What was my humidity using the Wet Bulb Temp?

[NicksDad1970]

I'm tring to figure out what my humidity was for several hours.

My understanding is if I give 2 factors someone should be able to tell me what the humidity was (like x% humidity)

The temps are celsius

13 08
16 08
17 06
18 03
19 03
21 03
15 07
18 05
21 04
22 05
23 06

The way the guy explained it to me was if it's 13 08. Then if the 2nd # is the same as the first then it would be 100% humidity.

My first # was the temp in celsius and I don't know exactl what the 2nd # means.

I obviously don't know anything about the weather. But I do need to find out the humidity for these times.

[Silent A]

First, I assume the second number is wet bulb temperature in celcius.

I'd like to give you a simple formula, but unfortunately there isn't one.

T = air temperature (C)
Td = dew point temperature (C)

RH (%) = 100*exp(17.27Td/(237.3+Td))/exp(17.27T/(237.3+Td)

Which is pretty easy if you have the dew point temperature, but you have the wet bulb (which is different). It's pretty easy to calculate wet bulb T from dew point T, but not the other way around:

Tw = wet bulb T (celcius)
P = actual air pressure (Pa) - not the air pressure you get in weather reports (which has been converted to sea level, which is only ok for these equations if you live near sea level)

Tw = (G*T + D*Td)/(G+D)

G = 0.00066*P
D = 4098*611*exp(17.27Td/(237.3+Td))/(Td+237.3+Td)^2

because D is a function of Td, calcualting Td from Tw is a pain in the but.

link to an online calculator (http://www.connel.com/freeware/psychart.shtml)

and another (http://www.abc15.com/weather/index.asp?doc=calculators/wetbulb_drybulb.html)

Both these calculators assume air pressure is 101325 Pa (i.e average pressure at sea level)

[Silent A]

OK, the above has some inaccurate info about how to calculate wet bulb from dew point temps (blame it on a bad reference).


Given the following:

T = air temperature (aka dry bulb temperature, column 1 in your number list)
Tw = wet bulb temperature (column 2 in your number list)
P = actual air pressure in Pa (usually close to 101325 Pa at sea level)

If you're at elevation your approx air pressure is:

P = 101000 - 10*elevation in metres

so at 1000 m, P is around 91000 Pa

1) calculate A

A = 0.00066*P*(1+0.00115*Tw)

2) calculate B

B = 611*exp(17.27Tw/(237.3+Tw))

3) calculate C:

C = B - A*(T-Tw)

4) calculate D:

D = 611*exp(17.27T/(237.3+T))

5) calculate relative humidity, RH:

RH = 100*C/D (RH is in %)

6) get a mod to sticky this vital information to the top of this forum

[NicksDad1970]

Wow I am confused but very impressed. I'm not sure I'm going to be able to figure it out but that really isn't surprising.

Thanks so much for taking the time trying to help me out.

Robert