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Air masses
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:
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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
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assessing the relevance and accuracy of its content. The State of Victoria, the Australian Camps Association, the editors, contributors and creators of this site
give no warranty, express or implied that the material is accurate, authentic or complete and take no responsibility for any error, omission or defect and do
not accept liability for any loss, damage, cost or expense you or others might incur as a result of the information provided on this website or incorporated into
it by reference.
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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.
The information on this website is provided for information purposes only and on the basis that all people accessing the site undertake responsibility for
assessing the relevance and accuracy of its content. The State of Victoria, the Australian Camps Association, the editors, contributors and creators of this site
give no warranty, express or implied that the material is accurate, authentic or complete and take no responsibility for any error, omission or defect and do
not accept liability for any loss, damage, cost or expense you or others might incur as a result of the information provided on this website or incorporated into
it by reference.