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Climate: Trends and Variability

Climate is the average weather of an area over time. The climate of an area can be summarized and reported in many ways – here are some common metrics:

  • Daily mean temperature: calculated by averaging hourly temperatures, when available; otherwise calculated by taking the average of the daily minimum and maximum temperatures
  • Daily temperature range: calculated by subtracting the daily minimum temperature from the daily maximum temperature
  • Monthly mean temperature: calculated by averaging the daily means for each day of the month
  • Annual mean temperature: calculated by averaging the 12 monthly means
  • Annual temperature range: calculated by taking the difference between the warmest and coolest monthly mean temperatures
  • Daily precipitation: total precipitation measured over a 24-hour period
  • Average monthly precipitation: calculated by averaging the total precipitation of the same month over 30 years

Different average temperatures.Yet, depending on which days are averaged, the calculations of the climate for the same site might be drastically different. Consider this example for the University of Arizona. Average July temperature in 1986 was 86 degrees Farenheit. Average July temperature for the period 1985-1990 was 87.3F, and average July temperature for the period 2000-2005 was 89.2F. So depending on the year(s) considered, average July temperature on the campus of the University of Arizona varies by several degrees.

Understanding the factors comprising a calculation of a region’s climate is very important. A basic understanding of how data are summarized can help us make sense of the nightly weather report and possibly impact the timing of land management activities. They can also help us put each day’s weather conditions into context – is it unseasonably hot? Has this year been dry? Are we in a drought?

In this unit, we’ll explore some of the terminology and methods used to summarize and report climatological datasets.


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