D.C. summer heat versus the South: Almost as hot and more abnormal

It’s no secret that the last three summers were exceptionally hot in D.C. – the three hottest summers on record, in fact. Here is a more arresting fact: the heat  we experienced in 2010, 2011, 2012 and so far this summer, compared to normal, was more extreme than across a wide swath of the sticky South.

Populous areas of Texas, Florida, the Southeast and Gulf Coasts may have somewhat warmer average temperatures in the summer, but when considering the difference between normal and actual temperatures – or the difference from normal – D.C. has boasted more extreme warmth.

For example, Reagan National Airport has averaged just over 3 degrees warmer than normal over the last three summers, besting the average warm departure recorded in San Antonio, Texas (just over 2 degrees).

The Texas city did record a more extreme departure of nearly 4 degrees above normal in the summer of 2011; San Antonio, however, failed to exceed its average summer temperature of 84 by more than 1.7 degrees in 2010 or 2012. On the other hand, D.C. warmed beyond its average by 3.6, 3.4 and 2.7 degrees, respectively, in 2010, 2011, and 2012.

Gulf and Atlantic coastal cities – such as Miami, Fla., Mobile, Ala., and Charleston, S.C. – each experienced one cooler than normal summer since 2010, unlike D.C. and San Antonio. Not to go unnoticed, Charleston posted its warmest and second-warmest Junes on record back to back in 2010 and 2011 (83.4 and 83.2 degrees), and also tied its record for warmest July (83.7 degrees) in the consecutive summers of 2011 and 2012.


Comparison among average summer temperatures in 2010, 2011, and 2012 and the 30-year (1981-2010) climatological normal June-July-August temperature. Only D.C. has averaged warmer than normal by more than 2 degrees each summer. Chart by Rick Grow.

While the extremeness of Washington’s warmth is impressive in its own right, one should use caution when comparing summer temperature differences from normal (or anomalies) recorded for a more northern city with those logged in the South.

For one, southern areas are hotter and more humid on average compared with D.C. overall. Because of that fact, it is more difficult for them to warm much beyond the average temperature (more energy is required to warm an already hot, very humid air mass by a few degrees compared to a cooler, less humid air mass).

Secondly, the jet stream’s typical summer position well to the north in Canada prevents much change in the temperature pattern over the South. Cold fronts and associated areas of stronger winds both in advance and behind the front do not reach the region in most instances, inhibiting the transport of markedly warmer or colder air masses to Dixie.

Even this summer has started off with more anomalous warmth in the District – and especially across other parts of the northern Mid-Atlantic and Northeast – in comparison to the Southern states.

Through midweek, D.C. is running 1.3 degrees warmer than normal at this mid-summer stage; that warm anomaly will increase in size through week’s end as our heat wave continues. San Antonio is just less than 1 degree warmer than the 30-year average, while Miami and Mobile are actually running slightly colder.


Average temperatures to date in summer 2013 compared with the 30-year climatological mean temperature through July 16. Values bolded in red indicate warmer than normal temperatures, while blue bolded values denote colder than normal temperautures. Chart by Rick Grow.

Why are we holding onto warmth (albeit in a less intense form) for a fourth straight summer, while much of the South stays on the slightly cool side of normal?

Two large domes of high pressure – one over the Western U.S. and another just off the East Coast – have blocked disturbances over the Lower Mississippi and Tennessee Valleys, leading to an onslaught of wet weather. If not for the cloudiness and periodic heavy showers over the last several weeks, even more frequent heat might have snuck into the D.C. area.

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Jason Samenow · July 19, 2013

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