Saturation

[Cron]

Would be grateful if someone could give me a snappy explanation of the answer to this one. Thank you in advance for your help.


A moist but unsaturated parcel of air becomes saturated by
a) lifting the parcel to a higher level
b) lowering the parcel to a lower level
c) moving the parcel to an area with lower pressure and equal temperature
d) moving the parcel to an area with higher pressure and equal temperature

[Taiguin]

Hey mate,

I've not done meteorology but this is my opinion......

Saturation is the same as condensation where a vapour turns to liquid. Water vapour rises in the air and then condenses as rain clouds thus becoming saturated.

So as you go up in altitude the pressure increases and temperature drops so that rules out c) and d) because they imply the temperature stay constant. That leaves the other two and since they are only concerned with altitude it must be a) as discussed previously.

Hope this helps...and is correct.

T

[High Wing Drifter]

Hmmm, going out on a limb here.

Saturation of a parcel of air occurs if the temprature of the air is lowered below depoint or the pressure increases and temp doesn't increase comensurably to prevent saturation.

So A is the classic exaple of cloud from convection. But D is significant because it states that the temprature remains constant.

A & D are, in my mind, correct!

Unless that is that Oxford Blue, Send Clowns or Alex can provide their usual reason as to why I'm wrong...again :\ :O

[-IBLB-]

That's a negative, A is the only correct answer.



when air is rising, it expands.
for expansion you need energy
latent energy from the surrounding air is used -> the air cools.

The Relative Humidity is (e/E)100% or (x/X)100%.

both e and x remain the same, however E and X decrease due to the temperature decrease. So the relative humidity increases. When you reach 100% -> saturation.


A higher pressure has nu influence on saturation when the temperature remains constant, since it has no influence on e,E,x or X.


-IBLB-

[Solid Rust Twotter]

Don't think you need energy for expansion. It's just that the energy available has to occupy a larger volume, thus the drop in temperature. Cold air holds less moisture than warm air so the relative humidity will increase as the temperature drops, until saturation point is reached.

[Send Clowns]

Not bad, HWD :D

Lifting a parcel of air without exchanging energy with the environment (that is the meaning of "adiabatic") will always cause significant drop in temperature due to significant drop in pressure. This will eventually lead to condensation except for the driest air.

A drop in pressure without commensurate change in temperature (option C, while ambiguous, appears to mean this) will not cause condensation. Any effect of pressure alone on the capacity for air to hold water is negligible and not within the syllabus, as far as I am aware. Our met instructor is on leave so I cannot check, but I certainly never learnt anything about that when I sat the course (and I got 96%, so it is clearly not essential knowledge!). I suspect there is some effect, but not enough to worry a poor pilot!

Twotter

IBLB is actually correct, if a little over technical for most ATPL groundschool students, more like a physics lesson. The energy taken from the thermal energy goes into the work done in expansion. Imagine the air surrounded by a balloon - it would expand, needing energy.

[Dick Whittingham]

Whether or not water evaporates, that is the same thing as saying whether or not the air is saturated, depends on both the system temperature and the partial pressure of water vapour in the air. If you reduce the system temperature you move toward saturation. It is also true that if you increase the partial pressure of the water vapour you move toward saturation. And vice versa.

You can change the partial pressure of the water vapour by either changing the amount of water vapour in the air - as by evaporation of water into the atmosphere - or by changing the overall atmospheric pressure - as by going up to a greater height.

So far as we are concerned it is enough to know that when you take a parcel of unsaturated air aloft the cooling efect far outweighs the pressure reduction effect, so the parcel becomes nearer and nearer to being saturated.

I don't know of any CQB questions that deal with this esoteric point, but it is a fact that for any given water vapour content dew point temperatures will be slightly lower at height. Thus the statement that for any given water vapour content dew point remains constant is only true for constant atmospheric pressure - usually we mean sea level.


Dick W

[G-ANDY]

A nice simple question where the answer is definately A.

Air can hold a certain amount of water depending on it's temperature. Warmer air can hold more water vapour than colder air.

Thus, by cooling the air, it can hold less water vapour and the relative humidity increases. How does air cool? - usually by increasing in height (but not always!)

So to get an unsaturated parcel of air with say 40% RH to 100% RH (fully saturated - a cloud), then you must cool the parcel by lifting it to a higher level.

Pretty much like everyone else already said!

[EGCC4284]

Well I think it's C

[-IBLB-]

100% RH (fully saturated - a cloud)

100% Relative Humidity is saturated air, doesn't necessarily mean there is cloud formation.


-IBLB-

[Flamgat]

The word "saturated" on its own can be confusing. Without getting too technical on the gas laws and vapour pressures, it is will normally suffice to look at it as follows :

At 100% relative humidity the air is saturated as regards evaporation, that is to say no more water vapour will be absorbed into the parcel of air. You will have reached the wet bulb temperature.

You would still have to cool it slightly more to the dew point, for it to condense out as visible moisture. ie cloud, fog etc. That is why the dew point is colder than the wet bulb temperature under most circumstances.

[puntosaurus]

Maybe this will help, although as Flamgat says, saturation is a rather complex subject so what follows is not the full story.

Water Phase Diagram (http://www.lsbu.ac.uk/water/phase.html)

ISA at sea level is 288K and around 1015 HPa (~10 to the power 5 Pa). Temperature changes in the troposhere are of the order of 200-320K, and pressure from 200-1050 HPa. Although the phase diagram indicates sudden state transitions, in the atmosphere they are actually equilibria - vapour in equilibrium with solid and liquid. This is because the atmosphere is a mixture of compounds as opposed to the pure water situation shown in the diagram.

It's clear from the diagram that (a) is right since the transition is right to left with a slight downward slope to the left. The equilibrium is therefore pushed in favour of liquid (ie condensation/saturation)

If you're restricted to vertical movements (constant temperature), only rises in pressure will cause saturation (transition to liquid state) so (d) might be technically correct.

As several posts have pointed out temperature effects in the atmosphere dominate (the pressure scale is a log one on the graph) so (d) is unlikely to prevail in an atmospheric situation. However, since the question itself has posited the situation in (d) - however unlikely - I suppose you have to accept that it's a true answer.

It could certainly be true in the lab, so if you take the lawyer's approach you could appeal it since parcel size is not defined and hyperbaric chambers are not excluded as 'areas' ! Might be easier to give them the answer they want though.

[pugzi]

I suspect we lost the student, I certainly am and I teach Met!.

..wink...


x

[Cron]

Not lost, absorbed. All info being taken to the pub tonight and carefully considered. then I'll be lost. Grateful to all.

Cron.

[10002level]

Air does not necessarily cool as it is lifted (an inversion)

[-IBLB-]

air always cools when it is lifted.
In a situation where there is an inversion, temperature goes up with the altitude, but there is no unforced lifting.

[EGAC_Ramper]

Well I'll offer up my opinion on the answer being A....

then again I think Met is a Voodoo subject lol:p and I have it to look forward to this week,oh the joy!!


Regards:ok: