It is easy for anyone to describe whether they are hot or cold. But if we ask several people in the same situation, we are sure that the answers are very varied and that their thermal sensation is different. The ambient conditions, which include other variables apart from temperature, are also decisive.
To define the thermal sensation we have to analyse two parameters. On the one hand, the conditions of the human body, and on the other hand, the environmental conditions.
Human body conditions. Thermal comfort
First of all, it must be understood that the human body is at a certain temperature (between 36° and 37°), and that there is always an exchange of heat with the environment to maintain this temperature.
Therefore, one of the most important factors is the metabolic rate. This can be summarised as the heat generated by the body (as energy dissipation) when performing an activity, be it sleeping, walking, running or sitting.
The more energy consumed, the more heat is produced and therefore the more the body can withstand the cold.
Another factor is heat loss through direct contact with cold air. The skin and fur of animals has a thermoregulatory function.
As the air temperature drops, the surface blood supply decreases and the skin becomes colder. Thus, the skin and air temperature equalise, minimising heat loss. As a result, the inside of the body maintains an appropriate temperature and thermal sensation, as there is a small “thermal gap” between the skin and the inside.
The higher our body fat, the thicker the skin, the more tolerant we are of the cold.
The index of clothing (the clothes and shoes we wear) acts as a thermal insulator, so it acts in a similar way to the functions performed by the skin, making us enjoy a pleasant thermal sensation.
For this reason, it is in areas prone to being naked, such as the hands or the face, where we feel the coldest and where it is easiest to suffer from frostbite.
It is also important to know that losses through contact with water are much greater than through contact with air. It is therefore very important that our clothing wicks away perspiration so that it does not get cold on contact with the skin.
The last factor is the acclimatisation of the body. If a person who is not used to the cold is exposed to it, his or her body will react very strongly. That is why people from cold regions will not feel cold in conditions where others from warm regions will feel cold.
We can observe this when we go outside and have been on an indoor establishment. At first we will shiver and feel very cold. But when our body regulates its temperature, this thermal sensation of cold will disappear.
Environmental conditions. Meteorological variables
There are three environmental conditions: temperature, humidity and wind.
The temperature that defines this parameter is called the Globe and Wet Bulb Temperature (GWBT). This is a combination of temperature with other variables such as radiation or relative humidity. It is used to measure the exposure of the body to certain activities and is applied in the military, athletes or to assess the conditions of a job.
Its use is not very common as it requires a balloon thermometer, and it is common to use the temperature of the dry balloon, which is measured with a normal thermometer.
The humidity (heat index) is a variable widely used in hot environments, from 80°. When the humidity is very high, the thermal sensation can increase by several degrees, as the sweat itself evaporates on the skin. It is measured with a hygrometer and expressed as a percentage (%).
The wind chill factor is measured with the anemometer and is expressed in m/s or km/h. Its role in wind chill is the same as explained above for the skin.
The layer of air next to the skin that is kept in balance with the body is constantly replaced by cold air, which means that the body has to act all the time to regulate this temperature.
There are many formulas for studying this variable, developed by organisations and institutions in different countries. This is the table for the USA National Weather Official Service.