This depends in turn upon the degree of saturation of the air, which may be measured in terms of relative humidity. On the other hand, under conditions that lead to comfort, with only light activity the main evaporative loss is that from the lungs, with little or no contribution from perspiration, at least until the upper limits of comfort are reached. A net heat exchange by radiation takes place between any two surfaces when there is a difference in temperature between them. In summer the exterior wall and window surfaces of a room may be appreciably higher than air temperature, while in winter the reverse situation may exist. Thus, the net radiation exchange between a body and the surrounding room surfaces can vary independently of convective losses.
Air motion is another factor that can have a marked effect.
Increased air speed over the body and clothing surfaces can increase convective losses and, when there is perspiration, the evaporative losses as well. Thus, under conditions of high temperature and high humidity, discomfort can often be greatly reduced by increasing the air flow. It is of more than passing interest to note that under these conditions even high air speeds can be pleasant. With cooler conditions, however, even small localized air circulation may give rise to complaints of drafts. Comfort Index and Effective Temperature Air temperature alone is not the only determinant of the thermal influence of room conditions and should not be expected to correlate well with thermal sensations of comfort under all conditions.