THSW INDEX
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.
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 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.
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 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. 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.
• For temperatures below 50°F the
wind chill result is used (calculate the wind chill increment using the
difference between the air temperature and wind chill)
The
resulting value is the wind term, which will be added to the humidity term and
subsequently the sun term as indicated below.
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.
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.
"Media
Guide to NWS Products and Services", National Weather Service Forecast
Office,
Quayle,
R.G. and Steadman, R.G., 1998: The Steadman Wind Chill: An Improvement over Present
Scales. Weather and Forecasting, December 1998