Geography Reference
In-Depth Information
over oceans because the energy to maintain their strength comes
from the heat released when water vapour condenses to form
clouds. When the moisture source is restricted as the cyclone hits
land then the storm dies down. Tropical cyclones have high rainfall
intensities and, as they travel slowly, any one place by which they
pass can receive very high rainfall totals. The rainfall over two to
three days from a single hurricane can be several hundred
millimetres.
The climate of the trade wind belt also includes the monsoon
which occurs in Asia, west and east Africa, Australia and a weaker
version over south-western USA. The monsoons produce regions
that experience an exceptionally wet rainy season. Monsoon
regions are all associated with the switching of the wind direction.
In winter, winds blow off the relatively cold continent toward the
warm ocean (warmer air rising above the oceans draws in air to
replace it from the nearby continent). There are therefore stable,
dry conditions over land. However, as the land warms in the
summer then the wind reverses due to the low pressure (rising air)
at the surface that forms over the warmed continent. Air from over
the ocean flows toward the low pressure over land bringing with it
lots of moisture. These changes are also supplemented by changes
in the location of the jet stream above. In Asia the dry northerly
wind over India reverses direction in May/June, and warm, humid
air from the Indian Ocean flows from the south until around
October, bringing torrential rains. The rains are not continuous but
in some mountainous places the rainfall can be 10,000 millimetres
per year. Monsoons are not always the same each year and El Niño
years in Asia can be associated with the failure of the monsoon
rains sometimes leading to crop failure.
In contrast to the wet conditions near the equator, the major
deserts are commonly found around 30° latitude coinciding with
the zone of descending, dry air from the Hadley circulation cell.
The driest hot deserts are found in the western coastal regions of
the continents where the subtropical anticyclones are most intense.
The main features of hot desert weather are a wind which increases
aridity and high daytime temperatures (often over 35°C). The dry
air and clear skies produce large daily ranges of temperature (as
much as 20°C in some places) and night temperatures can even
drop below freezing in places.
