It looks as if the IAD temperature peaked at about 52 this afternoon (and is now higher), so the afternoon difference between DCA and IAD was only about 13 degrees. To my surprise, I found that the greatest difference in the daily maximum temperature at the two airports is four degrees higher. That occurred on December 9, 1975, when DCA recorded a high of 53 and IAD 36. So, today did not come close to that, although there is still the question of the greatest difference at the same time. By the way, I found a third case of a 24-degree difference in the daily minimums, which occurred on February 13, 1983, when DCA recorded 16 degrees and IAD minus 8.