Geography Reference
In-Depth Information
humid. North of 50° there are severe winters and relatively short
summers (maximum of three months with average temperatures
above 10°C). Precipitation tends to be distributed throughout the
year in the mid-latitude continental interiors but generally with a
summer maximum (mainly from convective showers and weak
frontal systems). Total annual precipitation is low (below 500 milli-
metres), but the cold winter and summer precipitation peak pro-
vides sufficient moisture for plants (e.g. the main wheat-growing
areas of North America). The regions above 50° in the mid-
latitude continental interiors are dominated by high pressure in
winter but mid-latitude weather systems at other times of the year.
Winter average temperatures can be less than -25°C in the coldest
month and are as low as -50°C in Siberia.
Tropical and subtropical climate and weather
Close to the equator the Coriolis effect is negligible and so the
weather is not dominated by the movement of large circulatory
weather systems. Here, air simply flows from high to low pressure.
However, flow from the north-east and south-east trade winds
helps air to converge into the intertropical convergence zone (see
above). Air meeting at this zone and the warm conditions forces
upward movement and the formation of clouds. Close to the
equator the weather is dominated by frequent convectional clouds
and plentiful rain with some parts of Amazonia and West Africa
receiving over 4,000 millimetres per year. Average temperatures in
the equatorial regions are around 27°C throughout the year. Equa-
torial climates are not the same in each place, however, as topogra-
phy and proximity to the oceans play a role in altering temperature
and rainfall.
Moving north or south away from the equator into the trade
wind region, between about 5° and 20° latitude, a rainy (summer)
and dry (winter) season becomes clearer. The trade winds produce
a steady, but not particularly severe, wind regime. However, tropi-
cal depressions can form over the oceans, some of which become
tropical cyclones. These cyclones are known as hurricanes in the
Atlantic and typhoons in the west Pacific. Tropical cyclones require
high sea surface temperatures (at least 27°C) but do not occur close
to the equator as the Coriolis force is too weak. They only form
