Download ATTACHMENT 10 – ADIABATIC PROCESSES

ATTACHMENT 10 – ADIABATIC PROCESSES
The term “adiabatic process” refers to the process of energy conversion in a volume of air
without exchange of heat with the surroundings. We know that
when a volume of a parcel of air changes the temperature changes.
If the parcel of air is compressed, its temperature increases. If it is
allowed to expand, its temperature decreases. This change of
volume is most frequently caused by lifting the parcel of air or its
subsidence: As a parcel of air “sitting” over a warm ground surface
is gradually heated after a while it becomes lighter than the
surrounding air. As soon as this light bubble of air, the thermal,
starts to ascend, we can disregard the heat exchange with the
environment, since the air largely functions as an insulator, and the
influence from the surroundings becomes marginal. We now have a
parcel of air of higher temperature (lower density) than the
surroundings, which has begun to ascend in the atmosphere. We
know that as altitude increase, the air pressure decreases. When the
air pressure of the surrounding air decreases, an excess pressure is
created inside the “warmer bubble”, which is equalized by the
increase of its volume - its expansion. As we mentioned above, an expansion leads to a decrease
in temperature inside the “bubble” => gradually the temperatures equalize and the “bubble” stops
to ascend at that point.
The opposite occurs in subsiding air motions. As a parcel of air becomes colder than its
surroundings, it becomes denser and starts to sink. As it sinks the pressure of the surrounding air
becomes greater and the parcel of air is compressed => its temperature increases.
The decrease or increase of temperature with altitude is termed “lapse rate”. We distinguish 3
types of lapse rate:
Dry Adiabatic Lapse Rate (DALR): provided the parcel of air is not saturated (it is dry),
the lapse rate will be 3°C per 1000 ft (1°C per 100 m), independent of temperature or
pressure of the surrounding air.
Saturated Adiabatic Lapse Rate (SALR): when saturated (humid) air is lifted or
subsides the lapse rate is affected by condensation or evaporation processes. When water
vapour condenses into water droplets due to the cooling of the air parcel, latent heat is
released. This release of latent heat will warm the air, and the lapse rate will be lower than
that of dry air. The opposite is true about evaporation, which occurs when saturated air is
heated = the increase of temperature will be less than that of dry air because some of the
compression heat will be used in the evaporation process (latent heat is absorbed). The
saturated adiabatic rate varies depending on the actual amount of water vapor in the air
that evaporates / condenses. The average value is about 1.8°C per 1000 ft at the surface
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ATTACHMENT 10 – ADIABATIC PROCESSES
and temperature +15°C, with the SALR value approaching that of the DALR at low
temperatures (little humidity).
Environmental Lapse Rate (ELR): the actual rate of change of the air temperature with
altitude in the atmosphere. ELR varies throughout the atmosphere, but its average value is
2 °C per 1000 ft (0.65 °C per 100 m).
Stability of the atmosphere can be defined using the relationship of ELR, DALR and SALR:
Stable atmosphere: when the wind causes a parcel of air to be lifted along the side of a
mountain - if the parcel of air returns back to its original position when this lifting force is
removed, the atmosphere is considered to be stable. In this case the parcel of air will always be
colder than its surrounding air at every level and therefore more dense - it will have a tendency to
sink back to its original position. ELR < SALR < DALR
Unstable atmosphere: if we look at the same situation as above - parcel of air is lifted by wind
along the side of a mountain. If the parcel of air continues to rise even if the initial lifting force
(wind) is removed, the atmosphere is considered to be unstable. In this case the parcel of air will
always be warmer than its surrounding air and therefore less dense - it will have a tendency to
continue rising. SALR < DALR < ELR
Conditionally unstable atmosphere: the same situation as above again - a parcel of air is lifted
by wind along the side of a mountain. Upon removal of this lifting force, if the parcel of air either
continues to rise if saturated (moist - Relative Humidity 100%) or return to its original position if
unsaturated (dry), we refer to this situation as “conditionally unstable” atmosphere. In this case
the stability of the atmosphere depends not only on the ELR as was the case with stable and
unstable atmosphere, but also on the moisture content of the air. SALR < ELR < DALR
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ATTACHMENT 10 – ADIABATIC PROCESSES
In summary we can say that:
Absolute stability = ELR is less than 1.8°C per 1000 ft. Typical weather is low visibility,
light or no turbulence, stratiform clouds and rather continuous (or intermittent)
precipitation. Clouds are typically of large horizontal extent with very little vertical
extent, frequently layered clouds.
Absolute instability = ELR is greater than 3°C per 1000 ft. Typical weather is good
visibility, moderate turbulence, cumuliform clouds and rather a showery precipitation.
Clouds are typically of a small horizontal extent but with a large vertical extent.
Conditional instability = ELR is between 1.8°C and 3°C per 1000 ft.
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