Vapour Pressure And Humidity

[sammie]

Ok so here is what I have in my notes

 "Vapour pressure is the pressure of a vapour in an equilibrium state, this means the number of molecules evaporating is equal to the number of molecules re-entering. All liquids have a tendency to evaporate in to gas and gas has a tendency to condense back in to their original state (either liquid or solid.) In general the higher the temperature the higher the vapour pressure, when air is at its saturation pressure it is said to be at its dew point.

The saturation vapour pressure can be calculated using the Clausius-Clapeyron Relationship, it characterises the phase transition between states of matter on a pressure-temperature diagram. The equation gives the curve which indicates the point of saturation.

(1/es)(des/dT)=Lv/(RvT^2) where Lv = latent heat of evaporation, Rv = gas constant for water vapour, and T = Kelvin temp.

There is a far easier way to calculate it by using a SVP Diagram, which you simply interpret."

What I don't get is the maths though to be honest does anyone get it at all? If anyone does get it care to explain in English pretty please? I'm not sure if its the way the lecturer has explained it or my note taking thats confused it all up but the whole concept of vapour pressure confuses me, I've read about 12 different explanations of what Vapour Pressure is, does anyone have a clear and simple definition of it?

Also, problem two, or should it be three? When temperature and dew point are plotted from a Radiosonde ascent the point where they meet or come very close together is where condensation is going to occur, since at the dew point the air becomes 100% saturated? So will that be where the cloud base is?

Thanks x

[johnholmes]

last bit first, as near as anyone needs to know it is, actually about 30mb higher up is where the actual cloud base will be; for convection that is.

re the first no need to get too involved and yes old Clausius-Clapyon, the favourite of all professional meteorologists in their learning days is the full explanation.

Don't get too hung up on this as it can be very complex.

Just use the dry bulb, wet bulb and their conversion to the dewpoint, forget the vapour pressure unless you really need it.

There are umpteen references to it if you 'Google'.

Send an e mail to the Met O and see what they reply with!

Edited October 1, 2008 by johnholmes

[J07]

...

I'd look up partial pressures and Dalton's Law for a handle on the first problem. And try to get your head around the concept of the hypothetical equilibrium they talk about, though it's not that easy an idea really.

There are other forms of the Clausius-Clapeyron equation where it's been solved for e_s(T), in other words that is the subject of the formula.

If you have to only use the equation then you don't need a deep understanding of it, but if you have to derive it, then unfortunately you will need to get a pretty good feel of the thing. In which case you can smash your head against the problem repeatedly until it breaks, or ask the lecturer or a classmate for a hint which I always found much more efficient!

For higher in the atmosphere where ice crystals can be present you can get cloud with lower saturations because the air may well be saturated with respect to ice but not water. This goes back to the original definition of vapour pressure, in the equilibrium you swap the plane surface of water for ice, and evaporation for sublimation.

It sounds odd but vapour actually finds it easier to turn to straight to ice than to liquid, one of the many strange quirks of water I believe.

[sammie]

Thanks both of you <3 in the lab we just used the wet bulb and got given charts and graphs to work out the vapour pressure and dew point then used them to calculate the relative humidity. So I don't know if I have to know all the equation stuff but it was mentioned in lectures, so it is examinable. However I've decided that I'm not going to get too hung up on it, they can't bring in therodynamics like that its the introductory module (ok that is how I'm validating this in my head.)

[johnholmes]

the main thing sammie is to understand what it is and how it used in meteorology. provided you have a clear understanding of its variables, like over ice or water, that is what you need.