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Reply 1
loser88
My exam is on Tuesday :eek: and I really just don't understand them- does anyone have any good links or anything that may help? Thanks in advance :smile:


Now this i can explain :cool:

I was revising these the other day (but couldn't use them in the perfect question on them in the exam 'cos i forgot one figure :mad:) But anyway:

The lapse rate is the rate at which air looses energy (in this case heat) as it rises. There's an environmental lapse rate which varies from place to place, although i think you can look up averages. This is for the atmosphere at that place as a whole and how it gets colder as you rise.

There's also adiabatic lapse rates. Adiabatic means 'a parcel of air', the implication being that it is considered as an isolated body which can't exchange anything with its surroundings (ie the rest of the atmosphere). There's a dry adiabatic lapse rate, that's when the air parcel is at less than 100% humidity (ie it can easily hold all it's water vapour with none condensing). It's 9.78 °C/km.

Then there's the saturated adiabatic lapse rate. This figure varies, but will always be less than the dry one (ie less temperature lost).

Air becomes cooler as it rises because it is under less pressure higher up in the atmosphere (less 'weight' of air stacked above it) and so it 'spreads out'. Under less pressure, more spread out, the available energy (which assuming as we do that it's not exchanging any outside itself is fixed) is therefore spread over a greater area, and is less at any one point. If there is less energy at a point, it is 'colder'.

Dry adiabatic or the environmental lapse rate work on the idea that pressure decreases at a fixed rate as you rise, and so therefore does temperature from the starting temp.

However, saturated works differently. When the air parcel reaches a temperature where the water vapour held in it condenses, it actually gains some energy. This is because of the latent heat energy 'stored' in the water vapour from when that energy was 'put into' it as it was heated and evaporated initially, and this is released again as it condenses (i think a physicist would hate me for putting it like that but it's how i understand it ^^). So while the dry adiabatic lapse rate would be a steady decrease in temperature as altitude increases and pressure decreases, a saturated one would be marked by a point (where the parcel reaches the dew point and the water vapout in it condenses) where the decline in temperature is halted or at least reduced or [??even reversed??], for a while.

So intitially an air parcel can usually be considered as following a dry adiabatic lapse rate as it rises, 'cos the water vapour has not released any energy yet. Once the dew point is reached you need to follow the saturated lapse rate.

Do you need to know stuve diagrams as well? They attempt to show all of the above, all on one graph. Seriously confusing. They will have temperature along the x axis and pressure and altitude on the two y axes (the two are of course related). They will show the straight parallell lines of dry lapse rates going from whatever temperature your air parcel starts at, but also the varying-angle saturated lapse rates. You can pick out where the air parcel rises enough to reach the dew point from its initial temp, and then follow them. I'll try to find a link to some examples and edit this with that link.

Hope that helps. I'll look for some links to proper explanations, but sometimes it's clearer in a student's own language.


((apologies if any of that is wrong...lol. All from memory, and it's midnight, and i wasn't very clear myself when i first heard it...)


EDIT: ok, i've found some links.

Here's lapse rates on wikipedia (yes, yes, i know, never consult wikipedia academically. What the hell. I like wikipedia, and i haven't got anything too horribly wrong from using it yet. It seems to be agreeing with what i've written above so i think i got that mostly right. http://en.wikipedia.org/wiki/Adiabatic_lapse_rate
And Stuve Diagrams: http://www.meso.com/wind-personal/glenn/171/Stuve2a.htm (scroll right to the bottom for a full one)

Hope this all helps a bit
Reply 2
Thank You Thank You Thank You!!!!! :smile: :d Life Saver *rep* :d

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