The Vegetation op Western Australia. 
XXXV. 
Tlie evaporating power of the air is expressed by what is termed the 
^saturation deficit. This is the amount by which tiie partial pressure of water 
vapour in a given volume of air falls short of the pressure at saturation point, 
and is thus a direct measure of tlie eva}Dorating power of the atmosphere. 
Tn the appended tables, the mean annual saturation deficit values are given, 
•as well as figures for the four consecutive wettest months. They are useful 
for })urposes of comparison, and attention should be paid to the relative 
values for inland stations. 
The Meyer Ratio, which is the ratio of precipitation to the saturation 
<deficit (expressed as an annual ratio), shows a high dc'gree of correlation with 
Precij)itation /P\ 
- 7 T 1 - I, and has been omploved in sevwal countries as an ecological 
Evaporation \E/ 
index, proving of much value in this respect. Mapping the distribution of 
this ratio with any degree of accuracy is almost impossible, since there are 
only 3f> stations in Western Australia from vhich data are }>nblished. The 
accompanying map may, however, prove of interest, although the absence 
of recording stations in the Fortescue district is unfortunate. What the 
map does show, however, is that the Meyer Ratio determines the approximate 
boundaries of the forest formations of the south-west, and there is a general 
correlation between its isoj:)leths and the boundaries of the vegetation pro- 
vinces. In the north, the 50 isopletli may be taken as representing the south- 
cun boundary of the monsoon woodland area. 
The 
Seasonal precipitation / SP 
VSSD/ 
Ratio is also given for 
com- 
Seasonal saturation deficit 
parison with the Meyer Ratio, with which it shows a certain correlation, but 
serves to emphasise still further the thermo-illuvial factor. This ratio is the 
seasonal equivalent of the Meyer Ratio, and as such perhaps serves as a better 
climatic index in areas which are characterised by a seasonal period of drought. 
Seasonal Precipitation /SP' 
The 
/o.r\ 
I Ratio is also important in indicating 
Seasonal Temperature 
types of vegetation, but is mainly of value only when considered in conjunc- 
tion with some other factor. 
Finally, amongst the climatic statistics, nn important consideration is 
tlie variability of the annual rainfall. In the accompanying tables this is 
c'xpressed as a percentage over the number of years during which records have 
been taken. It serves as an intlex for the drought factor, and in this respect 
is important when considering the climates of the Northern and Southern 
Provinces in contrast to the Eremea. 
Included in the climatic factor, atmospheric movements are of con- 
siderable importance. Especially in the dry interior, the value of wind as a 
desiccating agent determining the habit and structure of plants is very con- 
siderable. Any casual study of the vegetation will at once illustrate this 
fact. These winds are mainly of two types : tlie dry prevailing winds that 
dominate almost the entire interior, and the strong sea winds that mould the 
structure of certain coastal formations. These latter appear to exert their 
most powerful effects on the trees of the littoral between Oeraklton and Don- 
garra, and on the dune thickets of the south coast. Their influence is more 
fully discussed in the cha[)ter dealing with growth-forms. 
VI.- 
RELATIONSHIP8 BETWEEN THE C^LIMATE 
OF WESTERN AUSTRALIA. 
AND VEGETATION 
From what has already been stated it will be evident that Western Aus- 
tralia can conveniently be dividetl into three climatic provinces determined 
