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Published: 2008-06-09 09:00:35
Updated: 2008-06-09 09:00:35
Posted June 9, 2008
By John Mullinax
MIKE MOSS SAYS: John, The THSW (temperature/humidity/sun/wind) index appears to be a Davis instruments implementation of ideas published as part of the original Heat Index concept as developed by R. G. Steadman in a pair of 1979 Journal of Applied Meteorology articles on "The Assessment of Sultriness." The first article lays out the method that is still used as the basis of Heat Index calculations by the National Weather Service. Heat Index depends only on temperature and humidity, with an implicit assumption that it applies in the shade.
The second article also considered the effects on perceived temperature of variations in wind speed, barometric pressure and solar radiation. The THSW index is an application of some of this additional information to attempt to account for the amount of solar radiation being received by the instrument and the observed wind speed. As with wind chill, an increase in wind speed typically reduces perceived temperature, and of course exposure to direct sunlight increases it. Conversely, at night when there is no solar radiation, one can perceive it to be cooler than it actually is due to radiative heat loss to open sky - I don't know for sure if this is included in the calculations, but if you see a THSW reading that is lower than the air temperature at night when humidity is moderate and winds are calm, that may be the reason.
Davis makes an overview document readily available online that provides an outline of how they implement the various elements to arrive at a THSW, but it does not include the detailed formulas, look-up tables and so on. You may be able to e-mail them and have them share those with you if you're interested. Meanwhile, here's the address for the overview document. You'll find THSW discussed beginning on page 7...