Davis has not seen any precipitation this month and will not see any tomorrow. The rumor is that this will be our first ever rain-free February. I looked back through the 50 years of data I’ve used for a couple of these recent posts, and sure enough, in those 50 years, every previous February has seen precipitation. The remarkable thing about our rain-free February in 2020 is just how much of an outlier this is. Not only have all previous Februarys seen rain, but the previous minimum number of rainy days in February is 7!
Warning: This is terrible use of statistics! The climatological probability of precipitation for February days in 36% (i.e. it rains about 1 in every 3 days normally). Given the 36% likelihood of precipitation on any given day, the likelihood of a rain-free February is just 1 in 229,688 (i.e. (1-.36)28). If that February has 29 days (like 2020), the likelihood drops to 1 in 356,951. So, it’s significantly less likely a leap year would have a rain-free February, not just a little less likely as may be intuitive. Of course, to do this calculation well, I really need a lot more data and some way of accounting for the autocorrelation of precipitation…
As a fun check of the data I do have: Februarys with 28 days have an average monthly rainfall of 3.50” and Februarys with 29days have an average monthly rainfall of 3.67”, or almost exactly 1/28 more. Cool.
