Thermal Sensation

We explain what thermal sensation is and what factors influence it. In addition, we tell you how it is calculated and how it differs from temperature.

What is thermal sensation?

The thermal sensation or perceived temperature It is the degree of cold or heat that the human body perceives in its environment depending no longer exclusively on the temperature recorded by the thermometer, but also on environmental factors such as wind speed or the degree of ambient humidity.

This perceived temperature or temperature perception is normally expressed in an index published by the meteorological services of each country, to warn citizens that there is a notable discrepancy between the official temperature and that which they will feel when they are outdoors. This index It is usually expressed in the ordinary unit of temperature measurement, either on the Celsius (°C) or Fahrenheit (°F) scale. In some English-speaking countries, the heat index (“heat index”) in summer and the wind chill (“wind cooling”) in winter.

The concept of thermal sensation emerged in the middle of the 20th century, during the Second World War, thanks to the experiences of the North American explorers Paul A. Siple (1908-1968) and Charles Passel (1915-2002) in Antarctica, where they presented different bottles with water into the environment and measured the time it took for the liquid to freeze, depending on atmospheric conditions and wind.

His findings on heat loss were used by the American Weather Service, and from there emerged the first formula for calculating perceived temperature. Starting in 1960, this formula began to be used normally in meteorological services around the world and in 2001 it was revised to formulate it again in a more precise way.

Difference between temperature and thermal sensation

In general, Temperature is the degree of heat in the atmosphere at a place which is measured objectively and generally, through the use of thermometers. The temperature depends on factors such as solar radiation and the conservation of atmospheric heat (for example, cloudy days are usually warmer, since there is more energy in the atmosphere).

However, the temperature of a region can be perceived in subjective ways depending on the specific meteorological conditions of each location. Thus, when the wind blows or the atmosphere is humid, the official temperature values ​​can be very different from the degree of cold or heat perceived by people. This second environmental indicator is the thermal sensation or perceived temperature, and it does not have so much to do with the state of the atmosphere, but with the distorting effect that certain meteorological phenomena have on the environment, making it feel colder or warmer than normal. what it really is.

Put very simply, temperature is the official degree of heat or cold in the atmosphere; while Thermal sensation is the degree of heat or cold that bodies will experience in said atmosphere due to specific meteorological factors

Factors that influence thermal sensation

The thermal sensation can be conditioned by meteorological elements, and also by individual bodily factors, such as the metabolic rate of each individual. However, when proposing an index useful for all people, these last factors are not very useful.

On the other hand, the perceived temperature depends on various factors in winter and summer. During the cold months the degree of wind intensity is more relevant which contributes to the decrease in perceived temperature; while In summer the degree of atmospheric humidity is more relevant since the high levels of energy in the atmosphere contribute to creating a suffocating feeling.

However, when calculating the thermal sensation in summer, the action of the wind can also be taken into account, to the extent that it removes the layers of warm air and allows a greater flow of energy, dissipating the perceived heat.

How is thermal sensation calculated?

The calculation of the thermal sensation or perceived temperature, according to most meteorological services, is carried out through the application of a specific formula, designed to reflect the perception of cold or heat of the human body. However, There is no universal formula and meteorological services use those that best adapt to the type of climate in their specific regions.

In the case of Canada and the United States, the calculation formula used during winter (wind chill) is the following:

Yes_t = 13.12 + 0.6215 T – 11.37 V^0.16 +0.3965V^0.16

Where Yes_t represents the estimated thermal sensation, T the ambient temperature expressed in degrees Celsius (°C) and V the wind speed recorded by an anemometer located at a height of 10 meters and expressed in kilometers per hour (km/h).

The trend of these values ​​is expressed in tables like the following:

Instead, during the summer (heat index) the calculation formula used is the following:

Yes_t = T + 5 ((P – 10) / 9)

Where Yes_t represents the estimated thermal sensation, T the dry air temperature expressed in degrees Celsius (°C) and Q the water vapor pressure produced by environmental humidity, expressed in hectopascals (hPa) or the percentage of relative humidity (%).

The trend of these values ​​is expressed in tables like the following:

Continue with: Thermal pollution

References

• “Thermal sensation” in Wikipedia.
• “Feeling temperature” in Wikipedia.
• “What is thermal sensation and how is it measured?” in La Nación (Argentina).
• “What is thermal sensation” in Meteorology and Climatology of Navarra (Spain).