Thayer Watkins
Silicon Valley
& Tornado Alley

Rainfall in San Jose, California:
Statistics and Histograms

This is part of a study of the probability distribution of rainfall. It utilizes a historical record of daily rainfalls for the period from January 1, 1908 to June 30, 2000. The probability of occurrence of rainfall was computed month by month and the results are shown below.

As shown in the above chart the occurrence of rainfall is seasonal. The highest probabilities of rainfall are in January and February, although December is not far behind. The probabilities decrease throughout the spring months and reach a minimum in the summer month of July. After July the probabilities increase slightly in August and more so in September and rising throughout October and November.

Precipitation is scant in the summer not only because the chances of rain are much less then than in the winter rainy season but also because the average rainfall during a day of rain is much less in the summer. The graph below shows the average rainfall during a day in which rain occured for each of the months.

A day of rainfall in July involved a small fraction of the rainfall during a day of rain in January. August is a month of low precipitation also but for August the reason is the low frequency of the days of rain rather than that the rainfall per day of rain being low. The average for August is about as high as for October and only slightly lower than for April.

The September figure is higher than for October but this may be due to the heaviest rainstorm of the period. In September of 1918 it started to rain on the eleventh. On that day there was 0.47 inches of rain, On the twelve came the deluge, 4.32 inches fell that day. There was another 1.53 inches on the thirteenth and a trace of 0.03 inches on the fourteenth. Over this four day period 6.35 inches of rain fell. The prune plum crop of Santa Clara County was harvested and out on the ground in the drying frames when the storm hit. Most of the crop was lost and it was said, "Every corner of the Valley smelled like a distillery in distress." This epic storm significantly affected the statistical averages.

The monthly distributions of rainfall (for the days that had rain) are shown below. Note that the horizontal scale is in units of 0.1 inches up to 3.0 inches but that the last two categories are for rainfalls of 3.0+ to 4.0 inches and 4.0+ to 5.0 inches.

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