Finzolas

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

I always thought Relative Humidity was the sweat that drips off your chin when making love to your sister...

Slasher

This has been covered recently in another thread.

rudderrudderrat

Hi Zolas,

Air Density Calculator
Difference between 0% / 100% relative humidity is about 1% in air density.
That will be reflected in the QNH.

barit1

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

rudderrudderrat: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)

Nope. QNH reflects pressure, not density (which is a function of pressure & temperature as well as H2O content.)

rudderrudderrat

Hi barit1,
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
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). "