Watts in a Comfort Index: evaluating pedestrian energy exchange and thermal stress in urban environments

The thermal environment that is experienced by pedestrians in cities is commonly, and habitually, described in terms of temperature. In some cases it is the near-surface air temperature that is used, and in others it is a mean radiant temperature, often embedded within a physiologically equivalent temperature or similar comfort index which is used to quantify, in degrees on a temperature scale, the thermal effects of the environment on a person. While temperature is indeed our most familiar and intuitive measure of thermal states, it is important to remember that the human body’s thermal endings are not in fact sensors of temperature, but rather of heat flow – monitoring the rate of heat gain or loss from the body due to radiation, convection and evaporation (and to a lesser extent, conduction). This paper describes the validation and implementation of an alternative approach for assessing the thermal environment in urban spaces, using the Index of Thermal Stress (ITS). Rather than attempting to portray the
effects of sun, wind, temperature and humidity as a single point on an imaginary thermometer, the ITS is based on an assessment of the individual energy exchanges between a pedestrian's body and the surroundings – expressed in watts – and the physiological response that is required for the body to maintain thermal equilibrium.