Geography Reference
In-Depth Information
place over a large area and while there may not always be heavy pre-
cipitation there can be substantial amounts of precipitation over a
large area. Looking at vertical slices through fronts shows they tend
to gently slope at a rate of 1 metre of vertical rise for every 80 to 150
metres of lateral distance. Cold fronts tend to be steeper than warm
fronts and over time a cold front tends to overtake a warm front
leading to an occluded front . As a cold front passes there can be a
sudden drop in temperature experienced at ground level (5°C fall
within 30 minutes is not uncommon).
Box 1.2 Air masses
An air mass is a regional parcel of air which has developed over an area
where it has remained for a period of days and gained particular tem-
perature and moisture characteristics. There are four basic types of air
mass: tropical maritime, tropical continental, polar maritime and polar
continental. Additional extremes are Arctic maritime and Antarctic
continental. Continental air masses are relatively dry and maritime air
masses are relatively humid.
Polar maritime air is common in both hemispheres with source
regions in the high-latitude oceans. Polar continental air occurs in the
northern hemisphere while Antarctic continental air occurs in the
southern hemisphere. Tropical maritime air is common in both hemi-
spheres, but tropical continental air is less common due to the lack of
large land masses in the subtropics (e.g. North Africa is the main one).
India can be a source region for tropical continental air in winter. In the
summer the high pressure over Siberia combined with the Himalayan
mountain chain restricts the movement of tropical continental air
northwards.
Air masses are modified by the Earth's surface. If the surface is
colder than the air mass then the stability of the air will be increased. If
the surface is warmer than the air mass then stability will be decreased
and cloud formation and precipitation may result. For example, when
Arctic maritime air moves south over the North Atlantic the sea surface
is warmer than the air mass. This warms the lowest layers, decreasing
stability, encouraging convection and the formation of frequent
showers. Weather forecasters study the source areas for air, track their
movement over a particular region and predict how the air masses
might be modified by local conditions and how they might interact with
other air masses.
 
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