## Monthly Rainfall in January, February and March

The

Borrego Sunprinted the monthly weather records from the weather station at the State Park Colorado Desert District Headquarters for the years 1962 to 2011. The weather station was apparently just east of the Borrego Palm Canyon campground until 1977, when it was relocated one mile south to the building near the corner of S22 and Palm Canyon Drive. Those locations are close enough that they should have roughly the same rainfall on average.

Note that the rainfall drops off markedly to the east of this weather station site, perhaps by a factor of two at the Imperial County line, and increases markedly to the west of this weather station site, perhaps by a factor of two near Ranchita.This page first gives statistics on the monthly rainfall recorded in January and February in those years. Those months were combined since they had the same average rainfall per month.

Table 1 gives a basic summary of that data, and a histogram of the monthly rainfall in plotted in Figure 1.

## Table 1. Monthly Rainfall Statistics for January and February

Statistic Rainfall (inches) Minimum 0.00 Median 0.65 Average 1.20 Maximum 8.78 The median rainfall is the midpoint of all the monthly totals; half the months have less rainfall, and half the months have more rainfall. The average rainfall adds all the monthly totals and divides by the total number of months.

The median rainfall is the best statistic to use to get an idea of what is the typical rainfall, for the same reason that the median home price is always used to give the typical home price instead of using the average home price. In both cases, the values are not distributed on a typical bell curve symmetric about the median value.

For home prices, it doesn't make sense to average a one million dollar house with a $100,000 house, giving an average of $550,000 since there are few houses at the high end of that distribution. Similarly, it doesn't make sense to average a 45 inch rainfall year with a 5 inch rainfall year, giving an average of 25 inches, if the typical year has 20 inches of rain.

The average rainfall is always higher than the median rainfall, because the histogram of rainfall totals does not fit the usual bell curve in which the average is the same as the median, and there is an equal probability of having x inches above the average as having x inches below the average. Instead, there is an equal probability of having x

timesthe median rainfall as having 1/x times the median rainfall.I.e., with a median rainfall of 0.65 inches in a month, there is an equal probability of having 1.30 inches and having 0.325 inches in a month. In this simple case of three months with a rainfall of 0.65 inches in one month; 1.3 inches in another; and 0.32 inches in a third month, the median is 0.65 inches but the average is 0.76 inches.

Fig. 1 shows a histogram of the monthly rainfall totals for the 100 January and February months from 1962 to 2011.

Fig. 1. Histogram of the 100 monthly rainfall totals for January and February from 1962 to 2011.7% of all the months in this record have no rainfall at all; 32% have rainfall less than 0.20 inches; 50% have rainfall less than 0.65" and 50% have more; and 32% have rainfall above 1.0 inches.

Table 2 gives the statistics on the total rainfall from January through March, and Fig. 2 plots the January through March rainfall by year.

## Table 2. Total Rainfall Statistics for January through March

Statistic Rainfall (inches) Minimum 0.0 Median 2.3 Average 3.1 Maximum 12.5 Only a single year, 1972, had no rainfall from January through March. The wettest year was 1980, with a total of 12.5 inches. One third of the years had a total January through March rainfall less than 1.5 inches; half had less than 2.3 inches and half had more; one third had more than 3.7 inches.

Fig. 2. January through March rainfall totals from 1962 to 2011, along with a 7 year running median.Fig. 2 shows that the decades centered on 1980 and on 1992 were wetter than normal, with all other decades drier than normal. Data were not analyzed for 2012 to 2015, but those years were very dry, continuing the dry decades since ~1995.

I thank Kate Harper for providing me with a printed copy of the rainfall records printed in the

Borrego Sun, and providing the pictures in Fig. 1, and Fred Melgert for pointing out that the weather station had been relocated once.

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Copyright © 2016 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
http://tchester.org/bd/weather/rainfall.html
Comments and feedback: Tom Chester
Updated 5 January 2016
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