The Climate and Weather of Australia/Chapter 6
VI.—HUMIDITY AND WET BULB TEMPERATURES.
Except for the head stations, these records are not very numerous or reliable. An attempt has been made to collect the data for 1910; and the wet and dry bulb isotherms for midsummer (January) and midwinter (July) are shown in Figs. 53 and 54.
From the January isotherms, it will be noticed that in the hottest regions of Australia, the Pilbara Gold-fields, Western Australia, the humidity is by no means high.
The following table gives humidity figures for the hottest and coldest months at various stations. A comparison is interesting.
| Station. | Relative Humidities. | |
| January. | July. | |
| Nullagine | 35% | 50% |
| Broome | 71 l„ | 49 l„ |
| Cossack | 55 l„ | 59 l„ |
| Peak Hill | 29 l„ | 60 l„ |
| Coolgardie | 43 l„ | 72 l„ |
| Perth | 52 l„ | 78 l„ |
| Adelaide | 36 l„ | 76 l„ |
| Melbourne | 64 l„ | 80 l„ |
| Sydney | 70 l„ | 77 l„ |
| Brisbane | 66 l„ | 73 l„ |
From this it will be noted that the humidity reaches only 29 per cent. at Peak Hill and 35 per cent. in midsummer at Nullagine.
At Broome, further north and on the coast, the humidity is 71 per cent., and it increases to 78 per cent. at Port Darwin in January. All down the east coast the heavy rainfall leads to much greater humidity than on the west coast. But, as the rainfall decreases rapidly towards the interior, the region with humidities over 70 per cent. in midsummer is very narrow.
In the chart showing the wet and dry bulb isotherms for July (see Fig. 54) the north of Australia at this time is experiencing the dry season, so that high humidities can only occur in the south. Since the average dry bulb temperature in the southern moiety is only about 50 F., none of these humidities though well above 80 per cent. in places have any deleterious effect on health.
Humidity data for the six capital cities are given in the following paragraph, together with graph relating to the monthly humidity and temperature:
- (1) Perth.—At Perth the mean annual humidity at 9 a.m. is 62; the greatest monthly mean is 83, and is in June; and the lowest, 42, in January.
- (2) Adelaide.—At Adelaide the mean annual humidity at 9 a.m. is only 56; the mean monthly humidity has been as low as 33 in January and December, and as high as 94 in June.
- (3) Brisbane.—In Brisbane the mean annual humidity at 9 a.m. is 68; the lowest monthly mean recorded is 47, and is in September; and the highest, 85, in the months of March and May.
- (4) Sydney.—In Sydney the mean annual humidity at 9 a.m. is 73, the greatest monthly average, which occurred in May, 1891, was 90; whilst the lowest monthly mean, 52, occurred in the month of December, 1911.
CHARTS SHOWING HUMIDITY, ALSO WET AND DRY BULB ISOTHERMS.
GRAPHS SHEWING ANNUAL FLUCTUATIONS OF MEAN MAXIMUM AND MINIMUM TEMPERATURE AND HUMIDITY IN SEVERAL PARTS OF THE COMMONWEALTH OF AUSTRALIA.
Explanation of the Graphs of Temperature and Humidity.—In the above graphs, in which the heavy lines denote 'temperature' and the thin lines 'humidity,' the fluctuations of mean temperature and mean humidity are shewn throughout the year. These curves are plotted from the data given in the Climatological Tables hereinafter. The temperatures are shewn in degrees Fahrenheit, the inner columns giving the corresponding values in Centigrade degrees. Humidities have not been obtained for Port Darwin, Daly Waters, and Alice Springs.
For the thin lines the degree numbers represent relative humidities, or the actual percentages of actual saturation on the total for the respective temperatures.
The upper temperature line represents the mean of the maximum, and the lower line the mean of the minimum results; thus the curves also shew the progression of the range between maximum and minimum temperatures throughout the year. The humidity curves shew the highest and lowest values of the mean monthly humidity at 9a.m. recorded during a series of years.
Interpretation of the Graphs. The curves denote mean monthly values. Thus, taking for example, the temperature graphs for Perth, the mean readings of the maximum and minimum temperatures for a number of years on 1st January would give respectively about 83° Fahr. and 62° Fahr. Thus the mean range of temperature on that date is the difference, viz., 21°. Similarly, observations about 1st June would give respectively about 66° Fahr. and 51° Fahr., or a range of 15°.
In a similar manner it will be seen that the greatest mean humidity, say for March, is about 62° and the least mean humidity for the month 48°; in other words, at Perth, the degree of saturation of the atmosphere by aqueous vapour for the month of March ranges between 62% and 48%.
Fig. 55.
- (5) Melbourne.—The mean annual humidity derived from the 9 a.m., 3 p.m., and 9 p.m. observations in Melbourne is 71; the greatest monthly average, 88, in June and July, 1858; and the lowest, 49, in December, 1908.
- (6) Hobart.—Hobart's mean annual humidity at 9 a.m. is 71; the highest monthly mean, 92, in June; and the lowest, 51, in February and December.
From the above results it is seen that in respect to relative humidity, Sydney has the first place, while Hobart, Melbourne, Brisbane, Perth, and Adelaide follow in the order stated, Adelaide being the driest. The graphs shown on Fig. 55 show the annual variations in humidity. It will be observed that the relative humidity is ordinarily, but not invariably, great when the temperature is low.
N.B.—For the thin lines (relative humidities), the degree numbers represent the percentages of possible saturation.