Parameters Used: Temperature, Humidity, Solar Radiation, Wind Speed, Latitude & Longitude,
Time and Date
What is it:
Like Heat Index, the THSW Index uses humidity and temperature to calculate an apparent
temperature. In addition, THSW incorporates the heating effects of solar radiation and the
cooling effects of wind (like wind chill) on our perception of temperature.
The formula was developed by Steadman (1979). The following describes the series of
formulas used to determine the THSW or Temperature-Humidity-Sun-Wind Index. Thus, this
index indicates the level of thermal comfort including the effects of all these values.
This Index is calculated by adding a series of successive terms. Each term represents one of
the three parameters: (Humidity, Sun & Wind). The humidity term serves as the base from
which increments for sun and wind effects are added.
The Vantage Pro and Vantage Pro2 calculation is an improvement over the THSW Index in the
Health EnviroMonitor because the Health system:
• only calculates THSW Index when air temperature is at or above 68°F.
• assumes the sky is clear.
• assumes the elevation is sea level.
The first term is humidity. This term is determined in the same manner as the Heat Index. This
term serves as a base number to which increments of wind and sun are added to come up with
the final THSW Index temperature.
Note: Heat Index has also been referred to as "Temperature-Humidity Index" and "Thermal
Index" in some Davis products
The second term is wind. Depending upon your version of firmware or software, this term is
determined in part by a lookup table (for temperatures above 50°F) and in part by the wind chill
calculation, or uses an integrated table that is used both for calculation of this term and for wind
chill. With this in mind, the following criterion apply with later versions referring to Vantage Pro2
console firmware revision May 2005 or later or WeatherLink version 5.6 or later:
• At 0 mph, this term is equal to zero.
• For temperatures at or above 68°F and wind speeds above 40 mph, the wind speed is set to
40 mph. For later versions, there is no upper limit on wind speed.
• For temperatures at or above 130°F, this term is set equal to zero. For later versions of this
algorithm: WeatherLink uses 144°F as the threshold; Vantage Pro2 console firmware
143°F. This is based on a best-fit regression of the Steadman 1979 wind table. The
differences are reflective of the higher resolution used in the WeatherLink software.
28 - 8 Rev A 5/11/06
• For temperatures below 50°F (later versions use the new wind chill formula result here
(calculate the wind chill increment using the difference between the air temperature and
The third term is sun. This term, Qg, is actually a combination of four terms (direct incoming
solar, indirect incoming solar, terrestrial, and sky radiation). The term depends upon wind speed
to determine how strong an effect it is. The value is limited to between −20 and +130 W/m2
in the Vantage Pro2 console firmware and WeatherLink software versions 5.6 or later.
Steadman, R.G., 1979: The Assessment of Sultriness, Part II: Effects of Wind, Extra Radiation
and Barometric Pressure on Apparent Temperature. Journal of Applied Meteorology,