PPRuNe Forums > Flight Deck Forums > Tech Log > Relative Humidity PDA View Full Version : Relative Humidity Finzolas23rd May 2012, 09:56Hi all, I wonder how do you consider relative humidity for take off computations? Relative Humidity, especially at high temperature, would reduce air density and thus take off performances. I think about it especially for operations in the Gulf during summer season, with very high temperatures (>40°C) and high relative humidity (80%). I did not see any performance computation requesting RH and I do not remember if SCAP modules allowfor Relative Humity input . Is this included in QNH? Thanks in advance for your help! Zolas anotherthing23rd May 2012, 10:23I always thought Relative Humidity was the sweat that drips off your chin when making love to your sister... Slasher23rd May 2012, 10:25This has been covered recently in another thread. rudderrudderrat23rd May 2012, 11:12Hi Zolas, Air Density Calculator (http://www.denysschen.com/catalogue/density.aspx) Difference between 0% / 100% relative humidity is about 1% in air density. That will be reflected in the QNH. barit124th May 2012, 00:39Don't quote me on this, but my recollection is that AFM performance chapter assumes 80% RH; In dryer air, you might be able to do a bit better than the book. Worst case, 100% RH, a tiny bit worse. barit124th May 2012, 00:48rudderrudderrat:Difference between 0% / 100% relative humidity is about 1% in air density. True. H2O vapor is lighter than the average air molecule which it displaces. (2xH=2; O=16; total = 18. 2xO=32; 2xN=28; the mixture averages about 29) That will be reflected in the QNH. Nope. QNH reflects pressure, not density (which is a function of pressure & temperature as well as H2O content.) rudderrudderrat24th May 2012, 08:25Hi barit1, Nope. QNH reflects pressure, not density I agree. However, do you think a parcel of less dense air will remain stationary in the atmosphere? If you agree that it will rise, then the local surface pressure will fall. edit: Density Altitude Calculator - English/Metric (http://wahiduddin.net/calc/calc_da.htm) It's interesting to note that, "Calculations using dew-point are often more accurate than those using relative humidity because the dew point is fairly constant for a given air mass and changes very little until another air mass arrives, while the relative humidity varies greatly as the ambient temperature changes." & "The ICAO International Standard Atmosphere standard conditions for zero density altitude are 0 meters (0 feet) altitude, 15 deg C (59 deg F) air temp, 1013.25 mb (29.921 in Hg) pressure and 0 % relative humidity (no water vapor in the air). "