66 / Basics of Environmental Science
Over the continents and oceans, mixing of the air tends to equalize the pressure, temperature, lapse
rate, and humidity horizontally over a large area. Such air is called an ‘air mass’. Its characteristics
are determined by the region in which it formed, so air masses can be labelled. The first division is
between those formed over continents and those formed over oceans, the second between those
formed in arctic, polar, and tropical latitudes. This yields six types: continental arctic (cA), continental
polar (cP), continental tropical (cT), maritime arctic (mA), maritime polar (mP), and maritime tropical
(mT). Continental air is dry, maritime air moist and, as the names suggest, tropical air is warmer
than polar or arctic air; temperatures are more extreme in continental than in maritime air, owing to
the moderating influence of exposure to the oceans. Air masses do not remain stationary, of course.
They move, and as they do so they pass into new regions which modify their characteristics. A
continental air mass originating over North America becomes a maritime air mass by the time it has
crossed the Atlantic and reached Europe.
An air mass does not exist in a vacuum. It has borders, beyond which lie other air masses, with
different characteristics, and as it moves the air mass ahead of it must also move. Not all air
masses move at the same speed, however, so the boundaries are sites where one air mass is
displacing another. The theory describing these reactions was developed during the First World
War by scientists at the Bergen Geophysical Institute, Norway, led by Vilhelm Firman Koren
Bjerknes (1862–1951). Because of the predominant news at the time the theory was being
constructed, Bjerknes and his colleagues described the boundary between different air masses
as a ‘front’.
Fronts are identified as ‘warm’ or ‘cold’ depending on whether the air behind the front is warmer or
cooler than that ahead of it. They meet the ground surface at an angle, the frontal slope, and warm
and cold fronts slope in opposite directions (ALLABY, 1992, pp. 86–87). Warm fronts have a gradient
of about 1:1000; cold fronts are much steeper, sometimes with a gradient of as much as 1:50, and
they generally travel faster than warm fronts. Figure 2.22 illustrates typical conditions in the two
types of frontal region. In (1), rapidly advancing cold air moves beneath warmer air, forcing it to
rise; as the rising air cools, its water vapour condenses to form cumuliform (heaped) clouds, bringing
heavy showers and possibly thunderstorms. In (2), a gently sloping warm front is advancing more
slowly, so its air is lifted gently above the cooler, denser air and stratiform (sheet-like) clouds form,
bringing drizzle or steady rain.
Cool air is denser than warm air, so its pressure is higher. Air flows from areas of high to low
pressure, but because of the Coriolis effect and vorticity (the tendency of a flowing fluid to follow a
spiral path) it follows an approximately circular path with a speed proportional to the pressure gradient
(the pressure difference between the high and low areas). In the northern hemisphere air flows
anticlockwise (cyclonically) around areas of low pressure and clockwise (anticyclonically) around
areas of high pressure; in the southern hemisphere these directions are reversed. The difference in
speed between cool and warm air masses results from the way air moves around them: in the northern
hemisphere air flowing anticyclonically moves faster than air flowing cyclonically and in the southern
hemisphere the reverse applies.
At first, a front is simply a line, with cold air on one side, warm air on the other, and air flows in
opposite directions on either side. Some air crosses the front, however, and a V-shaped wave appears
in the front. As this grows more pronounced, the cold air is partly behind the warm air and overtaking
it. A low-pressure centre develops at the apex of the wave; this is a ‘cyclone’ or ‘depression’. The
undercutting cold air then starts to lift the warm air clear of the ground; at this stage the fronts are
becoming occluded. Occlusion continues until all the warm air has been lifted above the ground;
after that the depression dissolves and disappears. Figure 2.23 shows the way clouds are distributed
around the depression as it develops and dissipates.