The dry adiabatic lapse rate (DALR) is a term used in atmospheric science to describe the rate at which the temperature of a parcel of dry air changes as it rises or falls in the atmosphere. This rate is dependent on a variety of factors, including the amount of moisture in the air and the pressure and density of the surrounding atmosphere.
Specifically, the DALR is defined as the rate of cooling or warming that occurs when a parcel of dry air (i.e., air with no water vapor) is lifted or lowered without exchanging heat with its surroundings. It is called "dry" because no water vapor condenses or evaporates during this process, which would otherwise release or absorb heat.
The dry adiabatic lapse rate is usually represented in units of temperature change per unit of vertical distance, such as degrees Celsius per kilometer. The DALR is approximately 9.8°C per kilometer or 5.5°F per 1000 feet. This means that if a parcel of dry air rises or falls in the atmosphere, it will experience a temperature change of about 9.8°C (or 5.5°F) for every kilometer (or 1000 feet) of vertical displacement.
The reason for this is that when a parcel of dry air rises or falls, it experiences a change in pressure, which leads to a change in volume. As the volume of the parcel increases, the air molecules within it move farther apart and the parcel expands. This expansion results in a decrease in temperature, according to the ideal gas law (PV=nRT), which relates the temperature, pressure, and volume of a gas. Conversely, when a parcel of air sinks, it experiences an increase in pressure, which compresses the air and leads to an increase in temperature.
It is important to note that the dry adiabatic lapse rate is a theoretical value that assumes the air is dry and that no heat is exchanged with the surroundings. In reality, the atmosphere is not always dry and heat exchange can occur through various processes such as convection, radiation, and advection. Therefore, the actual rate at which the temperature changes with altitude may deviate from the dry adiabatic lapse rate, especially in areas where moist air is present.