HEAT INDEX FORMULA
(Updated 15 February 2019)
The Heat Index "Equation"
(or, More Than You Ever Wanted to Know About Heat Index)
Lans P. Rothfusz
Scientific Services Division
NWS Southern Region Headquarters, Fort Worth, TX
When summer spreads its oppressive ridge over most of the United
State's Southern Region, NWS phones ring off their hooks with
questions about the Heat Index. Many questions regard the actual
equation used in calculating the Heat Index. Some callers are
satisfied with the response that it is extremely complicated. Some
are satisfied with the nomogram (see chart).
Chart by Lans Rothfusz, MIC at NWS Tulsa, OK
But, there are a few who will settle for nothing less than the equation
itself. No true equation for the Heat Index exists. Heat Index values
are derived from a collection of equations that comprise a model. This
Technical Attachment presents an equation that approximates the
Heat Index and, thus, should satisfy the latter group of callers.
The Heat Index (or apparent temperature) is the result of extensive
biometeorological studies. The parameters involved in its calculation
are shown below (from Steadman, 1979). Each of these parameters
can be described by an equation but they are given assumed
magnitudes (in parentheses) in order to simplify the model.
# Vapor pressure. Ambient vapor pressure of the atmosphere. (1.6 kPa)
# Dimensions of a human. Determines the skin's surface area. (5' 7" tall, 147
# Effective radiation area of skin. A ratio that depends upon skin surface area.
# Significant diameter of a human. Based on the body's volume and density. (15.3
# Clothing cover. Long trousers and short-sleeved shirt is assumed. (84%
# Core temperature. Internal body temperature. (98.6°F)
# Core vapor pressure. Depends upon body's core temperature and salinity. (5.65
# Surface temperatures and vapor pressures of skin and clothing. Affects heat
transfer from the skin's surface either by radiation or convection. These values
are determined by an iterative process.
# Activity. Determines metabolic output. (180 W m-2 of skin area for the model
person walking outdoors at a speed of 3.1 mph)
# 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
# Clothing resistance to heat transfer. The magnitude of this value is based on the
assumption that the clothing is 20% fiber and 80% air.
# Clothing resistance to moisture transfer. Since clothing is mostly air, pure vapor
diffusion is used here.
# Radiation from the surface of the skin. Actually, a radiative heat-transfer
coefficient determined from previous studies.
# 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.
# Sweating rate. Assumes that sweat is uniform and not dripping from the body.
As an aside, these assumptions are important for the forecaster to keep in mind.
For example, a common perception is that wind is not taken into account in the
Heat Index. In actuality it is. It is assumed to be 5 knots. This may seem trivial but
a forecaster may be able to use this information creatively when writing Public
Information Statements regarding heat stress, heat stroke, etc.
# Ventilation rate. The amount of heat lost via exhaling. (2-12%, depending upon
# Skin resistance to heat transfer. A function of activity, skin temperature, among
# Skin resistance to moisture transfer. A function of the vapor-pressure difference
across the skin (and, therefore, relative humidity). It decreases with increasing
# Surface resistance to heat transfer. As radiation and convection from the skin
increases, this value decreases.
# Surface resistance to moisture transfer. Similar to heat transfer resistance but
also depends upon conditions in the boundary layer just above skin's surface.
These last five variables are used explicitly to derive the apparent
temperature. By an iterative procedure which relies on the
assumptions in the first list, the model is reduced to a relationship
between dry bulb temperature (at different humidities) and the skin's
resistance to heat and moisture transfer. Since these resistances are
directly related to skin temperature, we now have a relationship
between ambient temperature and relative humidity versus skin (or
apparent) temperature. As a result of this procedure, there is a base
relative humidity at which an apparent temperature (e.g., 90°F) "feels"
like the same air temperature (90°F). Increasing (decreasing) humidity
and temperature result in increasing (decreasing) apparent
temperature, and, yes, apparent temperature can be lower than air
temperature. Steadman (1979) developed a table based on this
relationship and the nomogram (above) summarizes that table.
In order to arrive at an equation which uses more conventional
independent variables, a multiple regression analysis was performed
on the data from Steadman's table. The resulting equation could be
considered a Heat Index equation, although it is obtained in a "round-
about" way. Thus, here is an ersatz version of the Heat Index
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR -
where T = ambient dry bulb temperature (°F), R = relative humidity
Because this equation is obtained by multiple regression analysis, the
heat index value (HI) has an error of ±1.3°F. Even though temperature
and relative humidity are the only two variables in the equation, all
the variables on the lists above are implied.
Steadman, R.G., 1979: The assessment of sultriness. Part I: A
temperature-humidity index based on
human physiology and clothing science. J. Appl. Meteor., 18, 861-873.