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Summary IEB Grade 12 Geography-Subtropical Anti-cyclones
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IEB Geography notes for Grade 12 These notes are on Subtropical Anti-cyclones
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Urban ClimatesReasons for differences between rural and urban climates
The microclimate of a city
The temperature, humidity and rainfall in a city are different to those in the surrounding rural
area.
A city has a different microclimate to that of a rural area.
Temperature variations
A city is warmer than the surrounding rural area:
o The city is built of artificial substances such as concrete, glass, bricks and tar. These
surfaces absorb more heat than soil and grass, which are typical surfaces in rural areas.
o A city has artificial sources of heat, for example domestic and industrial heating, air
conditioning, power stations and car engines. Even people give off heat when they
breathe.
o In rural areas, a considerable amount of heat is used in evaporation from water surfaces
and transpiration from plants. In a city, the water is taken away in drains and there are
fewer plants.
o High-rise have large surface areas. When the sun’s rays strike the sides of these
buildings in the morning, the heat is absorbed by their large surfaces.
o Cities usually have polluted air above them. This causes the city to cool down more
slowly at night than the rural area. The polluted air acts like a blanket, keeping the warm
air close to the city in the evening.
Humidity and precipitation in cities
Humidity
o The humidity of the air describes how much water vapour is in the air.
o Usually the air in cities is less humid than the air in rural areas. This is because water in
cities is carried away by stormwater drains.
o There are fewer plants in cities than in rural areas, so less water vapour is added to the
air through transpiration.
Precipitation
o Precipitation is greater over city areas than over rural areas.
o Precipitation includes mist, fog, clouds, dew, frost, rain and snow.
o Water vapour in the air will condense only if there are hygroscopic nuclei. These are
particles in the air that attract and condense water vapour. Smoke, dust and salt are
examples of hygroscopic nuclei.
o Over a city there is usually pollution that consists of smoke and dust. So the pollution
provides hygroscopic nuclei that increase the amount of condensation.
o Thunderstorms are also more frequent over city areas.
o Hotter air results in stronger upward convection currents and therefore more
thunderstorms.
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