Air masses and weather maps

A large body of air originating in a particular place is called an air mass. As a low pressure system moves across southern Australia relatively warm air from the northern part of the continent (a warm, continental air mass) is eventually displaced by relatively cool air from the southern ocean (a cool, maritime air mass).

The boundary line that separates two air masses is a 'front', from the military term describing the line between armies. Norwegian meteorologists developed the concept during the First World War. These frontal boundaries are always areas of rising air which produce weather changes.

There are two main types of front: cold fronts and warm fronts. The main feature of a cold front is an active wedge of cold air that attacks and undercuts a mass of warmer air. It moves in under the warmer air because it is colder, and therefore denser. These cold air wedges form a range of shapes, and move at different speeds.There are also different types of warm air (for instance, dry or moist), so it is not surprising that they produce many variations of weather.

In the case of warm fronts, warm air is the more active part of the system. It attacks the cooler air but being warmer and lighter, rides over it. The retreating cool air is replaced by the warm air. The slope of the front is much less than in a cold front, and it moves at about half the speed.

The temperature, moisture content and other properties of an air mass reflect the region where it originated. As it moves, its properties are modified by the surfaces it passes over.

Very hot desert air is cooled as it moves over lakes, oases or irrigation areas. Very cold polar air is warmed as it moves over temperate water.

Even after such modifications, these two types of air mass often still have quite different properties when they meet. A striking example is the arrival of the famous 'cool change' in southern Australia in summer. With a high pressure system over eastern Australia, the anticlockwise air movement brings an air mass from the hot dry interior over the southeast coastline. Temperatures under these north wind conditions are typically around 40°C for several days.

Outbreaks of Antarctic air have a preferred trajectory or path, often approaching from below southwest Australia. Air from Antarctica travelling at 30 knots can reach Victoria in 48 hours.

At these fronts, the hot northerly wind is abruptly replaced by a cold southwesterly wind. Clockwise-flowing air moves towards the front from the west. The arrival of the front, with air temperatures perhaps only 20°C, can drop the temperature by 10°C or 15°C in half an hour or less, with a further decrease in the next day or so.

Such a front may continue moving to the northeast across eastern Australia, but as it passes over the warm inland, its properties are further modified. If it reaches southeast Queensland its temperature may be little different from the prevailing northeast wind there. Its only effect is to change the wind direction and replace humid air by drier air.