The dry adiabatic rate (DAR) is 5.5 degrees F per 1000 feet. The saturated adiabatic rate (SAR) is 3.3 degrees F per 1000 feet. Why is SAR lower than DAR?
To understand why the saturated adiabatic rate (SAR) is lower than the dry adiabatic rate (DAR), we need to understand the basic processes of adiabatic cooling.
When air rises and expands due to a decrease in atmospheric pressure, it cools down. The rate at which the air cools down during this process is called the adiabatic lapse rate. There are two types of adiabatic lapse rates: the dry adiabatic lapse rate and the saturated adiabatic lapse rate.
The dry adiabatic rate (DAR) assumes that the rising air parcel does not have any water vapor. As the parcel expands, it cools at a rate of approximately 5.5 degrees Fahrenheit per 1000 feet. This rate is relatively constant because the presence of water vapor does not significantly affect the temperature change of the air parcel.
On the other hand, the saturated adiabatic rate (SAR) assumes that the rising air parcel is fully saturated with water vapor and condensation is occurring. When water vapor condenses, it releases latent heat, which warms the surrounding air. This latent heat release reduces the rate at which the air parcel cools as it rises. Therefore, the saturated adiabatic rate is lower than the dry adiabatic rate, approximately 3.3 degrees Fahrenheit per 1000 feet.
In summary, the saturated adiabatic rate (SAR) is lower than the dry adiabatic rate (DAR) because the release of latent heat from condensing water vapor slows down the rate at which the air parcel cools during its ascent.