- #1
Rensslin
- 18
- 3
If cold air is more dense, why is higher altitude air colder and less dense?
rjjd@att.net said:So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself? or would it expand and maybe pop?
DaveC426913 said:Well, the expansion due to heat will far outstrip the compression due to rise in air pressure, so it will most definitely expand.
Studiot said:Why do you think cold air is by definition 'more dense'?
Consider a pneumatic tyre just after you have pumped it up.
Is the air inside more or less dense than the surroundings?
And is it hotter or colder than the surroundings?
The cold is from the gases (propellants) in the can.rjjd@att.net said:I squirted a blast of air out of a can for cleaning keyboard dust. I noticed that the can got cold. I assume that the can being pressurized must have been endothermic, therefore the release of pressure must be exothermic. Thus, the temperature decrease.
When the can is held upright and activated, gas flows out through the nozzle. The pressure inside the can therefore drops, and is no longer sufficient to keep the contents as a liquid; so some of the liquid boils, until the equilibrium pressure is re-established. The vaporization of a liquid is endothermic; thus, heat is absorbed, and the can becomes cold.
No. There is an interplay between pressure, density, and temperature.rjjd@att.net said:Aren't pressure and density synonymous? At least when it comes to air?
Heat is not a thing, separate from the air. The air has a certain amount of heat, which it carries with it as it rises.rjjd@att.net said:Does the air move up through the heat while the heat stays relatively stationary?
If there is moisture in it, the moisture can condense. That's how a thunderstorm works.If warm air is thinner than cold air and it expands as it cools, does it somehow re condense?
The atmosphere has a pressure gradient to it, so it gets less dense as you go up. But at any particular altitude, warmer air is less dense than colder air.I'm trying to get my mind around cold dense air versus cold thin air.
It would collapse.So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself?
Not quite, no. At any altitude, any non-rigid volumes of air will have the same pressure, but may have different density.Aren't pressure and density synonymous? At least when it comes to air?
russ_watters said:It would collapse.rjjd@att.net said:So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself? or would it expand and maybe pop?
Actually, I read too fast and didn't see the cold mountain part, but thanks for bailing me out!DaveC426913 said:Huh. I thought it would expand but, running some numbers through the formulae, you're right - if we use a tall enough mountain.
Going from Everest to Sea level would triple the pressure, causing the volume of the gas to be reduced to one third.
Going from -20C to 20C is actually going from 253K to 293K, which would only cause an expansion by one seventh.
Net effect is a collapse.
DaveC426913 said:Huh. I thought it would expand but, running some numbers through the formulae, you're right - if we use a tall enough mountain.
Going from Everest to Sea level would triple the pressure, causing the volume of the gas to be reduced to one third.
Going from -20C to 20C is actually going from 253K to 293K, which would only cause an expansion by one seventh.
Net effect is a collapse.
rjjd@att.net said:If cold air is more dense, why is higher altitude air colder and less dense?
It will though it is not as simple as that. You tested pressure change only, did you test temp change? Were your crisps* exposed to below freezing temps a thousand metres up?billiards said:I confirm this as I have inadvertently done this experiment in reverse. Buy a packet of crisps at sea level. Drive up a thousand metres or so and look at your crisp packet. In my experience the crisp packet has expanded.
billiards said:I confirm this as I have inadvertently done this experiment in reverse. Buy a packet of crisps at sea level. Drive up a thousand metres or so and look at your crisp packet. In my experience the crisp packet has expanded.
DaveC426913 said:It will though it is not as simple as that. You tested pressure change only, did you test temp change? Were your crisps* exposed to below freezing temps a thousand metres up?
*whatever crisps are...
billiards said:Are you having a giraffe or what? My favourite are prawn cocktail.
I have noticed they do this too. I think it's endemic, not situational.BadBrain said:when I stuck the straw into them, they began to drip the juice out in a steady stream within about a minute, before I'd even sucked on the straw.
klimatos said:In atmospheric physics, there are two kinds of density.
One is mass density, that is, kilograms per cubic meter. The mass density of a volume of air depends upon its composition, its temperature, and its pressure. Colder humid air is less dense than slightly warmer dry air at the same pressure, because the mass of a vapor molecule is less than the mass of a dry air molecule. However, air at high elevations is less dense than air at lower elevations because the number density is less.
The second density is number density, that is the number of molecules per cubic meter. The formula follows Avogadro's Law and is n= P/kT. Here n is the number of molecules per cubic meter, P is the pressure in Pascals, k is Boltzman's Constant, and T is the temperature in Kelvins. Obviously, the number density of a parcel of air depends upon both the temperature and the pressure. The number density of high altitude air is less than that of low elevation air at the same temperature because the number density reflects the Maxwell molecular speed distribution for any given temperature. This is a common exercise in discussions of kinetic gas theory and statistical mechanics. That is, showing that the number density reflects the barometric formula.
Another way of saying this is that there are two types of air pressure: air pressure due to the weight of the air above the air mass you're standing in, which might be called "Gravitational Air Pressure", or "High-Density Air Pressure", and air pressure due to the average kinetic energy of the molecules
Studiot said:What a pity you have mixed up this reasonable statement with a bunch of misconceptions.
A really, really poor way to attempt to heat a gas is to shine a light on it (solar radiation).
Studiot said:The majority of the heating of the Earth's atmosphere comes directly from the surface, both land and sea.
The solar irradiation (apart form a few isolated frequencies) largely passes through the atmosphere and is absorbed by the ground/ocean.
Thus, hot air at the surface creates space for the more rapid motion of its molecules by pushing cooler air aside, keeping its own density as low as possible to accommodate the greater need for freedom of motion for its high-energy molecules.
Studiot said:Be aware that this is not your only misconception (or slack terminology whatever you wish).
The atmosphere is bound to Earth by the Earth's gravity.
The pressure energy due to the weight of the air column is due to gravity.
While that is fascinating, surely you don't suggest that heating due to liftoff of rockets (or any other sounds) affects the climate of our atmosphere...BadBrain said:as is sound (read about the unmanned Apollo 4 mission, which was flown before the installation of the launch complex's sound suppression system).
DaveC426913 said:While that is fascinating, surely you don't suggest that heating due to liftoff of rockets (or any other sounds) affects the climate of our atmosphere...
BadBrain said:That's NOT what I'm saying!
By sound pressure I am referring to acoustical overpressure, NOT to heating.
And I'm NOT claiming that this has any impact on a planetary scale.
All I'm claiming is that the building in which Walter Cronkite was seated suffered severe shaking and a ceiling collapse during the liftoff, which phenomena he reported on-air.
See: