Lately things have been quite wet around here in my part of Florida. This is about what one would expect if we are heading into an El Nino year, but El Nino is not yet official, I think. Anyway, I was curious about what the typical rainfall in our area tends to be from day to day. The nearest climate station is apparently up at the West Palm Beach International Airport. It has (somewhat incomplete) data from July of 1938 until roughly the present (data available at KNMI run up until the 16th as of now) and the daily rainfall history looks like this:
Figure 1, Daily Precipitation Totals in inches, WPBIA
You probably notice that point near the beginning that is higher than any other point: specifically, according to the data the rainfall total for April 17 1942 was more than fifteen inches! Well, I’m not sure about that particular data point since it is so exceptional, but I’m going to assume it, and the rest of the data, are accurate. It does not appear there is any obvious trend within the daily data, though of course this mostly because it is just so noisy. What does the annual variation look like?
Figure 2, Daily Precipitation Totals plotted against “Fraction of the Year”
The above plot shows rainfall totals plotted against the “Fraction of the Year” which here is taken as the sum of the month minus one divided by twelve and the day of the month minus one divided by thirty one divided by twelve. That plot has 26420 data points. So I decided to see what the 1321 point center averages of that as a series is, each point of which representing the average daily precipitation in approximately one twentieth of the year. But first, I put a second version of the above series at the end of it, so that I could then get a full annual cycle in a smooth. So here is a full cycle of one twentieth of the year centered averages, plotted against ay of the year (well, almost, it’s actual “Fraction of the Year” times 365.2425, rounded down to the nearest integer, plus one):
Figure 3, one twentieth year centered averages of the Daily Precipitation Totals
Interestingly, the annual cycle in precipitation at WPBIA has not one wet season peak, but two. The wet “half” of the year (the above curve’s first and last points above it’s overall average) lasts from roughly mid to late May to the end of October. The two wet peaks occur during the above average parts of the wet “half” of the year (In the same manner as as the dates for the above average half year were determined, only this time with the average as the average of points above the average for all points) from the end of May to till just after Independence Day (ie early July) and from mid-August to mid-October, with the sharp peak of the latter period occurring in in mid to late September. Now, obviously this is just on average, and the edges of these periods are fuzzy even in the smoothed data by a few days. But the double peak is an interesting feature of our local seasonal weather variation. Anyone have any idea why this may occur?