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A question on rising air

  • Joe
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Joe created the topic: A question on rising air

Hello everyone, I'm currently going through my first CPL subject (met) and wanted to get some clarification on what is probably something quite simple.

So rising air expands and cools as it rises, cool air is more dense than warm air (this is why warm air rises). If the rising/cooling air is expanding, isn't it also becoming less dense? Or is this expansion negligible in relation to the air around it and the increased density due to cooling?

If someone could point me in the right direction with this one, that would be great, thanks.
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Richard replied the topic: A question on rising air

Hi Joe,

As you already said, when you go up in the atmosphere, the air cools but also has a net drop in density because of the decreasing pressure. How the air temperature varies with altitude is dependent on a lot of factors such as air mass movements etc and the temperature behaviour of the atmosphere can be profiled on any given day using small weather balloons launched by the Met Office. In the standard atmosphere, ISA, the standard rate of cooling is 2 degrees Celsius per thousand feet but that is rarely the case in real life. This is called environmental lapse rate (ELR) and the ELR at any altitude tells you how rapidly the atmosphere is cooling.

Now, if a small body of air is heated (for example by a hot car park), a localised bubble of warm air forms near the surface. The heating causes expansion of the air, the bubble becomes less dense and so becomes buoyant compared to the ambient air. The bubble will eventually detach and begin to rise. As it rises it begins to expand because of the decreasing ambient pressure but it won't mix with the air around it because the air around it is of a different density. Air, being a good insulator, doesn't like to mix with air of different density.

As the bubble expands though, the heat inside the bubble is spread over a larger volume so any one point in the bubble starts to feel cooler and because of the expansion the net effect is the bubble becomes less dense. This cooling due to expansion is called adiabatic cooling and this rate of cooling is about 3 degrees Celsius per thousand feet (as long as the air doesn't saturate).

So, if the environment is ISA and the ambient air is cooling at about 2 degrees Celsius per thousand feet, but the bubble is cooling at a standard of 3 degrees per thousand, you can see the the bubble will eventually reach a level where it has the same temperature (and density) as the ambient air. Once that happens, all relative buoyancy will disappear and the bubble will simply dissipate.

The variation we get in the stability in the atmosphere comes from how rapidly the environmental air is cooling with height and whether the rising "bubbles" of air saturate or not. If the bubble saturates, it no longer cools adiabatically at 3 degrees per thousand feet, but rather at 1.5 degrees per thousand. This is because the latent heat released by condensation helps to keep the bubble warm as it rises. This gives it a better chance to remain warmer than the air around it and continue rising - all the way to the tropopause if the conditions are right.

Have a look in the textbook on cloud formation and environmental stability to get a more thorough description.

Cheers,

Rich
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Joe replied the topic: A question on rising air

Fantastic, thanks very much, Richard.

I understand to adiabatic process (i think) and the DALR/SALR/ELR relationships. I think i was just getting confused, thinking that the air was cooling as it rises due to mixing/contact with the environmental air, not as you say cooling due to the adiabatic process (makes so much more sense now!).

I was thinking that if the air was expanding because it was cooling, then continued cooling could decrease density to a point where the air would continue to rise (which as I type it out, I now realise makes little sense).

Anyway thanks for the quick and in depth reply, much appreciated.

-Joe
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Richard replied the topic: A question on rising air

No worries. Welcome to the magical mysteries of Met :)

Cheers,

Rich
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