Thermal sensation

גֵאוֹגרָפִי

2022

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.

The thermal sensation depends on environmental factors such as wind or atmospheric humidity.

What is wind chill?

wind chill or temperature perceived 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 environmental factors such as wind speed or the degree of humidity environmental.

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 the one they will feel when they are outdoors. This index is usually expressed in the ordinary temperature measurement unit, either in the Celsius (°C) or Fahrenheit (°F) scale. In some Anglo-Saxon-speaking countries, the heat index (“heat index”) in summer and the wind chill (“wind chill”) in winter.

The concept wind chill arose in the mid-twentieth century, during World War II, thanks to the experiences of American explorers Paul A. Siple (1908-1968) and Charles Passel (1915-2002) in the Antarctica, where they exposed different bottles with water to the environment and measured the time it took for the liquid to freeze, depending on the atmospheric conditions and the wind.

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

Difference Between Temperature and Wind Chill

In general, temperature is the degree of heat in the atmosphere in 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 tend to be warmer, as there is more energy in the atmosphere).

However, the temperature of a region it can be perceived in subjective ways depending on the specific weather conditions of each locality. Thus, when the wind is blowing or the atmosphere is humid, the official temperature values ​​can be very different from the degree of cold or heat perceived by the people. This second environmental indicator is the thermal sensation or the 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 hotter than usual. what it really is.

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

Factors influencing wind chill

Wind chill can also depend on individual body factors.

The thermal sensation can be conditioned by meteorological elements, and also by individual body factors, such as the metabolic rate of each individual.However, when it comes to proposing a useful index 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 intensity of the wind is more relevant, which contributes to the decrease in perceived temperature; while in summer the degree of humidity atmosphere, 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 air warm and allows a greater flow of energy, dissipating the perceived heat.

How is wind chill 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 suit the type of climate in their specific regions.

For Canada and the United States, the calculation formula used during the winter (wind chill) is the next:

St = 13.12 + 0.6215 T – 11.37 V0.16 + 0.3965 V0.16

Where st represents the estimated wind chill, T the ambient temperature expressed in degrees Celsius (°C) and v the wind speed recorded by an anemometer located 10 meters high and expressed in kilometers per hour (km/h).

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

Wind Chill and Wind Chill Table
Wind (km/h) Temperature (°C)
Calm 10 7,5 5,0 2,5 0 -2,5 -5 -7,5 -10 -12,5 -15 -17,5 -20 -22,5 -25 -27,5 -30
8 7,5 5 2,5 0 -2,5 -5 -7,5 -10 -12,5 -15 -17,5 -20 -22,5 -25 -27,5 -30 -32,5
16 5 2,5 -2,5 -5 -7,5 -10 -12,5 -15 -17,5 -20 -25 -27,5 -32,5 -35 -37,5 -40 -45
24 2,5 0 -5 -7,5 -10 -12,5 -17,5 -20 -25 -27,5 -32,5 -35 -37,5 -42,5 -45 -47,5 -52,5
32 0 -2,5 -7,5 -10 -12,5 -17,5 -22,5 -22,5 -25 -30 -35 -37,5 -42,5 -47,5 -50 -52,5 -57,5
40 0 -5 -7,5 -10 -15 -17,5 -22,5 -25 -30 -32,5 -37,5 -40 -45 -47,5 -52,5 -55 -60
48 -2,5 -5 -10 -12,5 -17,5 -20 -25 -27,5 -32,5 -35 -40 -42,5 -47,5 -50 -55 -57,5 -62,5
56 -2,5 -7,5 -10 -12,5 -17,5 -20 -25 -30 -32,5 -37,5 -42,5 -45 -50 -52,5 -57,5 -60 -67,5
64 -2,5 -7,5 -10 -15 -20 -22,5 -27,5 -30 -35 -37,5 -42,5 -45 -50 -55 -60 -62,5 -70
Moderate chill risk Severe chill risk Risk of extreme cold

In contrast, during the summer (heat index) the calculation formula used is as follows:

St = T + 5 [ (P – 10) / 9 ]

Where st represents the estimated wind chill, T the dry air temperature expressed in degrees Celsius (°C) and P the pressure of water steam produced by ambient humidity, expressed in hectopascals (hPa) or the percentage of relative humidity (%).

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

Table of wind chill with hot flashes due to humidity
 

Temperature (°C)

 RH (%)
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
20 16 16 17 17 17 18 18 18 19 19 19 19 20 20 20 21 21 21 21 21 21
21 18 18 18 19 19 19 19 19 20 20 20 20 21 21 21 22 22 22 22 22 23
22 19 19 19 20 20 20 20 20 21 21 21 21 22 22 22 22 23 23 23 23 24
23 20 20 20 20 21 21 22 22 22 23 23 23 23 24 24 24 24 24 24 25 25
24 21 21 22 22 22 22 23 23 23 24 24 24 24 25 25 25 25 26 26 26 26
25 22 23 23 23 24 24 24 24 24 24 25 25 25 26 26 26 27 27 27 28 28
26 24 24 24 24 25 25 25 26 26 26 26 27 27 27 27 28 28 29 29 29 30
27 25 25 25 25 26 26 26 27 27 27 27 28 28 29 29 30 30 31 31 31 33
28 26 26 26 26 27 27 27 28 28 28 29 29 29 30 31 32 32 33 34 34 35
29 26 26 27 27 27 28 29 29 29 29 30 30 31 33 33 34 35 35 35 36 36
30 27 27 28 28 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 36 37
31 28 28 29 29 29 29 30 31 31 31 32 33 33 34 35 36 36 37 37 37 38
32 29 29 29 29 30 30 31 31 31 32 32 33 33 35 35 36 37 37 38 38 38
33 29 29 30 30 31 31 32 33 33 33 34 34 35 36 36 37 37 38 38 38 39
34 30 30 31 31 32 32 33 33 33 34 34 35 36 36 37 37 37 38 38 39 39
35 30 31 31 32 33 33 34 34 35 35 35 36 36 37 37 38 38 38 39 40 40
36 31 31 32 33 34 34 34 35 36 36 37 37 38 38 38 39 39 40 40 40 41
37 33 35 35 36 36 37 37 38 39 39 39 40 40 40 41 42 44 44 45 46 46
38 34 35 36 37 38 38 39 39 40 41 41 42 42 43 43 44 44 45 45 46 47
39 35 36 37 37 38 38 39 40 40 41 41 42 43 44 44 45 45 46 46 47 48
40 37 38 39 40 41 41 42 43 44 44 45 46 47 47 48 48 49 49 50 50 51
Tolerable to warm warm to hot Very hot to suffocating
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