Why does dew point decrase?

[A Squared]

Why does dew point decrease?

We all know that when air is lifted, the temperature and dewpoint converge at approximately 2.5 degrees C/1000 feet Now I understand the process of adiabatic cooling, and that it occurs at about 3 degrees C per 1000 feet. The other part though I'm having a little difficulty with. As the air is lifted the *dewpoint* decreases at approximately 0.5 degrees/ 1000 feet, leaving you with a convergence of 2.5 degrees/1000 feet.

So my question is this, *why* does the dewpoint decrease? If you take a parcel of air and raise it 1000 feet, you still have the same number of water molecules and the same number of air molecules, why would the dewpoint be lower? All the references on my shelf merely state that the dewpoint decreases 0.5 C/1000 feet and leave it at that. No explanation. Any weather whizes out there? Or am I just missing something embarrassingly simple.

 

[bigD]

If you take a parcel of air and raise it 1000 feet, you still have the same...number of air molecules,

I'm not sure that you do anymore. If you were to take a given number of air molecules up to a higher altitude, they'd eventually reach a pressure equalibrium with the surrounding air molecules. So you end up with fewer air molecules per volume.

I had always assumed that this decreased density is the reason behind the lower dewpoint.

 

[A Squared]

>>>>I'm not sure that you do anymore.

You have to, matter is neither created nor destroyed (except by nuclear reactions)

If you take 3 cubic feet of air and raise it 1000 feet those molecules still exist, they will however, take up more than 3 cubic feet at the higher altitude.


>>>>So you end up with fewer air molecules per volume.

yes, I'd agree with that.

>>>I had always assumed that this decreased density is the reason behind the lower dewpoint.

That may be, if so, can you explain *why* the decreased density results in a lower dewpoint?

 

[CitationLover]

being a former nuclear engineer couldn't let this one slip....

nuclear reactions do not CREATE matter (it cannot be created or destroyed).

fission splits the matter apart (2 then becomes 1 and 1), while fusion fuses the matter together (1 and 1 become 2).

i think what bigd was saying was that if we have 1000 molecules and it rises then some of the molecules will stabilize (for simplicity lets say 100), thus higher up as the remainder rise you only have 900. of course i could be completely wrong in my interpretation.

i would imagine the dewpoint dropping due to the formula:

PV = nRT (as Pressure drops Temp must follow, since n and R are constants and Volume is not changing).

any weather geeks out there?

 

[52Vincent]

Dewpoint decreases with altitude as result of falling pressure which lowers the partial vapour pressure of water.

Hope this helps

 

[A Squared]

>>>> nuclear reactions do not CREATE matter (it cannot be created or destroyed).

OK, shoulda just stopped typing when I had it right, instead of throwing in the bit about nuclear reactions <g>


>>>>>i would imagine the dewpoint dropping due to the formula: PV = nRT

That's the Ideal Gas Law and it explains adiabatic cooling, but I can't see a relation to dewpoint.

 

[bigD]

You have to, matter is neither created nor destroyed

Well, that's why I said per volume. Effectively, for a parcels of air at a fixed volume, you have fewer air molecules at higher altitudes. So we agree on that. Sorry for the confusion.

Anyway, the dew point is just the temperature in which the air condenses to vapor, correct? I know that the total pressure has a pressure of vaporization factor in it, so if you drop the total pressure, you're going to lower the vaporization pressure as well. But I suspect you're looking for something more low level. I'm gonna have to look at my stat mech book at home to give you an answer on the particle level.

 

[A Squared]

OK, got it now, it's the pressure of vaporization. In simple terms, at a lower pressure it's easier (takes less energy) for water to exist as a gas. Same reason a pot of water boils at a lower temperature at higher altitudes.

Thanks guys for helping me out on this.

 

[FlyDeltasJets]

If it's red, don't fly through it.

That's all the weather I bothered to learn. It hasn't failed me yet!

 

[bigD]

RADAR snob!