The Vegetatiox of Western Australia. 
XXXI. 
1,110 mm. (4,300 points), and a secondary focus of much less importance 
in the Hamersley Range. The absence of meteorological stations on the 
Hamersley Plateau leaves us in ignorance regarding the local climatic con- 
ditions, but the area experiences an increase in rainfall compared with the 
lower country which surrounds it. From both of these two focal points there 
is a gradual diminution in the rainfall. The rainfall decreases sharply between 
Camden Sound and the southern and eastern margins of the Harm Plateau, 
being about 490 mm. (1,934 points) at Broome, and 567 mm. (2,229 points) 
at Wyndham, while at Wollal it falls to 249 mm. (978 points). The seasonal 
isohyets rise to 275 mm. (1,090 points) at Whim Creek and Tambrey, at the 
northern end of the Hamersley Range, and are doubtless higher within the 
Range itself. Over the whole of the north the rainfall is very low between 
the months of April and November, being everywhere less than four inches, 
and in places below one inch. 
A climatic feature of some importance, even if not of any marked seasonal 
reliability, is the occurrence, usually between the months of January and April, 
of tropical hurricanes which, bom in the Timor Sea, sweep in a south-westerly 
direction over the sea until they reach the approximate latitude of 18 degrees 
S., when they curve in a southerly or south-easterly direction and encounter 
the coast, usually between Broome and Onslow. These violent disturbances 
may expend their energy soon after reaching the land, or they may continue 
inland, bringing with them heavy rains, often extending as far south as the 
latitude of Wiluna, and occasionally to the Nullarbor Plain. They are quite 
unreliable in their incidence, sometimes occurring twice or more in a given vear, 
at other times they may be entirely wanting for two or more years. Their 
importance as the principal rain-bringing systems of the great central area, 
is considerable, smce they determine the habitat of the grassland and steppe 
of the arid region. 
While this northern summer monsoonal system is in operation, the south- 
ern half of Western Australia lies under the influence of high-pressure systems 
and summer drought, and except for occasional thunderstorms wdiich are either 
<lirectly or indirectly influenced by the low’-pressure systems of the north, little 
or no rain falls except in the extreme south-west and southern littoral where 
the karri forest receives 10 inches of rain, and the entire south coast five 
inches during this period, due to northw^ard surgings of the systems carried along 
by the “ roaring forties ” at all seasons in the Southern Ocean. 
As the apparent northern motion of the sun tow’ards tlie Tropic of Cancer 
progresses, so the thermal equator shifts to the north a\\ay from Australia, 
and the tropical area lies under a system of laigh-pressure and drouglit which 
continues from April until November or December. This northern “ shift ” 
of the atmospheric systems causes a similar northern shift of the so-called 
“ Antarctic ” systems, bringing into operation the cycle of whiter precipitations. 
Tlie winter rainfall cycle usually commences in May, either early or late in tlie 
month. Sometimes it commences in April, and is then more or less connected 
with the late cyclonic systems of the north-west (Ashburton-Fortescue) areas. 
If, as sometimes happens, a powerful southern system should coalesce w'ith a 
nortlK'rn active low^-pressure system, the result is a general rainfall o\’er the 
entire west coast, usually regarded as a propitious opening for the southern 
season. In other years the season develops more gradually from tlie south, 
gradually extending northwards, and being without any northern reinforcc'- 
ment nunains enfeebled : in such years the inland dry regions usually experi- 
ence low’ precipitations. Tlie season retires as it begins, the rainfall' systcnns 
gradually weakening, until in October the precipitations affect th<'' lower 
south-west only, and there is a return to drought in November. 
