In the northern Midlands, the annual evaporation is nearly 1,500mm due largely to the prevalence of winds coming from the Western Tiers, which become warmer and drier as they descend. This area of high evaporation extends to the lower Derwent Valley and the Huon. Mean monthly evaporation is nearly 200mm in the summer in these areas, but is closer to 30mm in winter. Evaporation in western, central and southern regions is much lower, normally less than 750mm each year, ranging between 15 mm per month in winter and about 100mm per month in summer.
The mid-latitude westerlies, a belt of winds squeezed between the subtropical ridge and the sub-Antarctic trough and affectionately known as the Roaring Forties, affect Tasmania directly. The greatest strength and persistence of these winds occur during late winter and early spring, but the speed and direction vary with the passage of high and low pressure systems.
In the summer months, when the westerlies are weak, afternoon sea breezes become the predominant wind in most areas. Periods of more humid north-easterly winds are most likely in the summer and early autumn.
Gales are most likely to come from the western quarter as deep lows pass just to the south of Tasmania.
The highest recorded wind gust in Tasmania is 176 km/h at Cape Grim on 28 July 1998. Higher, unrecorded gusts are likely to have occurred about the south-west coast.
FROST AND FOG
Frosts can occur throughout the year in all areas apart from the extreme coastal strip, where the frost season extends from about March to November. The dates of first and last frost vary markedly from year to year. Local factors, such as valleys where cold air pools and bright sun does not penetrate, heavily influence frost occurrence.
Fog is prevalent in Tasmania, particularly during autumn. It also occurs frequently in winter, especially in the inland river valleys.
Launceston is particularly susceptible to fog, while Hobart has occasional occurrences. Fog also occurs over the inland lakes, given the right conditions of light winds and cold nights. Occasionally sea fog is carried over coastal districts, especially along the northern and eastern coasts.