Met quest

[ElNull]

Hello everyone

I apologize if the question look dumb to some of you.

I understand at high atitudes as the air become colder (ie at temp below -40C) the air become colder and as air the air become cold it wont be able to hold water vapor. My question does this mean that air is "saturated" (full of water vapor) and thus cant hold more water? Or it has something to do with the ability of the cold air (has nothing to do with saturation)?

[MarkerInbound]

It's both. Yes the air is saturated and yes it's because the air is cold. If you take a cubic meter of air at sea level that has half the water vapor in it that it could hold, the relative humidity is said to be 50 percent. If you raise that cube of air it's temperature will normally drop 2 degrees per 1000 feet. As the temperature drops, the total amount of water the air can hold decreases and so the relative humidity increases. At some altitude the air cools to the point it can no longer hold any more water and it is said to be saturated. Now the air's RH is 100 percent.


Start at page 15 of the download, page 37 of the book.


http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2000-6A%20Chap%204-6.pdf

[ElNull]

Thanks for the explanation and for the link as well. But when the air is saturated at high altitudes then why wouldnt the excess water vapor become visible (clouds to form everywhere in the sky). Also, the 737 states not to use anti-ice when SAT is -40 or below (high altitude). I can explain this its the air at temp -40 or below that cant hold more water vapor because the air is saturated.. But wait, since the air is saturated wouldnt the excess water form an ice also?

[MarkerInbound]

When the air is saturated you do get clouds. So on a day with a completely clear sky you can say there is no altitude where the air is saturated. If you have high cirrus clouds only you know the lower levels are not saturated but the higher levels are. And if you have a low cloud layer and clear above, then you know the lower level is saturated but the RH is less when you get higher.


It's hard to think of different parts of the sky being different from the part right next to it because we don't see the difference. But on a day with 20 SCT the air where the cloud is either has more water vapor or is colder than the air without clouds.


Boeing and the engine companies have thought that at -40 the amount of water vapor in the air will not be enough to cause a problem. The water should already be ice crystals so it will not stick to anything. However Boeing and GE have found that even these ice crystals can cause problems in their latest engines on the 747-8 and 787.


A kg of air (a bit less than a cubic meter at sea level) at 20c can hold 15 grams of water. At -40c at altitude it now takes a few cubic meters to equal a kg but the air can only hold .1 of a gram of water.

[Turbavykas]

Air is not saturated at high altitudes! Why it should be? First there is very little moisture up there. When air ascends it gets saturated, clouds form, rain starts water drops back to the ground so you have dry air at the altitude. Second to get air saturated and fog or cloud to form you need temperature to reach dew point. Not to many people know but when air ascends not only pressure drops but dew point also drops! This has something to do with water molecules being far apart. So at altitude you have very dry air with very low dew point. But if it gets saturated sometimes very small ice crystals will form. That's cirrus clouds. But they are not thick for the reasons stated above as there is no much water left.
It's dangerous to use anti ice in these conditions because you will can melt ice crystals but water can freeze instantly as it is very cold outside and you will get clear ice icing down the wing which will be dangerous.

[ElNull]

Thanks for replies. Tubavykas, I thought clouds form for the same reason of the fog formation which occurs when temperature drops to reach dew point near ground (no space for more water).

From what I understood from your post, cloud formation is different and has nothing to do with air being saturated, but it is the cold air (at high altitude) loses the ability to hold water, so any ascending watervapor will not find a support from the upper cold air so it will condense.

Please correct me if I misunderstood and if I am correct, can you help me imagining why cold air loses the ability to hold watervapor as warm air? Thanks

[Turbavykas]

I didn't say that cloud formation is anything different. Clouds form when temperature drops to reach dew point. All I wanted to say that dew point also depends on pressure.