# Dew point to humidity

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I want to be able to convert dew point to humidity and all the formulas I found were either complicated, I didn't know what the variables meant or the formula was for humidity to dew point. I want dew point to humidity. Is there and easy conversion or something where I know what the variables are? I guess there are some coefficients? I can do it online, etc, but I just want to know the math.

Thanks.

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The simplest formula for converting the dew point to humidity is: RH is approximately equal to 100-5(T-TD), where RH is relative humidity, T is the current temperature, in degrees C, and TD is the current dew point, in degrees C.

Yes, it's an approximation (I couldn't find the right symbol for "approximately equal to", which actually looks like a wavey equals sign), but the error in the equation is 5% or less, which, in this application, is relatively meaningless.

Remember the correct order of calculation: Inside parenthesis first, then multiplication/division, then addition/subtraction.

SO: in this equation, do the T-TD first, then multiply by 5. Finally, subtract the result from 100. This will give you a close approximation of the relative humidity, in %.

Does all this rambling answer your question? It gives a formula, and defines the variables, as desired, yes?

Patâ˜º

[sIGPIC][/sIGPIC]

Had a thought...then there was the smell of something burning, and sparks, and then a big fire, and then the lights went out! I guess I better not do that again!

Sgt, USMC, 10 years proud service, Inactive reserve now :D

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https://edwilliams.org/avform.htm has formulas for most anything aviation. Since the dewpoint/humidity relationship also is dependent on current temperature, it's not such a simple thing. From the formulary:

The relative humidity, f (as a fraction) is related to the temperature, T and dewpoint Td by:

f= exp(17.27(Td/(Td+237.3)-T/(T+237.3)))

and to the frostpoint temperature Tf by:

f= exp(21.87(Tf/(Tf+265.5)-T/(T+265.5)))

Temperatures are in Celsius. Multiply f by 100 if you want a percentage. The above are based on an empirical fit to the saturation vapor pressure of water due to O. Tetens in Zeitschrift fur Geophysik, Vol VI (1930), quoted in "Principles of Meteorological Analysis" by W. J. Saucier (Dover NY 1983).

To keep it simple, the problem is that dewpoint represents the moisture available in the air mass, and the percentage figure for relative humidity varies with the actual air temperature, even though the moisture content (thus dew point) remains the same for the air mass. And note the text I changed to red. There's more on the website.

And you can rearrange the formula to solve for TD or T, if you desire.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Thank you very much. I like Phantom's easy conversion the best. I shall drill it into my head with the rest of the equations I have memorized. :)
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Dew point to humidity conversion formula still drilled into my head. Even though you get a ~5% variable, it's a great non-meteorological precise formula. Those coefficients and what not were beyond what I wanted to try and memorize. I just wanted something I can easily memorize and compute on the fly.

So far buried deep in my brain library is how to convert Celsius to Fahrenheit, nautical miles to statute miles, (same equation for Knots to MPH), and meters to feet. Next up is TAS. I found that equation here on the forum today and made a note of it on my local FTP.

sudo dork mode off.

:D

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https://edwilliams.org/avform.htm has formulas for most anything aviation. Since the dewpoint/humidity relationship also is dependent on current temperature, it's not such a simple thing. From the formulary:

The relative humidity, f (as a fraction) is related to the temperature, T and dewpoint Td by:

f= exp(17.27(Td/(Td+237.3)-T/(T+237.3)))

and to the frostpoint temperature Tf by:

f= exp(21.87(Tf/(Tf+265.5)-T/(T+265.5)))

Temperatures are in Celsius. Multiply f by 100 if you want a percentage. The above are based on an empirical fit to the saturation vapor pressure of water due to O. Tetens in Zeitschrift fur Geophysik, Vol VI (1930), quoted in "Principles of Meteorological Analysis" by W. J. Saucier (Dover NY 1983).

To keep it simple, the problem is that dewpoint represents the moisture available in the air mass, and the percentage figure for relative humidity varies with the actual air temperature, even though the moisture content (thus dew point) remains the same for the air mass. And note the text I changed to red. There's more on the website.

And you can rearrange the formula to solve for TD or T, if you desire.

Do pilots have the compute formulas on a card or something in the cockpit for all that? Because I don't think even I, Sir Sponge A Lot could memorize all that. Even so, I might confuse one equation with another which sometimes happens to me when I want to compute meters to feet and use nautical miles to statute miles instead.

Is there such a thing like a pilot calculator much like a scientific calculator? I love my scientific calculator. Use it all the time.

Edited by CRJ_simpilot
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Is there such a thing like a pilot calculator much like a scientific calculator? I love my scientific calculator. Use it all the time.

Yep, there are manual as well as electronic flight computers.

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Is there such a thing like a pilot calculator much like a scientific calculator? I love my scientific calculator. Use it all the time.

Yup! The Sporty's Electronic E6B Allows a lot of calculations like density altitude, true airspeed, time/speed/distance and so many more things that pilots need to figure.

The older non-electronic E6B is essentially a circular slide rule with specialized scales and has been around since before WW II. The back of it calculates the wind triangles.

And Sporty's now has an Android app to do these calculations.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Oh my god, that is so going on my 'things to buy' list when I get paid. Thank you. :D
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Man, I so wish I can fly for real. Unfortunately, I was handed schizophrenia in 2010 and I have a form of color blindness. So it's 2-0 so to speak in terms of the FAR. Yet, I fly perfectly well in the Sim and know if I'm too high or to low looking at the PAPI. Too me, if I see two red and two yellow? lights I'm on glideslope. Four yellows I'm too high, and four reds I'm too low. And I figured that out myself. I also figured out there is no coincidence between the PAPI and the ILS. Learned that early on. In fact, learned a lot early on. Though, I'm still far from perfect. Especially with mastering the fine art of using a VOR or DME to land. I need to try that some day and master it. Too reliant on the GPS or FMC currently.

There are so many great facets to aviation. For one, with simming it satisfies my computing/flying itch. I like complex systems and other technological things, I've been a radio communications hobbyist since high school (that's at least 24 years now mostly with scanners), and ever since I took my first flight aboard a Continental in 1988 from Bismark, North Dakota to John Wayne in California I wanted to fly. I love that felling you get when you're barrelling down the runway at about 200 MPH and all of a sudden like magic you lift off the ground. I just look out my requested window seat and look in awe at what a marvelous invention this machine is. Then I think how I'm not only in the pilot's hands, but maintenance's as well. LOL

If the FAR allowed it, I'd promptly head to Liberty University and take classes on aviation and pursue my dream to fly for a living. But that isn't happening.

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You say you figured out the PAPI and other things on your own, which is great, but if you go to the FAA publications web site you can read/download the Aeronautical Information Manual (AIM), which has a lot of that information in it (plus MUCH more), and such others as the Instrument Flying Handbook, the Pilot's Handbook of Aeronautical Knowledge, the Airplane Flying Handbook and more.

These are official FAA publications that pilots (and student pilots, or most anyone else) can download and read/study to really learn about flying, with no cost for these. Sure, there are many ground school packages you can find out there, and they cost some bucks, but this gets you essentially the same information just for the downloading.

You might be surprised to learn how many fine details there are which have never been apparent to you in the sim, even with the sim's lessons. Look'em over.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Which is rather informative.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!