Travelling Disturbances associated with anticyclonic circulation Grade 12 Climatology Travelling Disturbances associated with anticyclonic circulation

Travelling Disturbances Some weather conditions that arise from the South Atlantic, South Indian and the Kalahari anticyclones only occur at certain times of the year. They travel from one area to another. They include: Berg winds Moisture front (squall line), line thunderstorms Cut-off low

Moisture Front and Line Thunderstorms SA experiences hot conditions in the interior in summer. This causes a trough of low pressure over the interior. Winds are drawn into this trough from the South Atlantic and South Indian anticylones. The S.I.A provides warm, moist northeasterly winds, that blow into SA from the northeast. The S.A.A blows in from the south west. The two air masses meet along a zone called the moisture front, which coincides with the low pressure.

Moisture Front A zone extending from the northwest to the southeast through the interior of SA. The warmer air rises to great heights along the front. The air rises, cools and reaches dew point along the front. Huge cumulonimbus clouds form – heavy thunderstorms, hail and rain. Not the same as convection storms that occur in the interior during hot weather. occur at night as well. These can cause huge damage to crops and buildings. Only occur if K.A has risen to allow moisture into SA.

Costal Low pressure system Costal lows move around the coast from the west to east coast, usually during the autumn and winter. The air circulation is into the low in a clockwise direction. This causes an onshore flow behind the costal low, which brings in cloud and fog. Ahead of the flow there is an offshore flow causing clear warm weather (fire hazard).

South African Berg Winds Costal winters often see warm windless weather only for it to change and be followed by cold, wet, windy weather for a few days. This is caused by a Berg Wind followed by a cold front. Often associated with fire warnings.

Cut-off Lows South Atlantic anticylones can form a strong ridge behind a cold front. This blocks/cuts it off!!! This causes cold, moist air to flow in from the southwest behind the cold front. Very cold weather with heavy rain and snow on the mountains can spread from the Western Cape into the interior as far as Gauteng.