Some of the cameras show either a lot of fog or a lot of smoke. Earlier it really didn't look like fog because of how quick it was moving and it was not spread out like fog would be.
Live Webcam 1 - Reagan National Airport - Washington D.C.
Live Webcam 2 - 4K - Reagan National Airport - Washington D.C.
Live Webcam 3 - 4K - Reagan National Airport - Washington D.C.
Fog typically settles down near ground level from above. Smoke typically rises from ground level.
Fog happens when warm moist air passes by cold cool surfaces. Smoke comes from heat and heat rises up.
Example of fog
Fog normally is white looking like clouds usually are.