As the years 1888 and 1902 stand out beyond all others as drought years, a comparison between the two may be instructive. It is worth noting, to begin with, that the former was sandwiched between years of excessive rainfall, but that the latter was the culmination of a long stretch of years with low average rainfall which began in 1895. The year 1888 was characterized by extreme simplicity in weather types, there being scarcely any departure from the normal type. Pressures were higher than normal throughout the year in Melbourne and Adelaide, except in July, and expecially so in the Spring months. The low pressure systems were almost entirely of "Antarctic" type, there being few inland rains of tropical origin, even in summer, and during the seven winter months—April to October—practically no rains due to tropical incursions of low pressure systems. Cyclonic developments from "Antarctic" low pressures were absolutely wanting during the whole year, the tendency being towards a succession of high-pressure systems on the mainland, and low-pressure systems with centres far to southward passing along the south coast line. For the year as a whole, the departures from the average mean pressure were very large throughout Australia, averaging four-hundredths of an inch in excess at the six capital cities.
The year 1902 was also marked by pressures much above normal, but in other respects differed very strikingly from 1888. In 1888, the depressions followed one another over the southern waters with but little hindrance and without making many inroads upon the high-pressure systems upon their equatorial sides; in 1902, at all events during the five critical months, April to August, high-pressure systems were continually being built up over Tasmania and South-eastern Australia, apparently through the agency of large cyclonic depressions operating over Tasman Sea or New Zealand, many of which were of tropical origin. The result upon storm systems approaching from the west was generally disastrous; some were deflected too far south to affect Eastern Australia; some seemed to die out before arrival, and some were converted into feeble cyclonic circulations, which had little rain-producing effect. The last two months in both years were, however, a good deal alike, in that depressions of tropical origin became frequent; possibly owing to the approach of improved conditions.
The pressure excesses at the different capitals for the years 1902 were almost as large as in 1888, and bear out to some extent the tendency for high-pressure systems to form over Tasmania.
This would suggest two main dry weather types:—
- (a) High and low-pressure belts maintaining their respective latitudes, with little tendency to departure from normal position—probably the result of a general lack of humidity in the atmosphere. Of this, 1888 is most typical.
- (b) Unfavorable location of high-pressure systems over South-Eastern Australia and Tasmania, co-incident with the operation of great storm systems over Tasman Sea and New Zealand, and a tendency to the formation of slow-moving feeble cyclonic circulations advancing from the westward. This was most characteristic of 1902.
