Lapse Rate Definition

Lapse rate is the rate at which air temperature changes with increasing altitude in the atmosphere.

Positive and negative lapse rate

When the temperature decreases with an increase in altitude it is called a positive lapse rate, and zero when the temperature is constant with altitude, and finally, if the air temperature increases with altitude, it is called a negative lapse rate. (temperature inversion).

Lapse rate description

When air is forced upward in the atmosphere, air pressure decreases with height. For a given volume of gas, pressure divided by temperature remains constant (Boyle's Law). Therefore, as the air pressure decreases, the temperature also decreases and vice versa.

Types of lapse rate

Dry adiabatic lapse rate

As we know, atmospheric pressure decreases with increasing altitude and the volume of a parcel of air expands with increasing altitude and conversely, if a parcel of air descends from a higher altitude to a lower altitude, its volume is compressed at lower levels due to the higher atmospheric pressure.

The adiabatic lapse rate (ALR) is the rate at which the temperature of an air parcel changes due to the expansion or compression process associated with a change in altitude in the Earth's atmosphere, if no heat is lost or gained (exchanged) with the surrounding air during this process.

The Earth's atmosphere is not always completely free of water vapor, or moisture in the air. There are times when the air contains as much water vapor as it can hold, and this is called saturated air, or air with a relative humidity of 100%. However, there are also times when the air contains less water vapor than it is capable of holding, and this is called unsaturated air, or air with a relative humidity of less than 100%. The dry adiabatic lapse rate refers to the rate at which unsaturated air cools as it rises and expands. It is also sometimes referred to as the dry adiabat, DALR, or unsaturated lapse rate. It's important to note that "dry" in this context does not mean that there is no water vapor present in the air, but rather that there is no liquid water present. Water vapor can still be present in dry air.

Wet adiabatic lapse rate

When an air parcel that is not fully saturated with water vapor rises from the Earth's surface, it will cool at a rate of about -9.8 degrees Kelvin per kilometer (-5.4 degrees Fahrenheit per 1000 feet) as it expands. This rate of cooling is known as the dry adiabatic lapse rate. Eventually, the air parcel will cool to a temperature at which the water vapor it contains will begin to condense and turn into liquid water. This temperature is known as the atmospheric dew point.

When the air parcel reaches the dew point, it becomes fully saturated with water vapor, and the rate of cooling will slow down. This slower rate of cooling is known as the wet adiabatic lapse rate, which is also sometimes referred to as the wet adiabat, saturated lapse rate, SALR, moist adiabatic lapse rate, or MALR. This slowing down of the cooling rate is due to the release of latent heat as the water vapor condenses into liquid water.

The wet adiabatic lapse rate, which is the rate at which temperature decreases with altitude when an air parcel is rising and releasing heat through condensation, is not constant. This is because it depends on the amount of water vapor present in the air when it starts to rise. The amount of heat that can be released through condensation varies with the amount of water vapor present, so the wet adiabatic lapse rate also varies.

In the troposphere, the wet adiabatic lapse rate can range from about 4 K/kilometer (2.2 °F/1000 ft) in regions where the ambient temperature is about 25 °C (77 °F) to about 7 K/kilometre (3.8 °F/1000 ft) in regions where the ambient temperature is about – 10 °C (14 °F).

When an air parcel reaches its dew point and the rate of cooling slows to the wet adiabatic lapse rate, it will continue to rise until all of its water vapor has condensed. At this point, the rate of cooling will revert back to the dry adiabatic lapse rate.

Environmental lapse rate

The dry adiabatic lapse rate and wet adiabatic lapse rate are ideas about how temperature changes with altitude under certain conditions. In reality, the temperature at different altitudes in a specific place at a specific time is called the environmental lapse rate, which can also be referred to as the ELR, prevailing lapse rate, or ambient lapse rate.