Guest post by Indur M. Goklany

On June 29th, the temperature at Reagan National Airport (DCA) in Washington, DC, hit 104 °F and it was duly noted by all and sundry that this was the hottest June day EVAH. Typical was the Washington Post story:

D.C. shatters all-time June record high, sizzles to 104

Mark Richards, weather observer at Reagan National Airport, says the temperature at 2:48 p.m. hit 104, blowing by the old June record of 102 set on June 9 in both 1874 and 2011. We are now experiencing D.C.’s hottest June temperatures in 142 years.

Indeed, 104 °F was the highest temperature ever measured in June in the vicinity of Reagan National Airport. But was it the warmest day in Washington, DC, ever?

This is what Reagan National Airport looks like in the present.

Figure 1: Photograph from 2011. At left foreground is the Jefferson Monument. Behind it on the other side of the river, with the plane hovering over it is Reagan National Airport. Note the development, Crystal City, on the right hand side, also on the other side of the river.

But here is a photograph that shows us what this area look like a few decades ago.

Figure 2: This picture, taken in 1942, shows the Jefferson Monument under construction. There is no Crystal City on the right, nor is there any Reagan National Airport. In fact, as one can see, that area was still being filled in. In the 19th century, the area occupied by the Memorial and adjacent land was also water, since much of this is also filled-in land.

Clearly, comparing temperature readings taken in 2012 at Reagan National against those taken over past decades at the same location is not an apples-to-apples comparison. That is, the data are not homogeneous. And whether the claim that June 29th, 2012 was the warmest Washington June day in 142 years is correct (or not), that claim cannot be supported by merely looking at the temperature readings at the airport.

The two degree difference between the previous record reading and the June 29th one may well be due to both the urban heat island effect and the “airport heat island effect,” a much understudied phenomenon (despite the fact that anyone who has stepped on asphalt in the middle of summer knows that the only thing worse is walking on coals).

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