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| Giles has a dry climate with hot summers and mild winters. The annual average rainfall is 283 mm on an average 48 days and while the average rainfall is higher during the warmer months of the year, there is also considerable variation from year to year.  The climate of the region is strongly influenced by a band of high pressure known as the sub-tropical ridge. For most of the year this ridge is located to the south allowing east to southeast winds to prevail. Occasionally during the cooler months the ridge moves north, allowing remnants of cold fronts to pass over the region. While most fronts bring little rain to Giles they are sometimes linked to cloud bands to the north-west which can produce significant rainfall. However, the highest rainfall occurs in the warmer months of November to March, when a trough of low pressure extends southwards from the heat low in the tropics, and tropical lows associated with the monsoon trough move southward into the region.
By contrast winters are mild, with July average maximum and minimum temperatures being 19.9°C and 6.8°C respectively. Cold wet days with maxima below 15.0°C occur about once every winter on average. The lowest maximum temperature recorded was 9.0°C on 1 August 1975. Frost days, that is, when overnight temperatures fall below 2.0°C, occur about twice a month in a typical winter. Such nights occur on clear nights following a day of cool southerly winds. While ground temperatures are generally below zero under these conditions, it is rare to see a ground frost, as the air is so dry. The lowest temperature recorded at Giles was –1.6°C and occurred on 23 June 1981.
To underscore the extreme variation of the rainfall in the region, the lowest annual total was a mere 38 mm, and occurred in 1961. Some spectacular thunderstorms have been observed at Giles, which on average has 32 thunderstorm days per year, with the peak months for thunderstorm activity being November to February, which account for some 70 percent of events. It is not uncommon for very little rain to occur for months on end, particularly during winter. The longest spell without rain occurred, not surprisingly, in 1961, when no rain was recorded for 125 consecutive days between 17 May and 21 September. Almost half of the annual evaporation of 3457 mm occurs from November to February. The average daily rate in January is 14.4 mm per day while on a hot blustery day the evaporation can be well over 20 mm. During winter the average daily evaporation decreases to 4.2 mm. With an annual average of just 48 days of rain the air is generally dry, particularly in summer when afternoon relative humidity typically drops below 20 percent. Even during the cooler months the average 9 am relative humidity is only about 60 percent. |
January
is the hottest month, with an average maximum temperature of 37.2°
C. Temperatures above 40.0°C typically occur 7 times during the month,
although the highest temperature ever recorded of 44.8°C, occurred
on 27 December 1986. The average minimum temperature during January is
23.5° C.
The
wettest months are November to March, with February being the wettest
month with an average rainfall of 48.5 mm on five days. The heavier rainfall
during these warmer months is caused by the periodic southward movement
of the monsoon trough, and on occasion a rain depression from an ex-tropical
cyclone. The highest daily rainfall, 271.2 mm, occurred on 22 February
1974, when a slow moving tropical low interacted with an upper level trough.
The highest annual total was 843.4 mm in 2001, with 204mm falling in February
(a rain bearing tropical depression which was formerly TC Vincent) and
327 mm in December (associated with the monsoon trough).