Page:Hunt - The climate and weather of Australia - 1913.djvu/164

Rh sufficient quantity to produce saturation before the cooling by radiation or otherwise has reached its limit. Cooling by radiation being the most frequent cause of fog, it follows that the sky should be clear, and this most frequently happens during the passage of an anticyclone. But the air is then usually very dry, so much so that under ordinary circumstances inland in the winter months frost is the more probable result. The water vapour necessary for fog production is most likely to be present inland when a clear, calm night follows rain, and especially when the high-pressure system forms a wedge separating two "lows."

Fogs in Melbourne are most frequent in the early winter months, May—July. The conditions most favorable for their production are well illustrated by the three charts shown. It nearly always happens that the anticyclone is centred to southward. The effect of this is obvious. The gentle outflow of air from the high along the earth's surface reaches Melbourne from points between south and east, having just passed over a water surface, and so carrying a fair amount of moisture. The fact also that a cyclonic depression has very frequently been operative just before off the New South Wales coast and driven masses of humid air into Bass Strait would aid in this result. In connexion with this latter factor the influence of the mountains or hilly country stretching southwards from the Yarra sources into South Gippsland cannot be ignored, inasmuch as it causes the dissipation of the clouds of lower levels coming from the east or south-east, and thus aids in producing the required clear night sky over Melbourne. A very favorable type is that of 21st February, 1913. The tendency towards a tropical "dip" in the isobars seems to indicate sufficiently humid conditions inland. One of the densest fogs ever experienced in Melbourne was on the night of the 26th July, 1910. In this case heavy rains had just fallen over Gippsland, and the high is but the separation between two "lows." During very dense fogs the air movement is often from the north-east. This is not necessarily the direction of air movement which preceded the fog formation, but it helps to illustrate the usually small convectional action with light north—easterly breezes, as was noted previously in a paper dealing with observations upon the amount of dust ordinarily suspended in the atmosphere of Melbourne.