MID LATITUDE CYCLONES

Grade 11 & 12

Climatology

What is a mid latitude cyclone

How do cyclones behave

Explanation of the numbers above:

• The LP is caused by rising air.

• Air converges towards the LP area before rising.

• The centre is marked by an L on a synoptic weather map.

• Air rotates in a clockwise direction around a LP centre (for the Southern hemisphere; the opposite occurs in the Northern hemisphere).

• Isobar readings decrease towards the centre.

• The rising air leads to cloudy weather / rain.

Where do the mid latitude cyclones

form?

• These develop at the polar front, which is the meeting of cold polar air from the South, and warmer tropical air from the North (Southern Hemisphere).

The cold easterly winds meet the warmer westerly winds at the polar front.

The two air massess meet but they do not mix

Formation at the polar front

Initial Stage

The meeting of these different winds creates

a disturbance in the frontal surface which

forms a wave in the polar front. The wave

grows and intensifies, and forms a wedge

of warm air penetrating the cold air. Low

pressure forms at the point of the wave,

and air begins to rotate in a clockwise

direction (Southern Hemisphere).

Initial Stage (continued)

The Mature Stage

• The warm westerlies push into the cold air

as they are drawn clcockwise around the

Low Pressure.

• Where the warm air intrudes into the cold

air a warm front is formed

• The cold easterlies swing around due to

the convergence and intrude into the warm

air forming a cold front

The Mature Stage (continued)

Cross section of a Mature Cyclone

Revision

Influences

• Further facts:

• These weather systems move from West to East.

• Mid latitude cyclones occur in groups or families (usually about 3 to 5 in a family).

• They influence South Africa's weather: they bring rain to the S.W. Cape in winter and icy weather into the interior.

• They are responsible for winter and early spring rain on the plateau thus filling farm dams and softening the soil for planting.

• Remember:

Cape Town is a winter rainfall area, due to the influence of the mid latitude cyclones. The mid latitude cyclone has different fronts and sectors; thus as it passes overhead the weather will change.

Weather with the passage of a

cyclone

Weather

• Pressure:

Air pressure will increase as the cold front passes over you.

Why?

Because cold air is heavier and denser than warm air (thus has a higher pressure).

Understand and interpret the cross-

section of a mid latitude cyclone.

The synoptic chart