Percentages

One useful way of representing rainfall across a wide area like Australia is in terms of percentages of a mean rainfall over some base period. Percentages between 0% and 100% indicate lower than average rainfalls, while percentages greater than 100% indicate higher than average rainfalls. In some northern parts of the country, dry season rainfalls are often very low, and weeks or even months may go by without any rainfall being recorded. In these places and times of the year, the monthly rainfalls may be below average (that is, percentages less than 100%) in rather more than 50% of years.

The percentage of mean rainfall maps on this web site are all generated from monthly means calculated for each calendar month using the years 1961 to 1990.

Note that the month to date rainfall percentages maps compare the rainfall recorded in the month to date with the mean rainfall for the entire month. Consequently early in the month, the rainfall percentages will typically be quite low, unless the rainfall is unusually heavy at the start of the month.

Likewise, the year to date rainfall percentages maps compare the rainfall recorded in the year to date with the mean rainfall for the entire year.

The southern wet season is defined as the months April to November (inclusive). The southern wet season to date rainfall percentages compare the rainfall recorded in the southern wet season to date (that is, since the start of April) with the mean rainfall for the entire season.

The northern wet season is defined as the months October to April (inclusive). The northern wet season to date rainfall percentages compare the rainfall recorded in the northern wet season to date in a manner analogous to those for the southern wet season.

It is quite difficult to tell from a rainfall percentages map if the rainfall has been highest or lowest on record. To see how the rainfall in the current period compares with previous years, the rainfall deciles maps should be consulted.