Swamp Gas.
(but in today’s DC the swamp gas is so noxious and pervasive that it no longer is a believable excuse to cover for UFO sightings.
The official Air Force explanation for the DC flyovers given in an Air Force press conference was “temperature inversion”.
https://en.wikipedia.org/wiki/1952_Washington,_D.C.,_UFO_incident
If you believe that explanation I have some swamp gas to sell you.
Lol.