THSW INDEX Weather data – Derived Variables
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.
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. (THSW Index is not available on Vantage VUE).
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.
The second term is wind. This term is determined in part by a lookup table and in part by the wind chill calculation. With this in mind, the following criterion apply:
- At 0 mph, this term is equal to zero.
- For temperatures at or above 144°F, this term is set equal to zero. This is based on a best-fit regression of the Steadman 1979 wind table.
- For temperatures below 50°F, the wind chill depression (temperature – wind chill) is used.
The resulting value is the wind term, which will be added to the humidity term and subsequently the sun term as indicated below.
Note: The WeatherLink software (versions 5.2 and later) has a variable which does not include the sun term in its calculation. It is the “THW Index” or Temperature-Humidity-Wind Index. This value indicates the “apparent” temperature in the shade due to these factors.
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.
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
“Media Guide to NWS Products and Services”, National Weather Service Forecast Office, Monterey, CA, 1995.
Quayle, R.G. and Steadman, R.G., 1998: The Steadman Wind Chill: An Improvement over Present Scales. Weather and Forecasting, December 1998
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