The annual distribution of days with a high heat index provides insight into the health hazard of heat. Computed by combining temperature and relative humidity, the heat index provides a measure of apparent temperature, the temperature that reflects comfort or discomfort. Often, high temperature alone can be compensated for by evaporative cooling such as from transpiration. But if the air is nearly saturated with moisture, then that cooling potential is reduced and the apparent temperature increases. Here a standard heat index is used where 35 degrees is a high threshold beyond which humans not only feel uncomfortable but where health dangers increase rapidly.
The graph shows projected change in Number of Heat Days (Tmax > 35Â°C) per month by 2050 compared to the reference period (1986-2005) under all RCPs of CIMP5 ensemble modeling. Positive values indicate that number of heat days will likely to increase compared to the baseline, and vice versa. The shaded area represents the range between 10th and 90th percentile of the projection.
The calculation of the heat index requires both temperature and moisture output from climate models. Particularly the moisture field is not always as well reproduced by models as one might desire, and thus the uncertainty in the calculated heat index for any model is fairly high. Additionally, moisture was reported by fewer models than temperature, and therefore the total number of heat index projections is more limited. Together, the ranges of projected changes can be quite wide for any of the RCPs. However, comparing the medians generally provides the expected picture of more pronounced increases under the higher emission RCPs compared to lower emission levels. Not only are the expected counts of days higher, but also the duration of what could be called the heat season, the period of year within which days with excessively high heat index, is generally extended.