People consider it perfectly acceptable/understandable to "publicly" have a hard-on in areas of derect sunlight (i.e., a "clothing optional" beach), but it is still frowned upon to ask for sex from fellow tanners of the female persuasion, even though they, too, are totally "showing off their merchandise" for all to see, and thus it would be only natural for red-blooded studs in their vicinity to "want some" from them.
As with it's twin, wind chill factor, direct sunlight factor-- or DSF -- changes the temperature of an object when affected by it, in this case raising the temperature -- rather than lowering it -- in relation to the temperature of the air, due to the warming rays of direct, unimpeded sunshine on said object.
Despite the frigid temperatures at the airport this morning, Josh was able to get all the ice off his plane quickly by pulling it out of the hangar and into the sunshine where the direct sunlight factor raised the plane's surface temperature rapidly to melt it.