The physiological and behavioural responses of people to outside thermal conditions will be influenced by air temperature, radiant temperature including the sun, air velocity (wind) and humidity (also related to fog, rain, frost, snow etc). The responses will also depend upon the clothing worn and the activity carried out. A climate index is a single number that integrates the effects of the above factors to represent the thermal strain caused by the climate. The aim of the research presented in this paper was to evaluate existing thermal indices for use in outdoor climates. Laboratory experiments into the effects of solar radiation and field studies measuring weather conditions as well as physiological and subjective responses of people are presented. Comparison of the responses of people with values of thermal indices (Wet Bulb Globe Temperature (WBGT); Predicted Mean Vote (PMV); Wind Chill Index (WCI); provided an indication of the validity of current thermal indices. The results of a series of laboratory studies showed that the PMV index calculated using mean radiation temperature in the shade, and corrected for solar radiation (increase by 1 PMV scale unit for every 200Wm-2 of direct solar radiation) provides an index useful in the sun (PMVsolar). The field studies demonstrated that both the PMVsolar and WBGT indices provide good correlation with human response to outside conditions.