The heat and humidity in DC metro during the summer can be absolutely grinding. It's almost something alive.
I'll second that. I loathe going there for work in the Summer. I finally got wise to the "slow motion walk" which I use to walk the 300 yards from the Metro to the building I generally have to go to without breaking out in a sweat. Otherwise, not being acclimated, a normal walk will turn me into a human lawn sprinkler before I get through the front door, then another 30 minutes till the sweating finally subsides, then another hour or so until my clothes dry out.
