Updated each decade, the U.S. Climate Normals are 30-year averages of many pieces of weather information collected from thousands of weather stations nationwide. Each time they are updated, an old decade is dropped, and a new one added. Starting in August, when you hear that a day was hotter, or colder, or rainier than normal, that "normal" will be a little different from what it was in the past. The new 1981-2010 normals are available for download now (see below), though the National Weather Service will not use them operationally until August 1.

This time around, the 30-year window for the U.S. Climate Normals is 1981-2010: the decade 1971-1980 was dropped, and 2001-2010 was added. Since the '70s were an unusually cool decade, while 2001-2010 was the warmest ever recorded, it is not surprising that the average temperature rose for most locations. For the United States as a whole, it was not daytime highs (maximum temperatures) but overnight lows (minimum temperatures) that rose the most compared with the 1970s. Many areas of the country - parts of the Great Plains, the Mississippi Valley, and the Northeast - actually had cooler July maximum average temperatures in the 2001-2010 time period compared to 30 years earlier. At night, though, except for the southeasternmost part of the country, minimum temperatures in January were warmer. When temperatures were averaged for the entire year, every state came out warmer overall.

In Phoenix the normal average annual temperature increased 0.9 °F while Yuma warmed 1.0 °F. The normal annual precipitation decreased 0.26" in Phoenix and increased 0.16" in Yuma. Month-by-month, maximum temperatures increased in Phoenix primarily during the spring and fall months with a slight decrease during the summer and larger decrease during the winter. Minimum temperatures warmed all twelve months. Summer precipitation increased slightly with fall precipitation decreasing. The wettest month of the year changed from March to July. Similar changes occurred in Yuma, with the exception that minimum temperatures increased more uniformly throughout the year and precipitation values showed minor changes.

The general methods for calculating climate normals were established back in the 1930s by the World Meteorological Organization (WMO), and they are still followed by the United States and many nations all over the world. The consistency means that the U.S. Climate Normals are part of a reliable, long-term data record that can be used for studying natural climate patterns and climate change. The information also helps people understand what conditions they can expect wherever they may live.

You can graphically see the 1981-2010 normals, 1971-2000 normals, and how the differ by clicking on the gray boxes below. Phoenix Daily
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Probability of Precipitation Phoenix Monthly
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Precipitation Yuma Daily
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Probability of Precipitation Yuma Monthly
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