Re: Why is air cooler at higher elevations?
- From: John Bailey <john_bailey@xxxxxxxxxxxxxxxx>
- Date: Wed, 01 Aug 2007 08:41:24 -0400
On Tue, 31 Jul 2007 01:01:24 GMT, "Dan in Philly" <djr8@xxxxxxx>
wrote:
In a nutshell: If a parcel of air in moved to a higher altitude it
I realize that the atmosphere gets less dense at higher elevations. But I
don't understand why the temperature also gets cooler.
With all the molecules bouncing against each other, it seems like the higher
temps. at lower elevations would transfer to the higher elevations.
Dan in Philly
cannot exchange heat easily because air is a good insulator so it gets
cooler as it expands. The term is Adiabatic.
http://farside.ph.utexas.edu/teaching/sm1/lectures/node56.html
Classical thermodynamics
The Adiabatic Atomsphere
"Imagine a packet of air which is being swirled around in the
atmosphere. We would expect it to always remain at the same pressure
as its surroundings, otherwise it would be mechanically unstable. It
is also plausible that the packet moves around too quickly to
effectively exchange heat with its surroundings, since air is very a
poor heat conductor, and heat flow is consequently quite a slow
process. So, to a first approximation, the air in the packet is
adiabatic. In a steady-state atmosphere, we expect that as the packet
moves upwards, expands due to the reduced pressure, and cools
adiabatically, its temperature always remains the same as that of its
immediate surroundings. This means that we can use the adiabatic gas
law to characterize the cooling of the atmosphere with increasing
altitude. In this particular case, the most useful manifestation of
the adiabatic law is p^(1- gamma)*T^gamma = K
Hence, we can calculate, from the above expression, that the
temperature of the atmosphere decreases with increasing height at a
constant rate of centigrade per kilometer. This value is called the
adiabatic lapse rate of the atmosphere. "
Also
http://en.wikipedia.org/wiki/Lapse_rate
Lapse rate
"The dry adiabatic lapse rate (DALR) is the negative of the rate at
which a rising parcel of unsaturated air, such as a thermal, changes
temperature with increasing height. Unsaturated air has less than 100%
relative humidity, i.e. its temperature is higher than its dew point.
The term 'adiabatic' means that no heat is gained or lost from outside
the parcel. The DALR is approximately constant at + 9.78 °C/km (+ 5.37
°F/1000 ft, or about + 3°C/1000 ft). As a parcel of air travels
downward through the atmosphere, it experiences an increase in ambient
hydrostatic pressure. The pressure compresses the parcel and by the
ideal gas law its temperature increases. The converse applies for
rising air: it experiences a decrease in ambient pressure, it expands,
and its temperature drops. Air has low thermal conductivity, and the
bodies of air involved are very large, so transfer of heat by
conduction is negligibly small. At usual atmospheric temperatures,
heat exchange by thermal radiation is insignificant and by definition
convection does not exist within a parcel of air. Thus, the process
may be approximated as adiabatic (perfectly insulated)."
.
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