When temperatures are measured over a long time period, mean monthly values are fairly representative of typical values. That’s because recording errors can easily be excluded and each month usually has a well-defined range of values. Of course, this only applies when climate conditions are steady. If they are changing, a long-term mean will not necessarily be representative.
Rainfall amounts are different from temperatures since individual values over a month can differ by orders of magnitude, for example, 10.0 inches vs. 0.1 inches. This means that monthly rainfall averages may not necessarily be as good an indicator of how much rain to expect for that month as average temperatures are for monthly temperatures. We will see an example of this on the next page when we calculate the rainfall average for two months.