Knowing the sea-level wind movement along the weather map isobars, you can determine the likely route taken by the air which will be reaching you. The air temperature you experience, and the moisture content of this air, has been established over many days and is reasonably constant over hundreds of kilometres, so will provide useful information on the likely temperature and potential for rainfall. Figure 18.2 shows the main types of air mass that affect Australia:
- Generally prevailing westerlies: moist air over western coastal areas which condense where raised by topography.
- Cyclonic tropical: very strong, moist winds producing heavy rain over affected northern coastal areas
- Tropical continental: very hot, dry air which produces heatwave conditions when it moves south (hot northerlies in summer). The air has been heated by contact with the dry, hot land in central Australia.
- Southern maritime: a reasonably moist air stream with temperatures determined by the southern oceans over which it has passed.
- Polar maritime: very cold, partly moist air, originating over the Antarctic Ocean. It produces sudden cold snaps and heavy snow falls.
The temperature of an air mass is predominantly affected by conduction and convection, and a little by radiation. All ski tourers will have experienced the biting southerly wind under a cloudless sky. Convective temperature changes are evident in the large, fluffy ‘cottonwool’ cumulus clouds, and the thunderstorm (cumulonimbus) clouds.
At the surface, air exchanges heat with the ground. During a cloudless day, the ground absorbs radiation from the sun and heats up the surface layer of air via conduction. A parcel of air, heated by contact with the ground, rises (or convects upwards) and expands. The parcel cools. Water vapour may form a cloud at a ‘condensation level’, the height of which depends upon the amount of water vapour in the air. The resulting flat-bottomed cumulus clouds may develop into thunderstorms as the heat released by the condensation drives the air parcel higher. The tops of thunderstorm clouds may reach the tropopause at 9000–12 000 m.
Conductive cooling of an air mass occurs on clear nights. The earth cools by radiating energy into space. The air in contact with the ground cools. If there is much wind, this cooling effect is distributed through many hundreds of metres of the atmosphere, and little change in air temperature is noticed. However, on still, clear nights, particularly during the passage of the centre of a high pressure system, only the bottom few metres of the atmosphere will be cooled. Depending upon the moisture content of the air, fog may occur and, depending on the initial ground temperature, cooling may proceed to freezing point producing frost. In hilly country, the cooled air will drain into valley floors producing frost hollows. In such conditions, a warmer night will generally be experienced up on the valley sides.
A layer of cloud acts as a blanket, slowing the radiative cooling and generally resulting in warmer nights. Similarly, a tent pitched under a crown of trees will escape the frost that covers a tent pitched in the open.