Rainy days

Back in the day a meteorologist commented that it took 5 mm of rain to do more than lay the dust.  This was in the context of trying to maintain a vegetable garden in Canberra under level 3 water restrictions.  I thought it might be interesting to look at BoM records for Mallacoota to assess the number of days per year with falls of 5mm or more.  That is summarised in the following chart.

There is no trend in the data but the downward tendency of the last 4 years is quite obvious.  I have included the data for Gabo Island as a crude form of quality assurance and as the series have a correlation coefficient of 0.76 the pattern can be rated as quite similar.  This also "plugs the gap" in the mainland data from 1979-83 where several months are either totally missing or - in the case of 1979 and 1983 - monthly totals are given but many daily totals are missing.  Interestingly, where daily totals are given in 1980 and 1983 a high proportion are well over 20 mm: given the appearance of an arrow symbol in the table above many of these I believe them to be the situation described by BoM as

"For observations of daily rainfall which span more than one day it indicates that there is some uncertainty associated with the exact date on which the daily rainfall occurred."

This illustrates the need to look at the detail of the data and not just the numbers.

I also compared the number of days with falls >5 mm with the number >0.2 mm (the latter being to exclude days with a heavy fog rather than rain).

This illustrates the problem as being lack of sustained rainfall >5mm: the number of days with falls between 0.4 and 4.8 mm is about normal.