Sunday, August 23, 2009

Role Reversal: DC July vs. August Temperatures

Image: Washington, DC 1990-2009 July and August temperatures, CapitalClimate chart from National Weather Service data

On average, July is the hottest month in Washington, DC. The 1971-2000 base period average for July of 79.2° is nearly 2° warmer than the 77.4° August average. For much of the current decade, however, the months have reversed roles, and 2009 is turning out to be one of the most extreme examples in history. The August average through the 22nd is 80.7°, not only 3.8° above the cool July, but well above the historical July average.

In the decade of the 2000s, 7 out of the 10 years, including this one, have had warmer Augusts than Julys. There was only one such year in the 1990s; that was 1998. The most recent year before that was 1984. This means that in the last 25 years, 7 out of the 9 "upside-down" Augusts have all occurred in the most recent decade. In 139 years of Washington, DC records, the July-August reversal has been observed only 31 times, nearly one quarter of them in the current decade.

If the 3.8° excess of this August over July holds through the end of the month, it will be second only to the 4.4° difference in 1895. That July was the second coldest in history, however, while this July was much closer to average.

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