ADIABATIC TEMPERATURE AND DEWPOINT CHANGES
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METEOROLOGIST JEFF HABY
An adiabatic temperature change is the change in temperature a parcel of air undergoes when it
rises or sinks. The dewpoint changes as a parcel rises or sinks even though the amount of
moisture in the parcel of air remains the same.
When unsaturated air rises the temperature decreases by 10 Celsius (C) for each kilometer the
air rises. When air sinks it warms by 10 C for each kilometer. If the temperature of air
is 7 C and it rises 1 kilometer while remaining unsaturated it will cool to - 3 C once
it has risen 1 kilometer.
When unsaturated air rises, the dewpoint decreases by 2 C per kilometer. This occurs because thermodynamic
properties of the air such as pressure and volume are changing. If the dewpoint of a sample of
air is 16 C and it rises 1 kilometer while remains unsaturated the dewpoint will decrease to 14 C once it
has risen 1 kilometer. The dewpoint increases by 2 C per each kilometer the air sinks. Therefore, the
parcel of air that had the dewpoint decrease to 14 C will increase back to 16 C once the parcel sinks
back down 1 kilometer.
When air is saturated the temperature rate of cooling is not the same as when the air is
unsaturated. However, sinking air will always warm at 10 C per kilometer and the dewpoint
of sinking air will always increase by 2 C per kilometer. Once air begins the sink the
relative humidity will decrease below 100% since the temperature increases at a rate
more than the dewpoint increases when air sinks.
Saturated air cools at a rate that is less than 10 C per kilometer. The rate varies between
about 4 C per kilometer to very near 10 C per kilometer. The rate is closer to 4 C
per kilometer if the dewpoint of the air is very high and the rate is closer to 10 C per
kilometer if the dewpoint is very low. When the dewpoint is very low then the air is
almost dry even if it is saturated. When air has a high dewpoint and the air is saturated
there will be an abundance of condensation when the air rises. Since condensation warms the
air it partially cancels out the 10 C per kilometer cooling that unsaturated air has.
When working problems with saturated air, the rate of cooling will need to be given to
you since it is not constant. For example, if the rate of cooling for saturated air
is given as 5 C per kilometer and a parcel of air is 15 C then the temperature of the
air will cool to 10 C once it rises 1 kilometer.
The temperature and dewpoint fall together in saturated air. For example, suppose the
temperature and dewpoint are 8 C and air rises 1 kilometer and the rate of cooling
is 6 C per kilometer, after rising 1 kilometer the new temperature and dewpoint will
both by 2 C.
Below are 3 example problems. Use the information above to try to answer the questions. The answers
are given at the bottom of this page.
1. A parcel of air has a temperature of 31 C with a dewpoint of 9 C. What is the temperature
and dewpoint once this parcel of air is lifted by 2 kilometers?
2. A parcel of air has a temperature of 31 C with a dewpoint of 23 C. Once the air
becomes saturated the rate of cooling is 5 C per kilometer. What is the temperature
and dewpoint once this parcel of air is lifted by 2 kilometers?
3. A parcel of air has a temperature of 31 C with a dewpoint of 23 C. Once the air
becomes saturated the rate of cooling is 5 C per kilometer. What is the temperature
and dewpoint once this parcel of air is lifted by 2 kilometers and THEN sinks by
2 kilometers?
Answers
1. Temperature 11 C with a dewpoint of 5 C
2. Temperature and dewpoint are both 16 C
3. Temperature is 36 C with a dewpoint of 20 C
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