Temperature – How to Calculate Temperature Humidity Wind Index?

atmospheric sciencehumiditymeteorologytemperatureweather

I would like to know how to calculate Temperature Humidity Wind Index (THW Index)? I know how to calculate Heat Index and Wind Chill. I am asking this because my weather station Davis Vantage Pro2 calculates THW index but I could not find any information on NOAA. This is what it says under Help for my weather station:

The THW Index uses humidity, temperature and wind to calculate an apparent temperature that incorporates the cooling effects of wind on our perception of temperature.

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eHow.com has an explanation for how to calculate the THW index.

The Heat Index formula is complicated enough I don't want to try doing it in markup; but the wikipedia article you linked has that formula. From there THW is simple enough.

THW = HI - (1.072 * W)

Note that the formulas require a temperature in Degrees Fahrenheit and a wind speed in miles per hour.

Related Solutions

[Physics] How to calculate Vapor Pressure Deficit from Temperature and Relative Humidity

From [The ASCE Standardized Reference Evapotranspiration Equation]1

Given T is temperature in degrees Celsius, and RH is relative humidity:

Saturation Vapor Pressure (es) =

0.6108 * exp(17.27 * T / (T + 237.3))

Actual Vapor Pressure (ea) =

RH / 100 * es

Vapor Pressure Deficit =

ea - es

Why this is a meaningful measurement: "The strain under which an organism is placed in maintaining a water balance during temperature changes is much more clearly shown by noting the vapor pressure deficit than by recording the relative humidity." Anderson, D. B. 1936. Relative humidity or vapor pressure deficit. Ecology 17, no. 2: 277–282.

[Physics] How to calculate relative humidity from specific humidity and temperature

From Psychrometry in Refrigeration & Air Conditioning

Let $t_d$ be D.B.T. (dry bulb temperature) & $t_w$ be W.B.T. (wet bulb temperature) and $p_s$ & $p_w$ are saturation pressures at D.B.T. & W.B.T. respectively (refer to "Steam Table" for saturation pressures at given temperatures) then using Carrrier's Equation, the partial pressure ($p_v$) of water vapor is given as $$p_v=p_w-\frac{(p_b-p_w)(t_d-t_w)}{1544-1.44t_w}\tag 1$$ Where, $p_b$ is barometric pressure

Now, use the formula for given specific humidity say $W$ given as $$W=0.622 \left(\frac{p_v}{p_b-p_v}\right) $$ $$\implies p_b=\frac{0.622p_v}{W}+p_v$$ Substitute the value of $p_b$ in the eq(1) to get the value of $p_v$ Then apply the formula of $\color{blue}{\text{Relative Humidity}\ \phi}$ as follows $$\color{blue}{\phi=\frac{p_v}{p_s}}$$

Where, $p_v$ (partial pressure) & $p_s$ (saturation pressure at D.B.T.) are known.

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Related Solutions

[Physics] How to calculate Vapor Pressure Deficit from Temperature and Relative Humidity

[Physics] How to calculate relative humidity from specific humidity and temperature

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