Met Questions

[Greg2041]

Hi, whilst studying the groundschool notes for my PPL I came across the following:

Cold Air causes pressure to fall more rapidly with height whilst warm air causes pressure to fall more slowly with height.

Similarly:

Warm air holds more water vapour than cold air at the same pressure.

I am looking at the science behind these statements and would appreciate a simplistic explanation as none is offered in any of the textbooks.

Many thanks

Greg

[nick14]

The air pressure is really a measure of how much the air above you weighs. As you ascend, you remove the air you climb through from the weight felt by the altimeter.

If the air you climb through is cold, and hence has higher density, you remove weight at a faster rate and the altimeter will indicate a faster climb for the same actual rate of climb.

Think of it as the atmosphere having a constant weight, meaning the more the air below you at a given altitude weighs, the less the air above you will weigh. Denser (colder) air below you -> higher weight of the air below you -> lower weight of the air above you -> lower pressure -> altimeter overreads -> look out below

Copied from Fred

[Greg2041]

BUt why if air has a higher density, should weight be removed at a faster rate than warm air.

My OAT book say it is too complicated to explain so, I guess I might as well give up.

[beatnik]

Greg

I think you answered your own question. It's all relative.

Density of air is directly proportional to its weight.

In a column of cold, dense air, as you climb up into that column of air, you lose density/weight (its now below you) faster than you would if you climb up inside a column of warm, less dense/heavy air.

So if a cold column of air (say) 10,000 feet high, weighs 100kgs, and you climb 1,000 feet you lose 10% of 100kg = 10kgs. If you are in a warm column of air 10,000 feet high - say it now only weighs 80kgs. If you now climb 1,000 feet you still lose 10% = 8kgs So you are not removing weight as fast in warmer air.

Remember that warmer air has lots of more active air molecules, and because they are more energised and bouncing about, there are less of them in a finite column of air at any one time (they've bounced into the adjacent column of air) = less density of molecules = less weight.

[Greg2041]

Thanks Beatnik, makes perfect sense!

BTW any idea why warm air holds more water vapour than cold air at the same pressure?

Greg

[chrisN]

Greg, see:

About Humidity.... (http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html)

This met. man says "hold" is the wrong word, but he does give a scientific explanation if you follow it through.

I like his analogy – if a jar of marbles contains some red and some blue, the red ones don’t hold the blue ones any more than the blue ones hold the red ones. There are just a mixture of red and blue. Similarly, in the atmosphere, there is a mixture of nitrogen molecules, oxygen molecules, water vapour molecules, and a few others. The maximum number of water vapour molecules is related to how much energy there is available to evaporate them, which is related to the temperature.

Chris N.

[beatnik]

Thanks ChrisN - I hadn't got a clue. I only know that the H20 molecule is less dense than the other of "air's" molecules (Oxygen, Nitrogen etc), and hence air with greater water content is less dense than air with less water content. But that wasn't going to be of any use to answer Greg's question.

Nik

[gfunc]

Hi Greg,

Your second question sounds really simple, but in my (too many) of Met at uni, I've never had this fully explained. The most convincing explanation I've come across was actually from a chemist, it basically boils down water wanting to form bonds with itself (hydrogen bonds for you A-level fans) and the the kinetic energy (Brownian motion or something) of the other molecules smashing up these bonds.

So basically its a balance, at low temperatures the air molecules are less energetic and can't smash so many water bonds, so the water can form into the liquid phase, fall out and therefore reduces the specific humidity. At higher temperatures the air molecules are more efficiently smashing the water-water bonds and prevent phase change, which means more water vapour and higher specific humidity.

That sounds pretty convincing, but since it's from a chemist who's locked up with all sorts of subtances all day, I'd take it with a grain of salt.

Answers on a postcard please! - I prepare to stand corrected and have my chemistry based excitement destroyed.

Cheers,

Gareth.

[Checkboard]

It's not because of the temperature of the air. It's because of the temperature of the water molecules. Hotter water molecules are more energetic, and so a greater percentage of them have the energy to exist as vapour instead of existing as a slow, fat, lazy liquid. :)

[Deeday]

It's not because of the temperature of the air. It's because of the temperature of the water moleculesSorry if I'm pedantic, but that doesn't make much sense. The temperature is the temperature of the gas. Temperature is a statistical property defined for a whole system and expression of the average kinetic energy of all the molecules, not individual ones, of whatever substance they might be.
gfunc's explanation makes sense to me.

Deeday

[Checkboard]

I understand that- I was trying out point out why hotter atmospheres can contain more water than colder ones. There seemed to be some idea that the hotter air molecules have an greater ability to "hold onto" water. I was pointing out that it isn't the temperature of the air that permits higher water vapour it was the energy in the water molecules themselves (which are obviously at the same temperature). It was a way of re-defining a mental image of the problem I was trying to convey (obviously not very successfully). :}

[gfunc]

Checkboard,

I'm also a bit confused - isn't what you are talking about to do more to so with the boiling point of of a substance? You can can have water at 80C evaporating into the atmosphere, but if the air is at 0C it can only contain a partial pressure of 6.11hPa of water vapour before saturation - this doesn't vary with the water temperature (assuming the water doesn't heat the air of course!).

I admit that I don't know the actual answer, but my money is on the hydrogen bonds that lets good olde H2O have all these wacky properties. I'll have a look around my cloud physics (yawn!) book when I'm back in my office to see if that has anything.

Cheers,

Gareth.

[chrisN]

Gfunc, I think you have to work with the distinction between average temperature of the water, average temperature of the air above it, and energy of the water molecules that are evaporating or condensing - and the average comprises of molecules with more and less than that average energy level.

Water at 80°C contains molecules whose average energy is represented by that temperature. The average is a mixture of ones with less energy and ones with more. The ones with more energy at the very surface can fly off into the air above.

Air at 0°C contains a mixture of molecules bouncing around like little balls. Their average energy equates to that of 0°C, but some have more and some have less. The water molecules within that air (certainly not many of them at that temperature) will contain some with more energy and some with less than the average. The ones with less, particularly if they are close to the water surface, will tend to encounter the water surface and stay there.

The combination at the interface of water at 80°C and air at 0°C means that more high energy water molecules will escape than cold water vapour molecules in the air will get trapped/condense. This means that the volume of water will get less (net evaporation), its average energy will get less (it will cool down), and the average energy of the molecules above it in the air will increase (it won’t be 0°C anymore).

If you put your hand just above a pan of hot water when you are outside on a cold day, you will feel that it is a lot warmer than the general air temperature. You are feeling the higher energy molecules, a few of them water, and a lot of oxygen and nitrogen molecules which have been bounced by and acquired additional energy from high-energy (evaporating) water molecules – so the temperature of that layer of air close to the water has been increased.

Hope that helps.

Chris N.

[Greg2041]

Thanks guys. I guess I'm going to have to just accept certain bits of Met. I am progressing through the syllabus but far too slowly as I'm probably over-analysing everything.

I really do appreciate all the info though!

Greg2041