Lapse rate Totally Explained

Everything about Adiabatic Lapse Rate totally explained

The lapse rate is defined as the negative of the rate of change in an atmospheric variable, usually temperature, with height in an atmosphere. While typically applied to Earth's atmosphere, the concept can be extended to any gravitationally supported ball of gas.

Definition

A formal, peer-reviewed definition from the Glossary of Meteorology is: ť The decrease of an atmospheric variable with height, the variable being temperature unless otherwise specified.

The term applies ambiguously to the environmental lapse rate and the process lapse rate, and the meaning must often be ascertained from the context.

Types of lapse rates

There are two types of lapse rate:

Environmental lapse rate - which refers to the actual change of temperature with altitude for the stationary atmosphere (for example the temperature gradient)
The adiabatic lapse rates - which refer to the change in temperature of a mass of air as it moves upwards. There are two adiabatic rates:
- Dry adiabatic lapse rate
- Moist adiabatic lapse rate

Environmental lapse rate

The environmental lapse rate (ELR), is the negative of the actual change of temperature with altitude of the stationary atmosphere at a specific time and specific location. The ELR at a given place varies from day to day and even during each day. As an average the International Civil Aviation Organization (ICAO) defines an international standard atmosphere with a temperature lapse rate of 6.49 °C/1000 m (3.56 °F or 1.98 °C/1000 ft) from sea level to 11 km (36,090 ft). From 11 km (36,090 ft or 6.8 mi) up to 20 km (65,620 ft or 12.4 mi), the constant temperature is -56.5 °C (-69.7 °F), which is the lowest assumed temperature in ISA. It is important to remember that the standard atmosphere contains no moisture, and that the temperature of the atmosphere doesn't always fall steadily. For example there can be an inversion layer in which the temperature rises with increasing height.

Dry adiabatic lapse rate

The dry adiabatic lapse rate (DALR) is the negative of the rate at which a rising parcel of dry or unsaturated air changes temperature with increasing height, under adiabatic conditions. Unsaturated air has less than 100% relative humidity, for example its temperature is higher than its dew point. The term adiabatic means that no heat transfer (energy transfer due to a temperature difference) occurs into or out of the parcel. Air has low thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small.
Under these conditions, when the air is stirred (for instance, by convection), and a parcel of air rises, it expands, because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work; since the parcel does work and gains no heat, it loses internal energy, and so its temperature decreases. (The reverse occurs for a sinking parcel of air.)
For an ideal gas, the equation relating temperature T and pressure p for an adiabatic process is ť

p(z)^

Saturated adiabatic lapse rate

When the air is saturated with water vapour (at its dew point), the moist adiabatic lapse rate (MALR) or saturated adiabatic lapse rate (SALR) applies. It varies strongly with the moisture content, or the psychrometric constant.

Further Information

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