Pressure, volume and temperature

[chrisbell-nottheweatherman]

Hello.

I'm new here, although I 'd spend some time posting on the now-defunct BBC weather boards during the winter of '05/'06, using (if memory serves) the screen name cpbell or cpbell0033944, only leaving when one of the Farkin/Allen alter-egos started being abusive. I read about NetWeather then, but was intimidated as it was full of experts and, as a dunce, I wasn't sure whether I'd be welcome or not

I'm not at all knowledgeable on things meterological, (I'm a biologist by training - my maths wasn't anywhere near good enough to get into this fascinating subject) and I'll soon be starting postgrad studies so I won't be able to post that often. However, I'm enthusiastic and would like to understand the weather better than I do now, and, to that end, I've started reading the guides. However, I'm now confused. In the guide entitled "The Mechanism Behind a Sea Breeze", johnholmes states

Just one little bit of physics here: temperature and pressure are related through the Gas Laws, so when the temperature increases, all other things being equal, then the pressure decreases.

however, in the Learners Area, when Avain asks about how Lows and Highs Affect each other, Full Frontal Occlusion says

Temperature also affects pressure in that the individual atoms speed up with higher temperatures. This means that more collisions between atoms/molecules happen over the same time span than for a cooler gas. With greater numbers of collisions, the force exerted collectively has increased and hence the pressure within the gas is higher.

and these two statements seem to me to contradict each other. With memories of GCSE Physics being baffling, I found a kid's revision guide, where it states that Charles' Law shows that pressure and temperature are proportionally-related, which suggests that FullFrontal Occlusion is right.

Can anyone please enlighten me?

[Paul]

Welcome to the forum, I'm sure John and some of the other experts will be along shortly to discuss further

[chrisbell-nottheweatherman]

Thanks Paul!

[chrisbell-nottheweatherman]

Sorry, I forgot to mention my name (seems a bit silly for people to call me w0033944 ). I'm Chris Bell.

[BrickFielder]

John should probably answer this but here is my take on the subject.

A Gas or air heated in a confined area will have its internal pressure rise. So the higher the temperature the higher the pressure. This is what the gas laws show (A steam engine is a good exaple of the principle)

The problem with the atmosphere is that it is not in a confined area as such. One result of low level heating of the atmosphere is that the air rises. The more you heat it the more it rises and this is how storms and clouds form. As the air rapidly rises then something has to try and fill the hole left by the rising air. If the air round about does not move quickly enough (wind) then there tends to be less air and so lower pressure at the surface. The end result is higher pressure at the tropopause and low pressure at the surface. This happens not just at the cloud or storm level ,but at weather system level and even gloabl levels with the Walker and Hadley cells. This is linked to the Gas laws just not in the same direct way as the confined area example. Look up Baroclinic and Barotropic for further information.

Weather is all about air moving either up and down or towards low pressure and away from high pressure and different levels in the atmosphere can have different air movements. These movements always relate to the temperatures of the air masses at various heights above sea level. This is why SkewT charts and soundings are usefull tools for weather prediction.

[chrisbell-nottheweatherman]

John should probably answer this but here is my take on the subject.

A Gas or air heated in a confined area will have its internal pressure rise. So the higher the temperature the higher the pressure. This is what the gas laws show (A steam engine is a good exaple of the principle)

The problem with the atmosphere is that it is not in a confined area as such. One result of low level heating of the atmosphere is that the air rises. The more you heat it the more it rises and this is how storms and clouds form. As the air rapidly rises then something has to try and fill the hole left by the rising air. If the air round about does not move quickly enough (wind) then there tends to be less air and so lower pressure at the surface. The end result is higher pressure at the tropopause and low pressure at the surface. This happens not just at the cloud or storm level ,but at weather system level and even gloabl levels with the Walker and Hadley cells. This is linked to the Gas laws just not in the same direct way as the confined area example. Look up Baroclinic and Barotropic for further information.

Weather is all about air moving either up and down or towards low pressure and away from high pressure and different levels in the atmosphere can have different air movements. These movements always relate to the temperatures of the air masses at various heights above sea level. This is why SkewT charts and soundings are usefull tools for weather prediction.

Thanks BrickFielder. I have to admit you lost me when you started saying words like "tropopause", "Walker and Hadley cells", and "Baroclinic and Barotropic" but I think I get the gist of what you were saying.

Would it be perhaps less potentially misleading to unknowing idiots like myself to think more in terms of air becoming less dense as it's heated and rises? Certainly, a molecular eye-view of this would show greater kinetic energy within a body of warming air, thus the already huge gaps between molecules of the constituent gases (of course oxygen and nitrogen are molecular as gases, not atomic) increase, which lowers the density and causes that body of warm air to rise, rather in the manner of a hot air baloon. All I'm saying is that, to my absolute beginner's logic, to speak of the body of air's pressure having an effect on whether it rises or not seems less than relevant as it's not being contained,as you pointed-out.

P.S, I've just looked-up "Baroclinic" and "Barotropic", and found the following explanation that I could understand (i.e. it didn't contain endless equations):

Baroclinic and Barotropic

is this about right? I'm not, I'm afraid, going to be able to understand complex mathematical theories, I'm just interested in getting a qualitative, empirical understanding.

Thanks again.

[BrickFielder]

The tropopause is the top of the troposphere.

Tropopause explained

Walker and Hadley Cells

Habyhints on pressure and heights

Rising air

Air Masses

Weather is about cold air meeting warm, air rising and sinking and winds which blow from high to low pressure.

Your analogy of the hot air baloon is a good one and perhaps should should think about what would happen if the baloon met a band of very warm air as it was going up (it would stop rising) you also need to take into account that the air will cool in the baloon as you rise which is why you need to blast on the heater occasionally to keep the balloon up.

I hope I am not confusing you more but it can get quite complex.

[johnholmes]

Hello.

I'm new here, although I 'd spend some time posting on the now-defunct BBC weather boards during the winter of '05/'06, using (if memory serves) the screen name cpbell or cpbell0033944, only leaving when one of the Farkin/Allen alter-egos started being abusive. I read about NetWeather then, but was intimidated as it was full of experts and, as a dunce, I wasn't sure whether I'd be welcome or not

I'm not at all knowledgeable on things meterological, (I'm a biologist by training - my maths wasn't anywhere near good enough to get into this fascinating subject) and I'll soon be starting postgrad studies so I won't be able to post that often. However, I'm enthusiastic and would like to understand the weather better than I do now, and, to that end, I've started reading the guides. However, I'm now confused. In the guide entitled "The Mechanism Behind a Sea Breeze", johnholmes states

however, in the Learners Area, when Avain asks about how Lows and Highs Affect each other, Full Frontal Occlusion says

and these two statements seem to me to contradict each other. With memories of GCSE Physics being baffling, I found a kid's revision guide, where it states that Charles' Law shows that pressure and temperature are proportionally-related, which suggests that FullFrontal Occlusion is right.

Can anyone please enlighten me?

hi Chris

Been away a few days but drop me a pm and I'll try and help out and keep it simple for you.

welcome to this site, lots of interesting things in it and please do not feel intimidated. I know very very little about biology for instance!

regards

John

[chrisbell-nottheweatherman]

BrickFielder - thanks for the links. Some already reinforce what I'd read in the Guides, but most are very helpful. I see exactly what you mean about the hypothetical baloon meeting the layer of warm air, and also the need to blast hot air into the canopy at regular intervals. Oddly enough, back when I were a lad (he says putting-on flat hat), I'd always wondered why the air got colder the higher you went; to my logic as a kid, the higher you went, the closer you were to the sun, so the hotter it should be.

John - if I can work-out how to message you, I shall. I'm afraid I really am at a novice level here, so don't be afraid of patronising me.

[johnholmes]

hi Chris

I forgot that as a newcomer you have to do a certain number of posts before you are able to pm. That said I'll try and do my version of an asnwer to your original question as soon as I get the time. Hope you can be patient as I seem to have a lot of things to do away from Net Wx at the moment. Do keep reading the Guides it will help.

be in touch as soon as I can get the time.

regards

John

[chrisbell-nottheweatherman]

hi Chris

I forgot that as a newcomer you have to do a certain number of posts before you are able to pm. That said I'll try and do my version of an asnwer to your original question as soon as I get the time. Hope you can be patient as I seem to have a lot of things to do away from Net Wx at the moment. Do keep reading the Guides it will help.

be in touch as soon as I can get the time.

regards

John

Thanks John.

[chrisbell-nottheweatherman]

Ah, in my post of 12:42, I seem to have confused the reason why air temperature in the troposphere decreases with altitude, which I gather is due to warming of the Earth's surface radiating heat into the lower part of the troposphere to a greter degree than the upper part with the reason air cools as it rises: lowering atmospheric pressure due to reduced gravitational pull causes expansion of the volume of the parcel of air which requires an input of energy to do the work of increasing the distance between the molecules of the various gases, thus using-up heat energy in the parcel of air (hence the need for a hot-air baloon to use it's gas burners regularly to maintain altitude). Am I correct?

Edited July 19, 2007 by w0033944

[johnholmes]

making it a bit complicated, ignore the effects of gravity, but I will try and answer your P, V, T question over this weekend.

[chrisbell-nottheweatherman]

making it a bit complicated, ignore the effects of gravity, but I will try and answer your P, V, T question over this weekend.

John, when I mentioned gravity, I meant purely on the basis of the air becoming thinner with altitude, which I understood to be due to gravitational pull. I'd read that this was the cause of air pressure also dropping as one ascends through the troposphere, therefore, as a parcel of air rises, it encounters steadily lowering air pressure around it, which encourages expansion of the parcel of air, thus cooling it. Eventually, as I've read, the temperature of the parcel of air drops toards the dew-point, eventually causing condensation and thus causing clouds to be formed.

I understand that you're busy, and I look forward eagerly to reading your post over the weekend - above all I want to make sure that I'm reading an accurate description of these processes: my philosophy is that it's better not to learn anything than to learn incorrect information.

[johnholmes]

as a parcel of air rises, it encounters steadily lowering air pressure around it, which encourages expansion of the parcel of air, thus cooling it. Eventually, as I've read, the temperature of the parcel of air drops toards the dew-point, eventually causing condensation and thus causing clouds to be formed.

spot on mate,

I'll do something over the weekend, honest!

[chrisbell-nottheweatherman]

Don't worry John - that's why I said that I knew you were busy and that I was looking forward to your post over the weekend - I wasn't expecting an answer until then! Thanks though; it's nice to know that I've learnt something.

[BrickFielder]

Yes this refers to the adiabatic process of cooling and heating.

The adiabatic process

If a parcel of air cools as it goes up so that it becomes cooler than the surrounding air then it will stop rising. If however there is a steep drop in air temperatures going upwards (lapse rate) then a parcel of air will never cool enough to stop rising and will accellerate until it reaches warmer air which may be at the tropopause. This is why the contrast in temperatures between air masses at different heights can help to determines what type and size of cloud is formed.

You need to take into account diabatic heating and cooling as well where the sun can heat a parcel and moisture evapourating out of the parcel can cool it. So actual cloud formation depends on the moisture content of the air as well.

[chrisbell-nottheweatherman]

Yes this refers to the adiabatic process of cooling and heating.

The adiabatic process

If a parcel of air cools as it goes up so that it becomes cooler than the surrounding air then it will stop rising. If however there is a steep drop in air temperatures going upwards (lapse rate) then a parcel of air will never cool enough to stop rising and will accellerate until it reaches warmer air which may be at the tropopause. This is why the contrast in temperatures between air masses at different heights can help to determines what type and size of cloud is formed.

You need to take into account diabatic heating and cooling as well where the sun can heat a parcel and moisture evapourating out of the parcel can cool it. So actual cloud formation depends on the moisture content of the air as well.

Thanks BrickFielder - I now have a technical term to use in order to help describe this process.

Anyway, still staying on-topic, but moving the focus slightly, there's something else I want to clear-up if I may. Subject to John's weekend posting on the definitive link between P, V and T of a parcel of air, we seem to be broadly agreeing that thehotter a parcel of air is, the greater the pressure it has in terms of the pressure it applies to lower-pressure air around it. However, on reading the sections here and on the MetOffice's educational site regarding high and low pressure systems, it appears that high pressure areas tend to contain cooler air, whereas lows tend to carry warmth and moisture. :huh:

This baffled me until I spotted the mention of high pressure areas containing cool, heavy air. Does this literally mean that the barometric pressue that one might measure at ground level would be higher under a high pressure system beacause of a layer of cooler, and therefore "heavier" air sitting on top of what can (at least in summer) be warm air near the surface, thus acting like a saucepan lid, trapping the pressure? Would this then cause a high lapse rate, explaining why, if a parcel of air near the ground were to warm enough to escape the surface and to pass through the "lid" of cool air, it would be more likely to reach the tropopause before hitting its dew point?

[BrickFielder]

There are I think two different processes by which high pressure can form, which I will try to explain a little, but first I need to mention the different affects of the land and sea. Land warms quickly and cools quickly where as water cools and warms slowly. This mean sea are warm in winter and cold during summer and this can affect the overlying air to create weather.

In siberia during winter a pool of very cold air forms due to being so far away from the moderating warmth of the oceans. This air is cold ,dense and in a sense heavy air. This heaviness pushes down to record high pressure. These high pressure systems tend to be rather shallow.

Siberian High

During summer air heats over land but remain fairly cool over the ocean with the effect that air rises over land and sinks over the ocean. This is why the azores high is normally a dominate factor during the british summer.

AntiCyclones

I think the important point is that high pressure is associated with sinking air and winds blowing away from the center towards low pressure where air is rising.

[johnholmes]

yes its getting very complicated but well explained by Brick.

basics on this are air tries to flow from High to Low pressure, just as a river/stream runs downhill. So on a weather map areas of HP are 'hills' and LP are 'valleys' if that analogy helps.

Best you read the simple explanation about how this evolves, try our Guides again.

I will post re P, V and T during Sunday.

Edited July 20, 2007 by johnholmes

[chrisbell-nottheweatherman]

Thanks chaps!

[chrisbell-nottheweatherman]

Looking back now on this post, I cringe at my own ignorance when I asked such an elementary question! :o All I can say to defend myself is that I was unaware of the context of John's explanation.

[J07]

I don't think there's any shame in asking for clarification, you're a scientist yourself so you must know how important it is to grasp basics. *Especially* when it comes to thermodynamics, which is conceptually one of the hardest branches of physics, and meteorology is so much about thermodynamics and fluid dynamics.

“Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you anymore.”

— Arnold Sommerfeld, when asked why he had never written a book on the subject (c.1950) [11]

[chrisbell-nottheweatherman]

I don't think there's any shame in asking for clarification, you're a scientist yourself so you must know how important it is to grasp basics. *Especially* when it comes to thermodynamics, which is conceptually one of the hardest branches of physics, and meteorology is so much about thermodynamics and fluid dynamics.

“Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you anymore.”

— Arnold Sommerfeld, when asked why he had never written a book on the subject (c.1950) [11]

True, and I think that quote is me and meteorology as a whole!