MLCs? (4)
frontal depressions
wave cyclones.
Which way do MLCs rotate in the SH? Clockwise
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, 8/3/24, 2:23 AM
1. Initial
Warm, subtropical air from the warm westerly wind belt meets with cold, polar air from
the polar easterly wind belt along the polar front ( 60°S). The air masses move past
each other due to rotational force creating surface friction.
2. Developing
Wave-like movements develop across the polar front. The regular surface becomes
disturbed and a vortex of low pressure develops at the apex of the wave. The air
masses change direction and start to converge clockwise in the southern hemisphere
around the low-pressure center.
What are the 4 stages of development
3. Mature stage
(cyclogenesis) and what happens in each
A distinct low-pressure centre has formed with a clearly defined cold and warm front.
stage?
Behind the warm front, the mass of warm air turns towards the poles and forms the
warm sector. Behind the cold front, the cold air mass turns towards the equator and
forms the cold sector. The entire mid-latitude cyclone moves eastwards.
4. Occluded
While the mid-latitude cyclone is moving eastwards the cold front catches up with the
warm front (cold air is denser and therefore causes the cold front to move quicker than
the warm front). The cold air displaces the warm air from the warm sector upwards and
out of the cyclone until the cold front merges with the warm front and they form a
single occluded front.
Eventually the low pressure centre is surrounded entirely by cold air and the weather
How does a MLC dissipate?
system dissipates*. The polar front then assumes its original position
A warm front occlusion forms when a faster-moving cold front catches up with a
What is the difference between a cold and a
slower-moving warm front whereas a cold front occlusion occurs when the cold front
warm front occlusion?
overtakes both the warm front and the warm sector air, lifting the warm air mass.
positives:
- water table rises and periodic rivers flow
- Cape Town receives winter rainfall
- frost freezes soil which can kill harmful organisms
What are 3 positive and 3 negative impacts
that MLCs have in SA?
negatives:
- Berg winds may bring unwanted forest fires
- frost can damage crops resulting in stock losses
- excessive rainfall can cause flooding
What is a TC? A gigantic mass of revolving, moist air.
on ocean surface temps above 27 and between 5 and 20 degrees latitude because
more than 20 and the ocean temps would be too cold to sustain convection currents
Where do TCs form?
and thunderstorms where less than 5 degrees the Coriolis force would be too weak to
start any cyclonic movement
Asia - Typhoons
What are TCs called around the world? USA - hurricanes
AUS - willy-willy
late summer and early autumn
When do TCs occur?
(November to April) in SH
The diameter of the vortex is about 600-800km in a category 5 storm.
How big are TCs?
The central eye is about 30- 50km in diameter.
How long do TCs last? 3-8 days
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