The adiabatic lapse rate refers to the rate at which the temperature of a parcel of air changes as it ascends or descends in the atmosphere without exchanging heat with its surroundings. There are two types of adiabatic lapse rates: the dry adiabatic lapse rate and the moist adiabatic lapse rate.
Table of Contents
Dry Adiabatic Lapse Rate (DALR)
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- The dry adiabatic lapse rate applies to unsaturated air, meaning that the air parcel contains no water vapor or is not saturated with water vapor.
- As a parcel of air rises in the atmosphere, it expands and cools adiabatically due to decreasing atmospheric pressure. Conversely, when it descends, it is compressed and warms adiabatically.
- The standard dry adiabatic lapse rate is approximately 9.8°C per kilometer (5.4°F per 1,000 feet). This rate can vary somewhat depending on factors like the composition of the air and the specific heat of the air.
Moist Adiabatic Lapse Rate (MALR)
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- The moist adiabatic lapse rate applies to saturated air, which means the air parcel is carrying its maximum amount of water vapor and may be on the verge of condensation.
- The presence of water vapor introduces additional complexity because it can release latent heat when it condenses or absorb latent heat when it evaporates.
- The moist adiabatic lapse rate is generally lower than the dry adiabatic lapse rate and varies with temperature and pressure. On average, it is around 5 to 6°C per kilometer (2.7 to 3.3°F per 1,000 feet).
It’s important to note that these lapse rates are idealized and assume certain conditions, such as no mixing of air with its surroundings and no external heat sources or sinks. In the real atmosphere, various factors like humidity, cloud formation, and the release or absorption of latent heat can influence the actual lapse rate observed in different atmospheric conditions.
Comparison
| Characteristic | Dry Adiabatic Lapse Rate (DALR) | Moist Adiabatic Lapse Rate (MALR) |
|---|---|---|
| Applies to | Unsaturated air | Saturated air (near or at dew point) |
| Water Vapor | Absent | Present |
| Temperature Change | Approximately 9.8°C per kilometer (5.4°F per 1,000 feet) | Generally lower, around 5 to 6°C per kilometer (2.7 to 3.3°F per 1,000 feet) |
| Reason for Change | Due to adiabatic expansion or compression as air parcel rises or descends without heat exchange with surroundings | Influenced by the release or absorption of latent heat during condensation or evaporation of water vapor |
| Latent Heat | Not involved | Latent heat release or absorption due to water vapor condensation or evaporation |
| Environmental Conditions | Commonly used in dry and clear air conditions | Relevant in humid or moist atmospheric conditions |
| Typical Rate Value | Fixed at 9.8°C per kilometer under standard conditions | Variable, but generally lower than the dry adiabatic lapse rate |
| Application in Atmospheric Science | Used in understanding the temperature changes of unsaturated air parcels in the atmosphere | Used in understanding the temperature changes of saturated air parcels with condensing water vapor |
These rates are theoretical and can vary based on specific atmospheric conditions. The values provided are average or standard values and may not represent the exact lapse rates observed in all situations. Additionally, in the real atmosphere, lapse rates can vary with altitude and geographic location.