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SALR, latent heat question

  • Cessna182
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Cessna182 created the topic: SALR, latent heat question

G'day,

Am just reading about the SALR, and it states that the amount of latent heat released causes the cooling rate to slow down.

If heat is released from the parcel, wouldn't that cause the cooling rate to speed up since the heat is no longer keeping the parcel "as warm"anymore? I know it's wrong, but the way I'm visualising it is almost like the heat is now leaving the parcel, so it should get colder quicker.

Would appreciate it if someone can shed some light on this
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Richard replied the topic: SALR, latent heat question

Hi Cessna,

Air is a good insulator and so doesn't like to mix with air of a different density. The parcel of air is rising in the air around it and is rising because it has a lower density. One way to visualise it, is to think of it as behaving like a "sealed" bubble, isolated from the atmosphere around it until it reaches a level where the environment air has the same density as itself.

As it rises, the bubble cools adiabatically at 3 degrees per 1000ft (DALR) but, if condensation is occurring, latent heat energy is being released into the bubble. The bubble will still try to cool adiabatically at 3 degrees per 1000ft but at the same time, it will be kept warm by the latent heat being released by condensation. It won't cool as rapidly anymore. This is why we end up with a new lapse rate, the SALR of 1.5 degrees per 1000ft.

Cheers,

Rich
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  • Cessna182
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Cessna182 replied the topic: SALR, latent heat question

Gotcha. Makes sense when you say it's released inside the bubble which reduces the cooling rate.
There's also a paragraph inside another book which states 'when water vapour condenses latent heat is released into the surrounding air, this warming it'. Is that statement also correct?
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Richard replied the topic: SALR, latent heat question

Yes it is. Again, visualise it as releasing heat into the air surrounding the new droplet, inside the bubble. It is a simplified model, but quite closely represents what is going on.

Cheers,

Rich
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  • Cessna182
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Cessna182 replied the topic: SALR, latent heat question

That all makes sense now, thanks again Richard
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