what it is and how it can be measured. The main indices used «3B Meteo

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Central Italy: peak heat and heat between Saturday and Sunday
Central Italy: peak heat and heat between Saturday and Sunday

It would have happened to all of you at least once to say: What’s up today! But what is heat really? Strictly speaking, it is just a scientific term linked to the relationship between humidity and atmospheric temperature, quantifiable by a precise mathematical relationship, and represented by different indices, including heat. The latter, in fact, allows us to estimate the level of physiological discomfort that our body feels during the summer season, in correspondence with high temperatures and humidity. That is, it provides an apparent temperature value, which is what our body actually perceives.

In general, cloudy conditions occur mainly at temperatures above 30 ° C and with humidity levels above 40%. Under these conditions, in fact, the human body transfers heat to the outside through perspiration, or body sweating, which is strongly linked to weather conditions and is influenced by a myriad of other factors such as wind, clothing, age, weight, sex. of the individual, etc … In hot conditions, the sweating produced by the body to lower the body temperature cannot evaporate in the surrounding environment because it is already saturated, forcing it to rise with possible consequences for the body. But are there any indexes that quantify it?

HUMIDEX INDEX – There are several indices that can be used to quantify the climatic well-being of man in relation to humidity and temperature. Among the most important is certainly the Humidex scale or index (H), which tries, taking into account the air temperature and relative humidity, to calculate a single value capable of describing the discomfort, for humans, that occurs on humid, hot days. This equation was formulated in Canada after several studies in 1963 and is used in a temperature range between 20 ° C and 55 ° C. Later, in 1979, thanks to his own physiological studies in humans under various climatic conditions, Steadman developed a table that correlates the temperature, humidity, and temperature perceived by our body.

Humidex Index - source: Environment Canada
Humidex Index – source: Environment Canada

HEAT INDEX – This is how he defined the Heat Index (HI) through a multiple regression analysis, intended to describe the phenomenon of “suffocating heat”, understood as the effects of temperature and relative humidity on humans at the level of the sea. This index is applicable at temperatures equal to or greater than 80 ° F (27 ° C) and relative humidity equal to or greater than 40%.

HI: c1 + c2 T + c3 R + c4 TR + c5 T2 + c6 R2 + c7 T2 R + c8 T R2 + c9 T2 R2

where: HI = heat index (in degrees Fahrenheit; T = wet bulb temperature (in degrees Fahrenheit); R = relative humidity (in percentage) and c = constants.

Heat Index - Noaa National Weather Service
Heat Index – Noaa National Weather Service

MOLÈS INDEX – Another widely used index is the discomfort index proposed by Thom, Discomfort Index (DI), considered one of the best estimates of effective temperature. The latter is defined as “an arbitrary index” that combines, in a single value, the effect of temperature, humidity and the movement of air on the sensation of heat or cold perceived by the human body.

DI = 0.4 (Ta + Tw) + 4.8

where: Ta = dry bulb temperature and Tw = wet bulb temperature.

Thom Discomfort Index - Eurometeo.com
Thom Discomfort Index – Eurometeo.com

SUMMER SUMMER INDEX – The most recent welfare index, presented at the 80th AMS Meeting held in Long Beach, California on January 11, 2000, represents a new version of the Summer Simmer Index (SSI), published in 1987 by John W. Pepi and is called NEW SSI. This is an adequate index to describe the conditions of heat stress during the heat season. Applicable when the temperature is above or equal to 22 ° C and is sensitive up to a temperature of 53 ° C.

SSI = 1.98 (Ta- (0.55-0.0055 (UR)) (Ta-58)) – 56.83

where: Ta = air temperature in ° F and HR = relative humidity.

Summer Simmer Index - blog.gcwizard.net
Summer Simmer Index – blog.gcwizard.net

However, there are other less used indexes but in any case well formulated and correlated by tables relating the effects on humans to different temperatures. These include:

1. SCHARLAU INDEX which experimentally defines, in the absence of wind, the limit temperatures of the air, in relation to the atmospheric humidity, beyond which the organism of an average man and his accuses annoyances.

2. THERMOHYGROMETRIC INDEX (Thermohygrometric Index) able to calculate approximately the value of Thom’s discomfort index, using directly, in addition to air temperature, relative humidity; thus avoiding the calculation of the wet bulb temperature

3. EQUIVALENT TEMPERATURE INDEX suitable for identifying conditions of thermal well-being or discomfort in a wide range of climatic conditions and corresponds to the temperature that a mass of air would assume, maintained at constant pressure, if the water vapor it contains condenses and if the heat is latent of the released condensation was used to increase its temperature

4. RELATIVE VOLTAGE INDEX (RSI) suitable to describe the conditions of heat stress and that taking as a reference model an average man, sitting, dressed in a full work suit, in good health, 25 years old and not acclimatized to heat.


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