Oh man, this is awful. The weather instruments here at The Sun were reading 93 degrees at 10 a.m. and they're still going up. The heat index numbers have been fluctuating between 109 and 112 degrees as the breeze causes humidity numbers bounce around. 

The overnight low at BWI was 80 degrees. A check with meteorologist Andy Woodcock, out at the NWS forecast office in Sterling, Va., found that this was only the third time since the official observations moved to BWI (Friendship) Airport in July 1950 that overnight temperatures have failed to drop below 80 degrees.

The other occasions were on June 26, 1952 and July 22, 1972.

The list is much longer if you include all dates since weather records began in Baltimore in 1871. Back then, the readings were taken in downtown Baltimore, where heat-island effects cause higher daytime temperatures, and retard overnight cooling.

Development and paving at BWI may be contributing to a heat island effect out there, too.

Here at Calvert & Centre streets, the low this morning was 84 degrees. It was 85 at the Maryland Science Center. 

Mercy.