The clo: A utilitarian unit to measure weather/climate comfort

A number of indices have been devised to assess human comfort under a variety of atmospheric conditions. In the USA the most widely used, particularly by the broadcast media, are windchill for cold conditions and the Heat Index for hot. Both provide measures of instantaneous conditions and are useful as weather stress guides; each is evaluated in this presentation. Thereafter, the clo (a measure of comfort that provides an indication of the clothing required under a given set of conditions) is examined as a potential alternative comfort index to those in current use. Derived many years ago, the clo expresses the resistance to heat transfer by clothing and is expressed relative to units of thermal insulation. Derived clo values are compared with values of Heat Index and windchill for the same data sets at a selected station and significant relationships are obtained. The results indicate that the clo value could be used as an alternative to the current weather stress indices. Of note is that the clo permits use of a single index applicable to all seasons. The clo, however, may also be used as a guide to relative comfort representing long-term average as well as extreme, instantaneous atmospheric conditions. It serves as a climatic index by providing a seasonal clothing index. To demonstrate its potential value, seasonal clothing guides for the USA, Europe, and Australia are provided.