Geography Reference
In-Depth Information
month in the polar tundra is above 0°C but below 10°C. Winter
temperatures are generally low (average below -25°C). Mean
annual precipitation tends to be less than 300 millimetres. The
weather is dominated by the prolonged winter with dry, clear high
pressure conditions. Although summer sunshine is weak in polar
tundra regions, the long daylight hours can result in melting of the
snow cover for short periods, allowing the upper soil layers to
thaw. This provides a short growing season (see Chapter 5).
Mid-latitude climate and weather
The mid-latitudes are dominated by weather systems that move
across the Earth. Air speeds up in high-level Rossby waves (I'm
sure you remember the chocolate pudding icing example from
earlier). This faster movement causes sections of air to spread apart
(there is divergence of the air). Upper air convergence occurs
where air slows down in the Rossby wave. If the divergence of air
high up in the troposphere is greater than the convergence of air
down below near the Earth's surface then there will be a fall in
surface air pressure and air will rise. This, along with the Coriolis
force, leads to the formation of a large, circulating, rising mass of
air flowing in an anticlockwise direction. This is known as a
depression. If the opposite occurs then there will be a zone of high
pressure at the surface with descending air flowing in a clockwise
direction. This is known as an anticyclone. Descending air in anti-
cyclones results in the air warming which means that it is less satu-
rated (less close to the dew-point temperature) and typically means
clear conditions. However, on some occasions, there can be a
temperature inversion
whereby colder, moist air is trapped
below the warmer air and a layer of cloud or fog can form, espe-
cially in winter. During summer the air below the inversion is
warmed sufficiently to cause the cloud or fog to dissipate.
A weather front is the boundary between two masses of air of dif-
ferent densities (and temperature). A cold front is where a cold air
mass is moving into a warm air mass whereas a warm front is the
reverse. Box 1.2 outlines the characteristics of air masses. The rising
air in depressions is concentrated along the warm and cold fronts.
Rising air will produce condensation of water vapour leading to the
formation of cloud and precipitation. Rising air in depressions takes
