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Mid-latitude Cyclones Cheat Sheet by

Weather systems that occur at the synoptic scale

What are they?

Mid-la­­titude: 30° to 60° degrees N or S of the equator.
Cyclones: A low-pr­­essure system
They are also called extra-­­tr­o­pical cyclones or frontal depres­­sions

Where to they develop?

Mid-la­titude cyclones develop at the polar front, usually over the sea.
The polar front is where the cold polar easterlies meet the warmer westerlies of the subtro­pics.


- They consist of a pair of fronts (cold/­warm) linked to a central area of low pressure
- They carry a lot of moisture.
-Their winds spiral clockwise in the Southern Hemisphere
- Very large systems: up to 2000km across
- They travel eastwards (with the westerly winds in the westerly windbelt)
- Travel at 10km/h over areas of ±1200km per day
- Takes ±48 hours for the system to pass
- Occur in families of 3-5, and travel between 2-8 days apart

What conditions are necessary for MLC to form?

MLCs are powered by large temper­ature differ­ences in the atmosphere
There must be a large temp contrast between the subtro­pical westerly and polar easterly air masses - so the polar front develops.
There must be distur­bances in the jet stream that make the cold air push into the warm air
--> This causes the warm air mass to rise up and over the cold mass and creates a low-pr­essure cell into which the wind spirals


You need to know the four stages

1. Initial Stage

A stationary polar front forms. Wind shears in opposite directions along the front.

2. Develo­pment Stage

A 'wave' or a kink forms in the polar front as warm due to a distur­ban­ce/­fri­ction. Warm air is forced to lift up over cold air. Air pressure starts to drop and the warm and cold air masses begin to swirl alound the low pressure

3. Mature Stage

The cold front bends, and a cold and form show develop.
The cold front leads the cold sector of air which pushes in the direction of the equator.
The warm front leads the warm sector of air which pushes in the direction of the pole.

Low pressure continues to intensify at the apex of the fronts.

Cold Front Conditions

The air pressure drops to its lowest, and then increases with the arrival of cold dense air (cold front)
Temper­ature and humidity (dew point) decrease
Cumulus and cumulo­nimbus clouds form
Heavy rainfall (occas­ionally snow) at the front
Surface wind direction backs (changes) and wind speeds increase
In the Western Cape, the wind direction changes from north-west to west to south-west to south. This is an anti-c­loc­kwise change.

Warm front conditions

Air pressure drops
Temper­ature increases and humidity (dew point) increases
Wispy cirrus clouds and stratus clouds form
Gentle rain from nimbos­tratus clouds
As the front moves on, the weather becomes mild, calm and warm (warm sector)

4. Occlusion Stage

An occluded front is a combin­ation of a cold front and a warm front.
A combin­ation of a cold front's tight bands of stormy weather and a warm front wide area of cloudi­ness.

Cold Front Occlusion

Occurs when the coldest air in the MLC is behind the cold front.
The warm front is uplifted along the cold front.
The warm front and its air mass (the warm sector) lose contact with the ground.
The air is forced to rise, cool and condense (clouds form). Rainfall occurs with conditions similar to a cold front.
*The most common form of occlusion

Warm front occlusion

When the overtaking cold front is lifted by the colder retreating air ahead of the warm front

It occurs when the coldest air is found ahead of the warm front. This causes the cold front to be uplifted along the warm front.
The air is forced to rise, it cools, condenses, and clouds form.
Rainfall occurs with weather associated with a warm front (temp rise, nimbos­tratus, contin­uou­s/heavy rainfall).


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