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?
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