Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: iSayHi on 16/02/2019 16:38:58

Title: How does humidity affect moist adiabatic lapse rate?
Post by: iSayHi on 16/02/2019 16:38:58
I am curious about how relative humidity affects the moist adiabatic lapse rate.

Using the lapse rate equation, I calculated that as humidity decreases, the lapse rate should increase, since the value of e, the water vapour pressure of the saturated air, decreases. (thus decreasing r, the mixing ratio of the mass of water vapour to the mass of dry air)

Am I right that as humidity decreases, MALR increases (thus temperature change is greater)?
Title: Re: How does humidity affect moist adiabatic lapse rate?
Post by: yor_on on 16/02/2019 18:29:30
You sure about that? It's about elevations isn't it? Atmospheric elevations? If you have molecules in the air and the atmospheric pressure decrease, with gravity or not. Can you then assume that the "vapour pressure of the saturated air, decreases"

Well, yea. I guess you can, but with a greater molecular density you should find a difference, right?.  And I think you need to count in more than just that, like if there is sun light acting on it or if it's inside a cloud, winds have their own 'pressure fronts' etc. And I think temperatures will have a say too.  Maybe one can argue that pressure is temperature, thinking of it. After all, that's 'kinetic energy' isn't it?

Ok, a 'equivalence' then :)

actually you need three things, a density, 'particles', and pressure to gain a temperature. Although, talking about light we need to exchange pressure for momentum.

Damn it, four then. We need a 'energy' too, after all, that's what they exchange. The scarlet pimpernel sort of, 'energy'. Actually 'that isn't good enough. You also have to consider the 'heat capacity' of whatever molecule you think of. If you compare methane to CO2 for example, there's a huge difference in the way they can assimilate and store, not to forget at what time span they release that heat. That should mean that with the same density but with a different composition of molecules you will get different answers.

Now that was tricky. I don't think you can assume that a lesser density or pressure will create a greater temperature difference? Temperature can be seen as a measure of the Suns (rays) momentum 'hitting' a molecule. Then it depends on what heat capacity it can store, right? More energetic molecules should 'wiggle' more than those with a lesser capacity for storing heat and so add to the Infra red radiation. It's tricky..

Maybe you could argue it as a general rule, depending on heat capacity though?
Title: Re: How does humidity affect moist adiabatic lapse rate?
Post by: iSayHi on 17/02/2019 03:38:26
Oof, that was a brain workout. Alright then, let's say that we don't look at temperature change. Let's think ultra simple and see how humidity affects the MALR.

With a decrease in humidity, there will be less water vapour in the air. This means that there will be less water vapour pressure of saturated air (less water vapour anyway, less force exerted).
This means the mixing ratio of the mass of water vapour to the mass of dry air will decrease, and given by the MALR equation, the MALR should be of a lower value. Heat capacity does come into play too.
Title: Re: How does humidity affect moist adiabatic lapse rate?
Post by: yor_on on 17/02/2019 11:34:44
Actually I think it's even worse I :)

Have a remembrance of molecules 'absorbing' sunlight differently depending on elevation. Not only frequencies and densities defining it but something more? Was a old paper I read, years ago.
Title: Re: How does humidity affect moist adiabatic lapse rate?
Post by: Colin2B on 18/02/2019 23:02:56
Let's think ultra simple and see how humidity affects the MALR.
Thinking ultra simple, a change in humidity canít affect the MALR. The terms dry and moist are misleading. Moist means saturated, dry means unsaturated, so moist = 100% RH, it canít be less. If itís unsaturated it is dry and dry air cools at about 10įC/km (DALR) but moist air (100%RH) usually cools at less than 6įC/km (MALR).

The important thing to remember is that it is the saturated air condensing that causes the difference between DALR and MALR. If the air is saturated it cools at the DALR minus the amount of heat added due to latent heat released by condensation. If that latent heat release is small, such as in cold air, then the MALR will be close to the DALR, but if the latent heat release is large - warm saturated air - the MALR will be around 4 to 6įC/km. So, the MALR is not constant because as a parcel of saturated air rises it is cooling and the colder that air gets then the less latent heat it can release.

However, you are correct; if the RH is lower than 100% the lapse rate is higher than the moist rate, but itís all or nothing. :)

Probably what you are really asking is how RH affects the DALR. Clearly if an unsaturated parcel of air rises and cools it can become saturated and the process then becomes the MALR. The point at which this happens will depend on the starting humidity and temperature.