On Apr 26, 12:18 pm, "Philip Eden" philipATweatherHYPHENukDOTcom
wrote:

Whatever, for anyone wishing to use these categories
in the modern era of tipping-bucket rain-gauges, it is
advisable to allow recordings of 0.2mm to pass without,
as it were, breaking the drought. A tip of 0.2mm is
very often the result of an accumulation of several
smaller quantities - indeed tipping-bucket tips may be
induced by vibration or by thermal expansion/contraction
of the rain-gauge housing or by insect life, when one of
the buckets is almost full. Relatively rarely is an isolated
0.2mm tip the result of 0.2mm of rain.

Philip

I have a tipping bucket raingauge (Davis VP 2) and was recently
annoyed to see, what would have been a 26-day drought "spoiled" by
falls of 0.2mm. There were only three recorded rainfalls between 22nd
March and 16th April - 0.2mm on 23rd March, 0.2mm on 6th April and
0.2mm on 15th April. So there was never more than 13 consecutive days
without recorded rain.

I often see the bucket has tipped to give 0.2mm, when it appears that
no rain has fallen, and in these cases I just clear the total and do
not record it. However in each of the above instances I was satisfied
that rain had actually fallen. There was approx 1 hr of very light
rain/drizzle on the evenings of both the 23rd and 15th, which wet the
ground and could easily have amounted to 0.2mm. I did not see any rain
on the afternoon of the 6th and was going to ignore that 0.2mm but
someone else was able to confirm that there had been a very brief
heavy shower, even though there was no sign by evening that any rain
had fallen.

I do not know of any other occasion during the 26 day period when rain
fell but I was away in Cornwall, from 7th to 14th April, and, even
when I'm here I won't notice every trivial rainfall, so it's certainly
possible that none of those 3 individual falls did actually amount to
0.2mm. Of the three falls I think the one on the 6th is least likely
to have amounted to 0.2mm. I was very close by at the time and neither
heard nor saw any rain and the person who confirmed the shower is
known to exaggerate rainfall rates (such as saying it is pouring rain
when the gauge is only registering 1mm/hr).

In view of the uncertainty of 0.2mm falls I think I may for my own
records regard a drought as 15 or more days without a fall greater
than 0.2mm. In any case 0.2mm of rain could hardly be regarded as
useful so it's hardly as if it's breaking a drought.

Richard Slessor,
Hazlehead, Aberdeen.