relative humidity and temp change formula?

[Js Cantrell]

Hi, I joined the forum to ask a question that I can't find the answer to on the web.  I'm looking for a formula to precisely answer the following question:  an air mass with 34% Relative Humidity (RH) and 84°F = what RH if that air is cooled to 72°F?  

For example, in a basement.  I know the Rule of Thumb : Relative humidity doubles with each 20 degree (Fahrenheit) decrease, or halves with each 20 degree increase in temperature, but is there an easy formula to answer precisely?    Thanks!

 

[Js Cantrell]

Bueller?  Bueller?  Anyone?

[A.J]

Hi there, welcome to the forum.....I've moved your thread to a more appropriate part of the forum, as the area you posted in is purely for non-weather related threads......hopefully, someone will see this now and be able to answer your question 

[Js Cantrell]

thank you ajpoolshark:)

[Nick L]

Hi @Js Cantrell. It's not quite so simple unfortunately. Relative humidity is the measure of how much moisture is in the air compared to how much it can hold. If there was a linear relationship between the temperature of an air mass and the amount of moisture it can hold, it would be relatively simple. However, that's not the case. There are formulae you can use to calculate it by hand using the figures you give, but it's not simple by any means, you need to work out what are known as the vapour pressure and saturation vapour pressure first - both of which require the dew point temperatures.

Fear not, however, as there are online calculators if you're not too bothered about the nitty gritty formulae! 

http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

Using your figures, 84F and 34% RH would give a dew point of 52.63F. We'll assume that the dew point remains constant in your example, so we put 72F temperature and 52.63F back into the calculator and we get 50.52% RH.

Edit: I will add that those formulae require your temperature units to be in Kelvin.

Edited June 15, 2016 by Nick L

[Js Cantrell]

Awesome!  Thank you Nick L.  You're right, the math beyond me, but the calculator is just what I was looking for.  But I wouldn't have known how to use it if you hadn't explained that I need to derive dew point and then plug it back in.   Perfect!  Cheers! 

My reason for asking is I wanted to know what happens when warm, dry outside air is drawn into a long, narrow, cool basement (20' x 200') at one end with an intake fan while the  existing basement air is evacuated with an exhaust fan at the other end.  I have humidistats/thermometers  throughout, and once the air exchange is complete, I turn off all fans.  My goal is to reduce humidity in the basement, but I don't want it too hot down there either, so on those rare summer days here in Tennessee when we get low RH and moderate temp outside, I suck in outside air to help the dehumidifiers keep the basement dry.  The basement has concrete floor, ceiling, and walls, so the drawn-in air gets cooled down.  The calculator will help me determine when I should and shouldn't ventilate!  The rule of thumb I mentioned is ok, but something in me, curiosity I guess, just wanted to know a bit more.  Thanks!

Edited June 19, 2016 by Js Cantrell

[Nick L]

On 19/06/2016 at 15:50, Js Cantrell said:

Awesome!  Thank you Nick L.  You're right, the math beyond me, but the calculator is just what I was looking for.  But I wouldn't have known how to use it if you hadn't explained that I need to derive dew point and then plug it back in.   Perfect!  Cheers! 

My reason for asking is I wanted to know what happens when warm, dry outside air is drawn into a long, narrow, cool basement (20' x 200') at one end with an intake fan while the  existing basement air is evacuated with an exhaust fan at the other end.  I have humidistats/thermometers  throughout, and once the air exchange is complete, I turn off all fans.  My goal is to reduce humidity in the basement, but I don't want it too hot down there either, so on those rare summer days here in Tennessee when we get low RH and moderate temp outside, I suck in outside air to help the dehumidifiers keep the basement dry.  The basement has concrete floor, ceiling, and walls, so the drawn-in air gets cooled down.  The calculator will help me determine when I should and shouldn't ventilate!  The rule of thumb I mentioned is ok, but something in me, curiosity I guess, just wanted to know a bit more.  Thanks!

You're welcome