Best guess, freezing rain? Departed DCA several years ago in a Falcon 20 in moderate freezing rain. We did it right, knew it was coming, Filled the plane with fuel the night before and put in a heated hangar. Waited until the pax showed then called for taxi while still in the hangar. Ground said we were first for TO because everyone else were in deice. We taxied out and had a short wait for a plane landing. It was amazing the ice we picked up climbing out, clear above 4000. Glad I was not in a plane with boots! AA slid an MD80 off the runway an hour after we departed.