In S.F., the average inches of rainfall for Jan, Feb, Mar are 4.4, 3.2 &
3.1. Assuming one day of rain is about 0.4 inches on average, that is 11
days, 8 days and 7.75 days of rain in these months, historical average.
Total average days of rain for Jan-Mar is 26.75. Total days is 90.25
(counting Feb as 28.25 for leap years). Thus, a rough average
probability for any given day from Jan to March to have rain in S.F. is
about 30%. It's a bit lower for Oakland & GGF.
We have had 4 autocrosses so far, in 3 of which it rained. The
probability to get rain on all three of these days is 0.3 * 0.3 * 0.3 =
0.027. Thus getting rain on all three of these days is only 2.7% likely.
And that's if they are all in SF. Some were in GGF and Oakland, which
get slightly less rain, so the probability is actually a bit lower.
Furthermore, now it looks like rain tomorrow morning on the school. This
further reduces the probability to a number so small that it stretches
the limits of credulity to imagine that it has occurred naturally.
Can anybody explain what is happening? Is there some phenomena going on
causing it to rain on every event this year?
Michael R. Clements
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