Overview of Historic Climate Variation

Here we examine the long-term variation of total annual (or calendar year) precipitation, days of precipitation, and precipitation intensity in the Mojave Desert region. The results of this analysis are illustrated in the following figures:

  1. Number of weather stations reporting by year 1892 - 1996
  2. Range of standardized precipitation, 1898 - 1996
  3. Total Precipitation standardized units, 1892 – 1996
  4. Number of days with precipitation annually, standardized units, 1892 – 1996
  5. Precipitation intensity, standardized units, 1892 – 1996

Before 1898, the number of weather stations operating in the Mojave Desert was only 1-3 (Fig. 4), which is too small to calculate meaningful statistics. From 1899 on, however, the number of stations gradually increased, reaching 25 by 1947 and about 48 by 1959. To analyze precipitation, the summary data was standardized for each weather station (i.e., average = 0 and standard deviation = 1) and the standardized values were averaged station-by-station and year-by-year. These averages are the "standardized-anomaly index" of Katz and Glantz (1986). Generally, this index encapsulates the spatial pattern of precipitation or some other climate factor with a single number. A large positive value indicates above normal precipitation, conversely, a large negative value indicates below normal precipitation.