Windchill is strictly a comparison. The "baseline" for the comparison is conceptually an unclothed individual moving through calm air at about 4 mph (a pretty brisk walking pace!). The comparison works like this:
The wind is blowing at 20 mph and the temperature is 20 deg F. If I were out naked, how would this make me feel? The answer (from the formula!) is that you'd feel like you were naked, and walking briskly through calm air at approximately -11 deg F. You might also feel pretty silly for having forgotten your coat and pants! The goal of the windchill temperature is to relate (perhaps extreme) conditions to something we have all likely experienced, namely: briskly walking naked through calm air at certain carefully calibrated temperatures! [If naked, would you walk anything but briskly?].More seriously: the windchill described here was developed around the time of World War II, partly to provide an easily calculated index for battlefield planning. It was formulated to determine risk factors when operating outdoors under various conditions, and gives a very rough idea (in easily assimilated terms) of potential problems caused by the combination of wind with cold.
Note that windchill described here is not dependent on relative humidity, both by definition and the observed fact that humidity has very little effect on cooling, particularly at low temperatures. Further, it is implied that if there is no motion, there is no cooling (thus the comparison to a reference windspeed of 4 mph); additionally, the original work was done on a can (or plastic container) of water, with the assumption that there was general extensibility to exposed skin.
There is nothing "exact" about windchill: it is an approximation or estimate. Other indices of "physiological" or "perceptual" temperature exist.
Court, A., 1948: Windchill. Bull. Amer. Meteor. Soc., 29, 487-493.
Siple, P.A., and C.F. Passel, 1945: Measurements of dry atmospheric cooling in subfreezing temperatures. Proc. Amer. Phil. Soc., 89, 177-199.
Steadman, R.G., 1971: Indices of windchill of clothed persons. J. Appl. Meteor., 10, 674-683.