Excel Formula for Calculating Wet Bulb Temp.

Try to put psychometric tables into your data sheet. And also the air speed. They are as exact as you can keep your wet bulb wet.

You might be able to enter about 100 points and interpolate between. The link mentioned "Skew-T" software, which might be useful.

I believe dew point and wet bulb temperature are the same. They're both defined as the temperature at which the water vapour is saturated. So if you have dewpoint you're there. Unless somebody can correct me.

To take an example (approx figures) at 30°C and RH 60% - saturated vapour pressure = 40 mb. Actual pressure of the vapour = 60%*40 = 24 mb. Temperature at which saturated vapour pressure is 24 mb = 21°C, = dewpoint.

If you do want to put it on XL, saturated vapour pressure as function of temperature (°C) is given to a good approximation by P_v = A*e^(B*T/(T+C)) where A = 6.105 mb, B = 17.074, C = 234.011. Multiply by the RH to get the actual pressure of the vapour. Then rearrange the equation to give T in terms of P_v and that's the dewpoint. Strictly speaking the figure is slightly high as it ignores the fall in actual pressure of the vapour (proportional to absolute temperature by the usual gas law). You could iterate to get closer, but for the sort of RH likely in weather data (40 - 80% ?) it doesn't make much difference.

Cheers.........Codey

In post 1, I also mentioned speed of the air.

dewpoint and wetbulb temperature can come close but I stick to this:

The Wet Bulb Temperature and the Dew Point Temperature are two different quantities.

The Wet Bulb Temperature is defined as the steady state nonequilibrium temperature that is reached when a small amount of water is contacted under adiabatic conditions by a stream of gas.

It is virtually equivalent to the adiabatic saturation temperature. As the water contained in the wick of the psychrometer evaporates, the convective heat flowing from the gas stream to the wick at a lower temperature balances the latent heat of evaporation of the water.

The Dew Point Temperature is the temperature at which an air/water vapor mixture is saturated with water vapor.

If the partial pressure of water vapor increases above the saturation partial pressure at constant temperature then condensation occurs.

It is possible for these quantities to be the same but it is not true in general. As humidity increases these two values begin to converge.

Hello dvmdsc

You may well be right, as indicated in #3, I was admitting some uncertainty.

Wikipedia gives one definition of wet-bulb temperature (among others) - "Thermodynamic wet-bulb temperature: the temperature a volume of air would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the volume of air." Perhaps that's the same as your adiabatic saturation temperature, but I can't get my head round it, maybe it's getting late or I'm getting old!

Can you give a calculation for wet-bulb temperature in terms of dry-bulb and relative humidity? Might help me (and others) to understand, and to compare with my calc for dewpoint.

Cheers...........Codey

Thanks for the reply.

However, dewpoint and wet bulb are not the same thing.

For example, at 70 degrees F dry bulb and 50% Relative humidity, the dewpoint is about 51 degrees F and the wet bulb is about 58.5 degrees F. That is from a psychrometric chart, for sea level.

I can get hourly weather data that includes wet bulb temperature, but I'll have to pay for it. Dang it!

Yes, that's right. I just looked again in more detail at the link with OP and it seems wet-bulb depression is only 1/3 - 1/2 dewpoint depression, so there's clearly more to this than I first thought.

Cheers.........Codey

First, two questions: (1) How accurate do you want or need the answer to be; and, (2) over what dew-point temperature range are you working?

If you need an accurate answer then you will need to perform the tried & true "iterative" mathematical solution using the 'Psychometric Equation':

e = e(w) - A·p·(t - tw)

...but, if an approximation is sufficient, and dew-point temperature is always positive (ie: tdew > 0), this 6-step approximation, which uses only dry-bulb temperature, Tdry, and relative humidity, RH%, is reasonably accurate:

1. Convert Tdry from ºF to ºC: tdry = (ºF-32)·5/9
2. Convert RH% from percent to decimal value: rh = (RH%/100%)
3. Calculate dry-bulb vapor pressure: e(dry) = 6.112*EXP[17.62·tdry/(243.12+tdry)]
4. Approximate wet-bulb vapor pressure, e(wet), as square-root of rh times e(dry): e(wet) ≈ e(dry)·√(rh)
5. Calculate approximate wet-bulb temperature, twet, from approximated wet-bulb vapor pressure: twet ≈ (237.7×Log[e(wet)/6.108])/(7.5 - Log[e(wet)/6.108])
6. Convert twet from ºC to ºF: Tdry = 1.8*ºC+32

Although this is just an approximation, it is usually within ±1-2 degrees as long as the corresponding dew-point (in ºC) temperature value is above zero.

OOPS...that's Psychrometric not Psychometric ! ! !

Er, um, <...dew point...> and <...wet bulb temp...> are the same thing.