Relative Humidity
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Relative Humidity
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
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
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Hi Zolas,
Air Density Calculator
Difference between 0% / 100% relative humidity is about 1% in air density.
That will be reflected in the QNH.
Air Density Calculator
Difference between 0% / 100% relative humidity is about 1% in air density.
That will be reflected in the QNH.
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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.
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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.)
Difference between 0% / 100% relative humidity is about 1% in air density.
2xO=32; 2xN=28; the mixture averages about 29)
That will be reflected in the QNH.
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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). "
Nope. QNH reflects pressure, not density
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). "
Last edited by rudderrudderrat; 24th May 2012 at 19:21.