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.

Formula:

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.

HUMIDITY FACTOR

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

WIND FACTOR

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 wind chill)):

SUN FACTOR

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.

REFERENCES

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, July 1979.