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Finzolas
23rd May 2012, 09:56
Hi 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

anotherthing
23rd May 2012, 10:23
I always thought Relative Humidity was the sweat that drips off your chin when making love to your sister...

Slasher
23rd May 2012, 10:25
This has been covered recently in another thread.

rudderrudderrat
23rd May 2012, 11:12
Hi 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.

barit1
24th May 2012, 00:39
Don'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.

barit1
24th May 2012, 00:48
rudderrudderrat: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.)

rudderrudderrat
24th May 2012, 08:25
Hi 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). "