View Full Version : Fohn winds
9th Jul 2004, 21:00
Moist air is less dense than dry air at the same temperature and pressure.
Why is it then that if moisture has been lost on top of a hill through precipitation then the cloudbase on the lee side will be higher than on the windward side?
If the top statement applies would this less moist air mass not be more dense and so the cloudbase be lower on the lee side?
Any feedback would be most appreciated!
Thanks for your time
9th Jul 2004, 22:18
When we talk about the Fohn winds we refer to air that is stable.
As air approaches the windward side of the mountain range it is forced up by and cools at the DALR of 3°C per 1000 feet. When the temperature of the air then reaches the dew point temperature clouds form. The rising air then cools at the SALR, 1.98°C per 1000 feet (slowed down by the release of latent heat). Precipitation may occur here. If it does then the moisture is removed from the air, hence it is less moist.
So the air being stable now wants to flow down the other side of the moutain range and will warm adiabatically at the SALR until the temp is above the dew point and the clouds disperse. The air will continue to warm now at the DALR until it reaches the bottom of the lee side, and since it had a larger height to warm in, the air is warmer.
so basically the drier the air, the lower the dew point, so the higher in the atmosphere the cloud base will be....
Hope this helps
Moist air less dense than less moist air??
This would appear to be a wrong basic assumption.
The Water Vapour content of air is a variable and being vapour is of the same density as the air in which it coexists.
Fascinating is the medium in which we fly.
10th Jul 2004, 12:34
Water Vapour is never the same density as the air in which it coexists. This by virtue of the fact that although water vapour may be in effect a gas the water molecules are still larger and heavier than the component gases of "Air". If it were of the same density we wouldn't be having this discussion and nobody would have to remember anything about adiabatic lapse-rates to calculate cloud base heights etc..
A parcel, or rather volume, of air, if moist, contains water vapour in gaseous form which exerts its own vapour pressure (this is the key to the whole subject). This results in a certain amount of air being displaced out of the volume in question, e.g. 1 cubic metre. Therefore the remaining AIR in this volume is less and as a result is less dense than if there were no water vapour present.
Warm, moist air is less dense than cold dry air. This causes all the problems with engine power losses in warm, high altitude environments because the weight (direct result of molecular density) of the fuel-air mixture is upset. Remember that it is the weight of the charge, not the volume, that enters the combustion chamber that is important.
10th Jul 2004, 15:21
Buy the Oxford met CD written by Steve Francis and the UK met office at Bracknell.
It explains everything and is well worth the investment.
10th Jul 2004, 20:12
The air descends on the lee side because, notwithstanding the water vapour changes it is still colder and more dense than the surrounding air mass.
Actually, air with water vapour in the mix is less dense than dry air for a given T and P. This is because water vapour has about half the density of dry air.
However, at max, there will be only 4% of water vapour in any met air mass, so the reduced density from the water vapour is tiny compared with any effect of changes in temp or pressure.
10th Jul 2004, 23:01
Thankyou all for your replies,but just after i posted this thread i turned the page of my text book to find that i wasn't looking at what happened on top of the hill after precipitation but what happened when the air mass slid down the other side of the hill!!
I'll learn by this mistake before the exams me thinks!!
p.s.Milt,considering the subject referred to here i think it was quite clear that when saying'moist air is less dense than less moist air' i was referring to the fact that moist air is less dense than dryer air at constant pressure and temperature.I was just trying to make it clearer that 'wetness' had been lost through 'rain' on the hilltop!!:}
12th Jul 2004, 13:26
The effect caused by a fohn is not due to any difference in density, though density will change. The point is that as the air cools as it rises it reaches the dew point (becomes saturated) and condensation occures. As it crosses the summit of the mountain it warms as it decends. However, because it has lost so much of its moisture on and below the summit the dew point of the air is lower, so the height at which the air becomes unsaturated is higher than it was on the windward side. From that point down the air warms at the unsaturated lapse rate, so the air at the foot of the mountain is warmer than it was on the other side of the hill, as well as being dryer.
For most calculations the Saturated Adiabatic Lapse Rate is 1.8 degrees per 1000 feet (0.6/300 m) - 1.98 degrees per 1000 feet is the ISA environmental lapse rate.
12th Jul 2004, 13:28
That to me sounds like the voice of a flight instructor :-)
12th Jul 2004, 15:06
I try my best