Thermal sensation: what it is, how it is measured and what factors influence it

A mass of polar air that arrived in Brazil this week dropped temperatures in several regions of the country.

The city of São Paulo recorded, on Wednesday (18), the maximum temperature of 12.6°C, the lowest since 1961 for the month of May, according to the National Institute of Meteorology (Inmet). The Federal District recorded a minimum temperature of 1.4°C this Thursday (19), the lowest rate since the beginning of measurements in 1963, according to Inmet.

However, the indices recorded by thermometers can be perceived even more intensely by individuals, according to several variables. The call Thermal sensation can be calculated in different ways depending on momentary weather conditions and there is no universal or definitive model.

In Brazil, the thermal sensation is usually associated with two models used by the scientific community, which consider the combination of air temperature, relative humidity and wind speed in the calculations.

One of the indicators is the Cooling Index (or Wind Chill), which expresses the cooling effect of moving air or wind at different temperatures. The data is widely used in winter, when the lowest air temperatures are recorded.

In low temperature situations, the wind has a greater influence on thermal discomfort, as it continuously removes the layer of warm air that surrounds our skin and works as an insulator. As the human body temperature is warmer than the environment during this period, a phenomenon occurs in which we begin to give heat to the environment more quickly, increasing the sensation of cold.

Another method used is the Heat Index, which considers the effect of humidity on temperature. The greater the humidity and temperature of the air, the greater the thermal sensation, because with the air saturated with water it is more difficult to sweat, a process responsible for “cooling” our body. This model is adopted in the analysis of hot periods, such as in summer, when the air temperature reaches maximum values ​​and the relative humidity remains high for most of the season.

The study of thermal sensation prediction models or thermal comfort indices reveals an extensive and varied amount. There are more than 200 counted in the bibliography and the amount continues to increase as new elements are included in the calculations.

Factors that influence thermal sensation

The trend is to use indices that consider environmental factors, type of clothing, physiological and sociocultural characteristics. Thermal sensation also considers combinations of weather elements such as radiation, air temperature, relative humidity, wind speed and direction.

The physiological and metabolic characteristics of each individual are also part of the calculations. An overweight and sedentary person will be more affected by high thermal sensation than an individual with ideal weight for their age and height and who exercises, for example.

Clothes will also influence the thermal sensation, as they can contribute to or impair body temperature regulating functions. Sociocultural characteristics are also part of the variables related to thermal sensation: people who live in locations commonly affected by intense cold acquire tolerance to cold and may perceive sudden drops in temperature differently.

Source: Mauricio Sanches Duarte Silva, PhD in Environmental Engineering Sciences at the School of Engineering of São Carlos (USP), with an emphasis on thermal comfort and climate indices for tourism.