Page:Hunt - The climate and weather of Australia - 1913.djvu/100

Rh systems we may have the "highs" contoured by isobars in such a way that the "col" is either distinctly anticyclonic, the wind circulation of the preceding high almost merging into that of the following one, or distinctly convectional and rather forming connexion between the circulations of two low pressure systems than between those of two "highs." It is for the latter type that the word "trough" is here used. In the case of a col, the isobars of the two "highs" are decidedly convex to one another along their common axis; in the case of a trough the rear isobars of the preceding "high" or the front isobars of the following "high", or both, tend to straighten and the convexity is more apparent in the isobars of the adjacent low pressure systems, producing often an hour glass appearance. The chart of 31st May, 1909, shows the straightening of the rear isobars of the preceding high very well. When a marked north and south lie of the isobars is thus produced it almost invariably happens that much cloud is formed over the trough area and rain falls to a considerable extent over inland districts, a result probably partly due to the cooling of the southward flowing mass of the air in the trough front. The following table gives the results in detail. The rainfall is a mean derived from twenty typical stations, ten in the northern wheat-growing area of South Australia, and ten in Northern Victoria, and refers only to the seven months' period, April-October:—