In my 45 year observation of DCA the discrepancy between it and BWI and Camp Springs and my station is as follows for spring, fall and winter.
Clear, windless night with average or lower humidity = +7 to +12.
Cloudy =+5 to +10
Rain-=+4 to +8
Raging snowstorm=+3 to +4
Vey windy: 20 gusting to 40 all night+2 to +3 ,
There are a lot of theories. . I have lived it for decades