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: : Thermal Sensation : :

Comfort Index, Thermal Sensation and Heat Waves Impact on Peolple

By: Lic. in Physics Ernesto Urriola


Matter can emit electromagnetic radiation by the molecule and atom agitation . The electromagnetic radiation spectrum encloses the γ rays, the χ rays, ultraviolet radiation, visible light, infrared light, radio waves, radar waves, etc. The thermal radiation emitted by agitation is related to the matter temperature, and generally, is called heat and light. In humans the eyes are the sensors to capture light and the body (skin) is the heat sensor.

The thermal sensation is the aparent sensation that people have in function to the paramters that determine the environment in which they move, for example:

The thermal sensation depends on the relationship between the heat that produces the body's metabolism and the one that dissipates to the environment. If the first is higher, the sensation is hot, and if the second is higher the sensation is cold. All mechanism that increases the heat losses from the body gives a sensation of coldness and the opposite.

Humans have a very effective heat regulating system that guarantee that the body's core temperature maintains to aproximatelly 37 °C. When the body begins to heat or colden too much, two processes begin:


The two most important sensor systems for the control system are located in the skin and the hypothalamus. The hypothalamus has a heat sensor that initiates the cooling function when the body temperature in the core exceeds the 37° C. The skin sensors are cold sensors that start the body defenses against cooling qhen the temperature drops below the 34 ° C.

From ancient times we have known that atmospheric weather and climate affect health and human welfare. Hipocrates, 2500 years ago, in his work about airs, waters and places, made difference in the climate regions and its relationship with the health state, that's whay we can call him the father of bioclimatology.

When a thermal environment in a room or a determined space is measured is important to remember that humand can't feel the temperature of the premises, but the heat that his body looses. The parameters that must be measured are those that affect the energy loss. These are:

The thermal sensation is valued in practice through the comfort indexes. The indexes came up by the need to observe the effect of the variables that affect the physical interchange between the human body and the environment and over the fisiological and sensitive responses in persons. At first, the most variables we include, the more precise the evaluation will be. However, in the practice we've learned that the most complicated are not always the more exact, and the simpler are not always the easier to apply.

The word comfort refers, in general terms, to a ideal human state that suposes a welfare, health and comfort for humans, in which there's no distraction or discomfort in the environment that distubes physically or mentally to the user.

There have been many specialists and international organisms that have dedicated to the study of this matter. For example the OMS defines comfort as "A state of complete physical, mental and social welfare". But these studies haven't been only oriented to conceptualize the term comfort but also have formulated clasifications depending of the energies that affect it (luminous, thermic, acustic). That way, they have analyzed the different parameters as the factors that fall in the welfare sensation, elaborating tables, formulas and even some design standarts have been marked considering the comfort levels.

These comfort levels can be quantified by the so called comfort indexes.

Comfort index types

Evaporation Recent studies about the bioclimatic comfort are still having the basic approaches signaled by Morgan and Baskett (1974): the analytic or rational, based in the human energetic balance, and the sinthetic or empiric approach, based on various meteorologic variables. The empiric indexes ignore the decisive role of human fisiology, the activity, the clothe, and other personal data(height, weight, age, sex). The rational indexes are most recent, they usually are developed by informatic techniques, and depend in the human energy equilibrium (HÖPPE, 1993). Here, the heat transfer applies as a starting point to describe the diverse flux interchange of sensible and latent radiation, together with some empiric expressions, that describe the fisiological regulating effects.

Comfort index types:


The humidex proposed by Lally and Watson (1960), uses the air temperature and the water vapor to characterize humid and hot environments. Was adopted by the Atmospheric Environment Service of Canada and changed to celcius degrees (MASTERTON y RICHARDSON, 1979). Is an index that pretends to involve in one datum the combined effect of the air temperature and the atmospheric humidity, that's why the measurement unit is celcius degrees. The humidex is the heat sensation that the human body perceives in a determined period of time (estatic sensation). Has been used mainly to prevent morbidity and mortality in people.

Humidex = Ta+ 5/9* (pa-10)


Ta= Dry bulb temperature (°C)

pa= Partial vapor pressure (hpa)

Aparent Temperature (TA)

Is the temperature that combines the air temperature and the relative humidity; the TA is a measure of how much heat feels or preceives an average person in various temperatures and relative humidities.
TA= -9.93122+1.186145T+0.122310HR

T= air temperature (°C)

HR= Relative Humidity (%)

Effective Temperature (TE):

Equivalent temperature to the calm air temperature that experiments a sedentary, healthy, dressed with work clothing, in the shadow subject if relative humidity was 100%.

TE= Ta-0.4(t-10)(1-HR/100)


Ta= air temperature (°C)
HR= Relative Humidity (%).

Thermic Comfort Index Charts

The way of showing to the public these comfort indexes, is usually, through the thermic comfort charts.

These charts are given in terms of relative humidity and the temperatures in °C, in which we tend to quantify the effect of these variables in the thermal sensation og people, chart #1 is an example.

Chart #1

Chart 1

In which you can observe the humidex index vs the temperature and the relative humidity percentage. The color band indicates the people's thermic sensibility range. From the green band, you can begin to feel some discomfort, until you reach the red band where the danger of suffering a heart attack is inminent.

The importance of these comfort indexes are that these can be used to determine possible rises in electrical consumption, because if the people are in a state of discomfort, they will turn to technologies such as air conditioning and heating. Studies performed in U.S.A. , Mexico and Chile reveal that when the temperatures are above or below normal, the electric consumetion increases considerably.

In higher latitude countries, heat waves studies are made to forsee electric consumptions above normal, for example:

The strong heat lead to that at 11:00 am a consumption of 53,500 MW, that exceeded the old record from December 10th 2003, when the intense cold lead to a consumption of 53,400MW.

Until now the hottest estival period in the last 500 years has been in 2003, when at least 3000 deaths were registered due to high temperatures.

According to a study from the Science magazine, the european winters are getting hotter. This study was made by researchers in the Berna University, in Switzerland, and its conclusions signal that the annual average temperature in the last three decades have been hotter than the last five centuries.

The increase of these temperatures it's not limited to the places with latitudes higher than Panama (located in the intertropical zone, near the equator, reason why it's considered tropical weather).

The most dramatic and relevant impact will be in the tropics, according to a group of researchers from the Washington University. This idea is structured in tha base that the tropic's organisms doesn't usually experience high temperature variation because of its low stationality, that's why small changes in temperature can have a bigger impact than other zones with higher stationality.

The tropic temperatures doesnt fluctuate so much, so small temperature changes predicted by the climate change models would be huge considering the tolaration of the tropical organisms. Maybe only a two degree change is acheived, but many organisms of the tropical areas have never experienced such change.

In the other hand, the higher latitudes usually register noticeable fluctuations in temperature, from hot summers to cold winters, that way the animals and plants are used to the wide range of temperatures. Plants that bloom earlier and animals that change their migration and hibernation patterns in response to the temperature rises, can be an example of the adaptability of life in the higher latitudes.

To prove how much these mild temperatures changes could affect the tropics, the researchers simulated temperature changes in different species of pants and animals, and they gave these conclusions:

Graphic #1.

Graphic 1

Maximum Temperature Variation in Panama in three Decades

Taking in count the importance an relevance that represents a temperature increase of only 2 degrees in the tropics, the management department of Hidrometeorology in ETESA in Panama, is directed to study which has been the behavior of the maximum temperatures in the Republic of Pamama.

In graphic #1 you can see the behavior of the maximum temperature in the Republic of Panama in 37 years. This corroborates the information that says that in the tropics there's not much variation in temperatures comparing to other regions of the world.

But something that cannot be denied is that the perception of the people in Panama is that every day that goes by, seems that the temperatures are higher, it can be the fact that the limit temperature frequency increases.

Chart#2 Maximum Temperature frequency

Temperature in °C 1971-1980 1981-1990 1991-2000 2001-2008
30,0 - 30,9 94 121 108 63
31,0 - 31,9 120 109 123 118
32,0 - 32,9 157 154 148 122
33,0 - 33,9 121 109 148 115
34,0 - 34,9 42 49 77 89
35,0 - 35,9 6 11 31 31

In chart #2, you can see the daily maximum temperature values in a period that goes from 1971-2008 in the Republic of Panama. They were grouped by the frequency with the higher value was repeated forming an accumulative, that is, is the count of one or more days that reached certain temperature.

In chart #2, the clasification is done by the following criteria:

Criteria A - At least two consecutive days that coincide or exceeds the limit temperature.

The number of cases found in the period from 1971-2008, as you can see on the chart #3

Chart #3 Temperature Frequency. Criteria A

Temperature in °C 1971-1980 1981-1990 1991-2000 2001-2008
30,0 - 30,9 72 94 71 46
31,0 - 31,9 76 67 88 85
32,0 - 32,9 99 84 88 79
33,0 - 33,9 74 60 85 74
34,0 - 34,9 28 36 38 53
35,0 - 35,9 3 10 15 21

Criteria B - At least two consecutive days coincide or exceeds the limit temperature.

Chart #4 Temperature Frequency. Criteria B

Temperature in °C 1971-1980 1981-1990 1991-2000 2001-2008
30,0 - 30,9 22 27 37 17
31,0 - 31,9 44 42 35 33
32,0 - 32,9 58 70 60 43
33,0 - 33,9 47 49 63 41
34,0 - 34,9 14 13 39 36
35,0 - 35,9 3 1 16 10

Comparing tables 3 & 4, you can see that as the time passes by the frequency of repetitions of maximum temperatures in 2 and 3 days periods increases.

Is important to stand out that the last column is missing 2 years of data to complete the decade, ans still is meaningful compared to the 1970's and 1980's decades. This indicates that in Panama City the maximum temperature cycles, as the years go by, become more frequent. This contrasts with the sense that panamanians feel more heat.

This increment in the maximum frequency temperatures in the decades of 1990's till 2008 is concerning because when these maximums extend by more than 3 days, a discomfort climate is created in people, which causes thermal stress, which is denominated tension and risk originated by heat due to a thermal overcharge. This can be influenced by diverse factos as humidity, air speed and environment temperature.

The constant exposition to elevated thermic chages can cause fisiological effects to the worker, such as: bad mood, fatigue, tiredness, concentration loss, etc. wich causes a diminish of the worker's performance and a higher quantity of labor accidents.

Graphic #2

Graphic 2

Graphic #2 gives a better perspective of the problem: you can see that in temperatures of 33,0°C – 33,9 °C it has been without much changes in all this time, but in the temperatures of 34,0-35,9°C you can see an increase in time. All this applied to criteria A, that is two consecutive days with high temperatures.

In graphic #3, you can see the behavior of the maximum temperature cycles that last three consecutive decades. Is more evident that the cycles are increasing.

Graphic #3

Graphic #3As you can see in graphic #3, the temperatures of 34,0°C and 35,9°C that in past decades weren't very common, are presenting more frequently in the past few decades.

The frequency in maximum temperature cycles has been practicly triplicated in the last decades, wich is preceived as an increase in the temperature causing, inside Panama city, a high index of thermic discomfort. Here's the thermic comfort index, the humidex and the aparent temperature calculated for Panama (Tocumen station) in the month of May 2009.

Humidex index calculated for the maximum registered temperatures in Panama City in April 2009.

Chart #5 Humidex index for Panama

Chart #5

In chart #5 we can see that, as the relative humidity and the temperature increases, the comfort indexes increase.

Color ranges

Aparent temperature (TA) for April 2009 in Panama City.

Chart #6 Aparent Temperature Index

Chart #6

Color Ranges

Once analyzed charts #5 and #6, you can see that Panama city is in its maximum temperatures in the thermic discomfort zone.


The thermic stress is a decease that could affect most of the panamanians, and affects fisiological aspects such as bad mood, fatigue, tiredness, lack of concentration, etc. This leads to a reduction in the worker's performance and a higher number of work related accidents.

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