Endless Summer XII: September's Sweltering Summary

For related record temperature posts, see:

• Record Heat
• Record Cold

The recent all-time record high temperature at Los Angeles got all the press attention, but that event was just one of literally thousands of daily high temperature records set in the U.S. during September. This continues a trend of heat records which has prevailed for nearly all of 2010 so far.

Following the blistering first 3 weeks of August, in which heat records outpaced cold records by a 7:1 ratio nationwide, a cooler surge in the final week brought the final ratio to 3.5:1. This was still much above the 2:1 average of the previous decade.

In September, daily high temperature records numbered nearly 2400, roughly 1/3 above the previous monthly high for the year, which was slightly less than 1800 in April. With only 459 daily low temperature records, the ratio of heat records to cold records was 5.2:1, the highest for any month since April's 6.1:1 dominated the spring statistics. The year-to-date ratio has now risen to 2.6:1. Every month this year from March onward has had at least twice as many heat records as cold records, except for May, when the ratio was 1.4:1.

September also had a large number of record high minimum temperatures (1,286) vs. low maximums (557), a ratio of 2.3:1.

The analysis from NOAA's Earth System Research Laboratory shows that average temperatures in September were warmer than the 1968-1996 base period over nearly all of the U.S. (Green/yellow/orange areas are warmer than average, and blue/purple areas are cooler on the map.) The notable exception was over the extreme northern Plains and the immediate Pacific Coast. This was consistent with the pattern prevailing for the entire June-September period.

Images (click to enlarge):
- Total number of daily high and low temperature records set in the U.S. for spring 2010 (March-April-May) and monthly from June through September 2010, data from NOAA National Climatic Data Center, background image © Kevin Ambrose. Includes historical daily observations archived in NCDC's Cooperative Summary of the Day data set and preliminary reports from Cooperative Observers and First Order National Weather Service stations. All stations have a Period of Record of at least 30 years.
- U.S. surface temperature departure from average (°C) for Sept. 2010 and June-Sept. 2010 from Earth System Research Laboratory/NOAA