Temperatures at 3, 4 and 5 p.m. at BWI airport Thursday (circled in black). High temperature (covering the afternoon hours) during the period also circled in black. (National Weather Service, adapted by author)

Thursday night, he wrote on his Facebook page: “Something is fishy with BWI reported high temps. [Thursday’s] high was 62F, but 3 straight hourly (3pm, 4pm, and 5pm) were 59F (the highest). No way temps jump up and then back down 3 degrees between hours.”

I replied: “We had highly variable cloud cover today. Could have easily and likely did go from mostly cloudy to mostly sunny in between obs and a 3 degree rise isn’t that big of a stretch...”

Justin wrote back: “I am on [chatting] with NWS Sterling right now The issue is that the 62F was reported at 3:45pm, then 3:54pm was back down to 59F after holding that solid temp for 3 hours... They are looking at 5 minute obs, which are in Centigrade. 15C at 1535, 1545 up to 17C, then back down. It was the wind direction for the brief warm up that went due west or 270 degrees vs 310 or so the rest of the day. This is a flaw in the location... due west is a runway ‘artificially’ adds heat in the afternoon if the wind flows from there.”

I then posted: “I will admit that’s a little bit strange--and maybe there’s an issue... though I did notice how quickly it warmed and cooled today as sun went in and out of clouds. Keep us all posted as to what you find out...”

After thinking some more about Justin’s runway theory, I have a hard time buying it. To me, it’s much more likely the sun popping out from the behind the clouds (mostly) drove the temperature spike, not a mere 40 degree shift in the wind direction. With temperatures mainly in the upper 50s Thursday coupled with considerable cloud cover, how would the runway be hot enough to cause the temperature to rise 3 degrees in such a short span even if the wind did blow straight across it?

The blog popular with climate change skeptics, WattsUpWithThat, has weighed in on the issue and is convinced asphalt caused the temperature spike. Many climate change skeptics think observed warming in the U.S. in recent decades in the U.S. may be overstated because analyses haven’t sufficiently corrected for “artificial” warming from contaminants like runways near airports (that heat up faster than the grass they paved over).

Anthony Watts, the blog’s curator, who has analyzed siting issues with weather stations across the country, concluded without reservation the wind/asphalt coupling was responsible for the spike.

“Seems an open and shut case,” he wrote.

Not so fast. I queried the National Weather Service Office in Sterling about this via email. Chris Strong, the warning coordination meteorologist, replied, and sided with my perspective. Here’s an excerpt from our email conversation:

Jason: I think it’s entirely plausible the temp spike was due to change in cloud cover - not artificial UHI [urban heat island or “runway”] effects.

Chris: Agree with your statement. Having a 3 degree inter-hour temperature rise is not an uncommon thing. The BWI ASOS [Automated Surface Observing System] does an accurate job of measuring the environment of BWI airport - it is calibrated quarterly by our technicians. While it would be ideal for temperature measuring to have nothing but a uniform, open, grassy field over our entire world, the ASOSs at airports report the weather where the FAA needs it. Our measurement of temperature is representative of the spot where it is measured.

So Chris seems to agree with me that the sun had more to do with spike but qualifies his answer a bit, leaving open the possibility the “environment” of the sensor could play a role.

Reasonable enough.