It’s been hot in the United States for June/July 2012, breaking many daily and all-time heat records across the country. In the nations capital, temperatures above 95F have been seen for the first 10 days of July, also a record. During such heat waves there is always talk of the Heat Index. Most people have a general notion that this means it feels hotter than the actual temperature, but what does this actually mean? Since the human body gets it’s energy by burning calories, it needs a way to get rid of the excess heat generated. It can efficiently achieve this by simple radiation up to about 70 or 80F. Above this temperature range the body doesn’t have enough surface area to radiate the excess heat fast enough. This is when you begin to sweat and the process of evaporative cooling takes place. When the air is dry this works quite well, but when the air gets humid the evaporation can not take place because the air is already saturated with water. In this case the body can neither rely on radiative cooling or evaporative cooling and you begin to overheat. This can become a dangerous situation resulting in Heat Illness, and is the physical basis for concept of the heat index. Daniel Engber of gives a nice description of how the heat index works.

This makes a certain amount of sense, but where did the index come from and how is it determined? A decently accurate short history and discussion can be found in an online article by the Capitol Weather Gang. Basically, a New York banker came up with the idea back in 1937. I was able to trace this back to a Time Magazine 1938 article called “THE WEATHER:Humiture Wave”. An excerpt from this article is as follows:

The weather served to publicize a new word: humiture. The invention of a 38-year-old official of Manhattan’s National City Bank, Osborne Fort Hevener, it was first used by his friend Frank L. Baldwin in the weather column of the Newark Evening News. Humiture is a combination of temperature and humidity, computed by adding the readings for both and dividing by two. Weathermen called it a “fool word” but according to Mr. Hevener (who last week escaped the humiture by motoring to Quebec) this figure “gives the man in the street a better index of the summertime torture to which he is being subjected.” Peak Manhattan humiture: (with temperature 76 and humidity 98% of saturation) 87.

Subsequent stories appeared in New Yorker 1954 and New Yorker 1959 Magazines. Hevener published his story in an article called “All about Humiture” in Weatherwise 1959. Granted, Hevener’s concept was simplistic, but provided some relative measure of the modern concept of a Heat Index. It wasn’t until the late 1970’s that a  TV meteorologist in Jacksonsville, Fla., George Winterling, came up with a revised and adapted version of  humiture, which he apparently published in the an article called “Humiture-revised and adapted for the summer season in Jacksonville, Fla.”  (Bulletin of the American Meteorological Society, vol. 60, pp. 329-300, 1979) and began using it in his on-air weather reports. I have not been unable to find this article, but still looking for it. It is reported that the work was based on Robert G. Steadman’s seminal studies, humorously titled “The Assessment of Sultriness”:

Part I (Journal of Applied Meteorology, vol. 18, Issue 7,pp. 861-873, 1979)
Part II (Journal of Applied Meteorology, vol. 18, Issue 7, pp.874-885, 1979)

From these articles there are no less than 20 parameters and assumptions that go into a model that determines the heat index. They are listed here for your viewing pleasure:

a.) Dimensions of a human: Determines the skin’s surface area. (5′ 7″ tall, 147 pounds)
b.) Effective radiation area of skin: A ratio that depends upon skin surface area. (0.80)
c.) Significant diameter of a human: Based on the body’s volume and density. (15.3 cm)
d.) Clothing cover: Long trousers and short-sleeved shirt is assumed. (84% coverage)
e.) Core temperature: Internal body temperature. (98.6°F)
f.) Core vapor pressure: Depends upon body’s core temperature and salinity. (5.65 kPa)
g.) Surface temperatures and vapor pressures of skin and clothing: Affects heat transfer from the skin’s surface either by radiation or convection, determined iteratively.
h.) Activity: Determines metabolic output. (180 W m-2 of skin area for the model person walking outdoors at a speed of 3.1 mph)
i.) Effective wind speed: Vector sum of the body’s movement and an average wind speed. Angle between vectors influences convection from skin surface (below). (5 knots)
j.) Ventilation rate: The amount of heat lost via exhaling. (2-12%, depending upon humidity)
k.) Skin resistance to heat transfer: A function of activity, skin temperature, among others.
l.) Skin resistance to moisture transfer: A function of the vapor-pressure difference across the skin (and, therefore, relative humidity). It decreases with increasing activity.
m.) Clothing resistance to heat transfer: The magnitude of this value is based on the assumption that the clothing is 20% fiber and 80% air.
n.) Clothing resistance to moisture transfer: Since clothing is mostly air,pure vapor diffusion is used here.
o.) Radiation from the surface of the skin: Actually, a radiative heat-transfer coefficient determined from previous studies.
p.) Convection from the surface of the skin: A convection coefficient also determined from previous studies. Influenced by kinematic viscosity of air and angle of wind.
q.) Surface resistance to heat transfer: As radiation and convection from the skin increases,this value decreases.
r.) Surface resistance to moisture transfer: Similar to heat transfer resistance but also depends upon conditions in the boundary layer just above skin’s surface.
s.) Sweating rate: Assumes that sweat is uniform and not dripping from the body.
t.) Other assumptions: Ambient vapor pressure of the atmosphere. (1.6 kPa), steady-state equilibrium conditions, wind gustiness neglected, all evaporation occurs at skins surface.

All this was taken together in a model of the human body to produce some tables, from which a multivariate fit can be made to produce an approximate equation for the Heat Index in terms of conventional independent variables, namely the ambient temperature in Fahrenheit (T) and the relative humidity in percent (R), which is accurate to plus or minus ~1 degree.

This is, in fact, approximately what The National Weather Service uses to predict the Heat Index. So, now you know the whole story of where is comes from and how it is determined. It should be mentioned that this applies to shade conditions and in the sun it can feel like ~15 degrees higher still. Of course it helps if you are 5’7, weigh 147 lbs, walking in a light breeze of several miles per hour, wearing long pants and a short-sleeved shirt. In addition, it is assumed that the following have been calculated correctly: vapor pressure, dimensions of a human, effective radiation area of skin, significant diameter of a human, clothing cover, core temperature, core vapor pressure, surface temperatures and vapor pressure of skin and clothing, activity, effective wind speed, clothing resistance to heat transfer, clothing resistance to moisture transfer, radiation from the surface of the skin, convection from the surface of the skin, sweating rate, ventilation rate, skin resistance to heat transfer, skin resistance to moisture transfer and surface resistance to moisture transfer.

That seems like a pretty tall order to take all that and derive something meaningful from it, so it begs the question: Is it really all that accurate or useful? Well, temperature is a relative thing after all, so perhaps it is not the actual numbers that matter, but the danger that such numbers could potentially pose for human health. I think that is the lesson and usefulness one can take from a weather report when a Heat Index is given. It advises a person to use caution, which is always a good thing for safety. Like any scientific model there are always variation in parameters, and not every human being is the same, but this determination of how hot it feels on a given day above 80F with less than 100% humidity for an average person is only as accurate as the input values. It’s just the limitations of science nature of the universe sometimes.

Note: Canadians don’t use Heat Index, but something called Humidex. It’s a similar way to determining how hot it feels, but is  derived from the dew point rather than the relative humidity. I am not entirely sure what the difference is in interpretation, or if any scientific comparison has been made.

Here’s a Heat Index calculator
Here’s a Humidex calculator

Just goes to show that there are probably more ways than one to assess sultriness!