|
E. Linacre |
9/'98 |
The dewpoint drops off with distance inland in Australia, in a remarkably even way (Fig 1). A comparison between the dewpoint maps and rainfall maps (Fig 10.12) for both seasons highlights the fact that water vapour availability is at least as important as upper level forcing in explaining rainfall amounts.
In January (summer) the dewpoints in the middle of the continent are 8-10ºC. This implies a mixing ratio of less than 8 g/kg near the surface. At this time values along the east and north coasts may exceed 24ºC, i.e. over 18 g/kg, and these areas receive more than 300 mm of rain per month. The corresponding mixing ratios in July (winter) are 5 g/kg and 11 g/kg. The driest areas are about 4 degrees further south in summer, because of the seasonal displacement of weather patterns.
Of course in winter the dewpoint values are lower because the air is colder, NOT because the relative humidity is lower. For instance, local residents will agree that the 12ºC dewpoint in Perth in January feels dry compared to the 8ºC dewpoint in July. The average 9am relative humidity is about 50% in January and 85% in July.
Fig 1. Monthly mean measured dewpoints (ºC) at 9am in January and July across Australia. No correction has been made for elevation to achieve sea-level equivalence. (Figure drawn by Jenny Kesteven of the Centre of Resource and Environment Studies at the Australian National University)