The climate of a region can be characterized statistically in a number of ways. One of the simplest statistical tools to use in this regard is the arithmetic average or mean. Also, often the relationship of the actual rainfall or temperature information observed in any given month to the longer term average is used by the media and the public to decide how "unusual" a given monthly rainfall or temperature is. Ultimately, to the average person, a series of what he or she thinks are "unusual events" suggests that climate may be changing.

However, there are problems with a mere usage of the average to characterize the climate of an area. These problems may result in a person misassessing the "unusualness" of events and wrongly concluding that "climate is changing." To illustrate this, we will examine the precipitation record of San Francisco.

Although the location of the San Francisco rainfall gage has moved many times in the last 150 y, its location has never varied by more than several miles, and its general location east of Twin Peaks has been consistent. Thus, one can trust that any variations in the rainfall over the last 150 y have been due to natural cycles and not to a change in the microclimates sampled by the station.

A first attempt at such characterization of the climate involves an examination the mean or average value for the period of record AND the range of the extreme values observed in the same period. In the case of the San Francisco rainfall record (Fig. 1), the maximum extreme value is around 50 inches, and the minimum extreme value is around 10 inches. This defines a sort of "envelope" which should encompass all the seasonal totals in the period of record.

Second, the average or mean value for the period of record is around 21.79". The fact that the mean value is more similar to the minimum value in the record reflects the fact that many seasonal rainfall totals for San Francisco were less than the mean value, with only a few very wet years.

In order to ensure that the average value of rainfall is truly representative of the present "climate", the so-called "normal" rainfall (or temperature, etc.) is defined. This is the 30 y average ending in the last year of the previous decade (i.e., 1961-1990; 1971-2000). The normal rainfall gives an estimate of the average rainfall during the most recent climatic past. The black curve on Fig. 1 gives a "moving" average of the 30 yr average since that calculated for the period 1849-1878. Note that the 30 yr average decreased in the 20th century, but has been on the upswing in the last decade.

An examination of Fig. 1 shows that typically, however, the average value does NOT occur in any given year and that the average itself has fluctuated somewhat over time. In fact, one of the interesting things about the climate of California (and, indeed, the Mediterranean climates around the world) is that the mean value is a very poor estimate of the amount of rainfall one would expect in any given year.

Compare Fig. 1 with a similar graph showing the annual precipitation for the period of record at Central Park. We will explore this topic more when we discuss "rainfall variability." You can see that rainfall values in any given year also deviate from the long term average at Central Park, but you may also notice that the swings appear to be less marked.

To see this better, take a look at the two charts below. These charts really are the basically the same as the two charts above, but instead of graphing the manner in which yearly or seasonal rainfall totals have varied across time, the charts show how much rainfall occurred in each season relative to the long term average. This is converted to a percentage and then plotted for each year instead of rainfall.

Take a look at the two charts and see if you can see that the average rainfall is a much more reliable indicator of how much rainfall might occur in any given year at New York, than it is at San Francisco. We will see that this is one of the most striking features that discriminates the rainfall climate for places experiencing the Mediterranean Climate from those in other parts of the world with similar annual precipitation amounts.