Topic 2 Flashcards
Pressure belts
bands of high pressure or low pressure that surround the earth at certain latitudes, for example Equatorial low pressure belt
Where do mid-latitude cyclones form
35-70 N+S known as the polar front. Always west to east
Mid latitudes in south africa
Clod fronts only reach south africa in winter
results in winter rainfall in the western cape
Air mass
a huge mass of air, extending for hundreds of kilometres with simular humidity and temperature
Polar front
A zone where the cold polar air mass and warm tropical air mass meet. 50* S
Jet stream
a band of very strong westerly winds in the atmosphere, above 9000m which partially controls the development of midlatitude cyclones
Cold front
The border zone between a cold air mass and a warm air mass. Winds in the cold sector blow against the cold front.
Warm front
The border zone between a warm air mass and a clod air mass. Winds in the cold sector blow against the warm front.
Warm sector
The area on the ground affected by warm air in a mid-latitude cyclone
Cold sector
The area on the ground affected by cold air in a mid-latitude cyclone
The development of a mid-latitude cyclone
Initial stage: Bend forms in the polar front, low-pressure cell begins to form and winds start to deflect and blow into the low-pressure cell. Wind + strengthening + Clouds
Mature stage: The bend in the polar front deepens and the pressure gradient increases, clockwise, fronts are now formed, and sectors are formed
Partially occluded stage and occluded stage: Warm air is lighter and warmer than cold making it rise and the cold sector wedges beneath it, the cold sector taking over - partially. Occluded = Almost all warm air above
Degeneration stage: Little gust of cold air on the surface, all warm air has risen, low pressure disappears and isobars even out, weather starts clearing
Anticyclones around south africa
South Atlantic, south Indian, Kalahari. Affect climate all year, descending warm air and dry conditions most of the year
Anticyclones around south Africa in winter
Have moved northwards, both anticyclone and the mid-latitude cyclones. South Indian + South Atlantic has moved closer to the coasts. Less distance the wind needs to travel to make contact with the coast = less water vapour being picked up and less rainfall. On the interior it is clear skies, sunny weather and no rain, freezing early mornings but can be in the high 20s during midday, several months of drought, interior caused by the kalahari anticyclone. The very cold air just above the plateau, Kalahari hp warms as it subsides. Above the plateau is an inversion layer. Warm air from the Indian ocean cannot blow onto the plateau due to the inversion layer.
Inversion layer
A layer of air where temperature increases with altitude.
Anticyclones around south Africa in summer
Kalahari anticyclone has risen 500m so surface conditions don’t reflect it. The inversion layer rises above the plateau allowing the moist Indian ocean air to penetrate inland. Anticyclones have moved southwards. South Indian anticyclone blows from further away from the coast picking up lots of moist air and bringing rain to the eastern part of SA. South Atlantic blows over cold ocean currents so it doesn’t pick up a lot of moisture resulting is Western side being dry. Anticyclones elongate to form a ridge south of the country resulting in strong southeasterly winds that are funneled between mountain ranges, down valleys and down streets sometimes causing gale-force winds.
Travelling disturbances
Anticyclones move throughout the year. Moisture front, line thunderstorms, bergwinds and cut-off low.
Moisture front
A zone extending from northwest to south-east through the interior of SA. Warm moist northeasterly winds from the Indian. Ocean converge with cooler winds from the southwest
Moisture front and line thunderstorms
Warm moist northeasterly winds form south Indian AS bow into the interior of the country. Cool southwest air from South Atlantic AS blows to the interior. Where they meet is a moisture front. Warm air is unstable, rises along the front, cools, dew point, condensation and cumulonimbus clouds form = thunderstorms, hail and heavy rain. Line thunderstorms occur along the line of moisture front. Not normal rain over the interior. Rain at night and day. Damage crops and buildings. This can only happen when Kalahari AS has risen, sometimes late in summer causing delayed rains.
Costal Low Pressure System
Move around course W to E coast, usually autumn and winter, clockwise into the low, bringing clouds and fog. in front = clear warm that can cause fire hazards
South Africa berg winds
A cold front approaching cape town = a strong pressure gradient with Kalahari HP. Blowdown from plateau to coast and warming adiabatically can cause 30+*C in winter
Cut-off low
South Atlantic AS sometimes forms a strong ridge behind a cold front, blocking it/ cutting it off = warm moist air flows southwestern behind a cold front = very cold with heavy rain and snow from the western cape to the Interior and as far as Gauteng.
Temperature variations between rural and urban.
City: Artificial substances like concrete, tar, and bricks. They absorb more heat. Artificial heating like aircon, power stations, cars and breathing. The city takes away water and fewer pants. High buildings with large surface areas absorb a lot of heat in the mornings. Polluted air is like a blanket causing a city to cool down slower at night.
More heat = stronger lift of air = thunderstorms. Hygroscopic nuclei increases the amount of condensation
Rural: Soil and grass absorb less heat. Heat is used in evaporation.
