Adiabatic lapse rate

The adiabatic lapse rate is the rate of temperature change that occurs in an atmosphere as a function of elevation, assuming that air behaves adiabatically (thermally insulated). This term is most commonly used to refer to Earth's atmosphere.

The relationship between change in altitude and change in temperature is expressed as a lapse rate. In general, a lapse rate is the rate at which an atmospheric variable (usually temperature) decreases with altitude. It is expressed as the negative ratio of the temperature change and the altitude change, thus:

\gamma = -\frac{T_2 - T_1}{z_2 - z_1}

where γ = lapse rate is given in units of temperature divided by units of altitude, T = temperature, and z = altitude, and points 1 and 2 are measurements at two different altitudes.

Types of lapse rate

There are three lapse rates used in meteorology:

Dry adiabatic lapse rate

The dry adiabatic lapse rate (DALR) is the rate at which a rising parcel of unsaturated air, such as a thermal, will lose temperature. Unsaturated air has less than 100% relative humidity, i.e. its temperature is above its dew point. The term 'adiabatic' means that no heat will be gained or lost from outside the parcel. The DALR is a constant 9.78 °C/km (3 °C/1000 ft, or 5.37 °F/1000 ft). The adiabatic lapse rate can be explained by simple fluid mechanics. As a parcel of air travels downward through the atmosphere, it experiences a rise in ambient hydrostatic pressure. The pressure compresses the parcel, doing work on it and thus increasing the energy and temperature in the parcel. The converse applies for rising air: it experiences a drop in ambient pressure, so it increases in volume, loses energy, and its temperature drops. Air has very poor thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small. Thus, the process may be approximated as adiabatic (perfectly insulated).

Moist adiabatic lapse rate

When the air is saturated with water vapour (reached its dew point), the moist adiabatic lapse rate (MALR) or saturated adiabatic lapse rate (SALR) applies. It varies with temperature and pressure, but is usually near 4.9 °C/km (2.7 °F/1000 ft or 1.51°C/1000 ft). The reason for the difference is that latent heat is released when water condenses. Even though there are no more than 10 grams of water in a kilogram of air at 15 degrees Celsius, water's high heat of vaporization creates a significant release of the energy when it condenses (and is an important source of energy in the development of thunderstorms). Until the moisture starts condensing, the parcel of air cools at the DALR so any air that is unsaturated can be assumed to be 'dry'.

Environmental lapse rate

The environmental lapse rate or temperature lapse rate, which refers to the change of temperature with altitude for the stationary atmosphere. The environmental lapse rate at a given place varies from day to day and even during each day. As an average the International Civil Aviation Organisation (ICAO) defines an international standard atmosphere with a temperature lapse rate of 6.5 °C per km (3.57 °F/1000 ft or 1.99°C/1000 ft) from sea level to 11 km. However, the temperature of the atmosphere does not always fall steadily. For example there can be an inversion layer in which the temperature hardly falls or even rises with increasing height

Significance in meteorology

As unsaturated air rises, its temperature drops at the dry adiabatic rate. The dew point also drops, but much more slowly, typically about 2 °C per 1000 m. If unsaturated air rises far enough, eventually its temperature will reach its dew point, and condensation will begin to form. This altitude is known as the lifting condensation level. The cloud base will typically be at this altitude.

The difference between the dry adiabatic lapse rate and the rate at which the dew point drops will be around 8 °C per 1000 m. Given a difference in temperature and dew point readings on the ground, one can easily find the lifting condensation level by multiplying the difference by 125 m/°C.

The varying environmental lapse rates across the earth surface are of critical importance in meteorology. They are used to determine if the parcel of rising air will rise high enough for its water to condense to form clouds, and, having formed clouds, whether the air will continue to rise and form bigger shower clouds, and whether these clouds will get even bigger and form cumulo-nimbus clouds (thunder clouds).

If the environmental lapse rate is less than the moist adiabatic lapse rate, the air is absolutely stable — rising air will cool faster than the surrounding air and lose buoyancy. This often happens in the early morning, when the air near the ground has cooled overnight. Cloud formation in stable air is unlikely.

If the environmental lapse rate is between the moist and dry adiabatic lapse rates, the air is conditionally stable — an unsaturated parcel of air does not have sufficient buoyancy to rise to the lifting condensation level, but once it gets there, it will gain buoyancy within the cloud.

If the environmental lapse rate is larger than the dry adiabatic lapse rate, the air is absolutely unstable — a parcel of air will gain buoyancy as it rises both below and above the lifting condensation level. This often happens in the afternoon over many land masses. In these conditions, the likelihood of cumulus clouds, showers or even thunderstorms is increased.

Meteorologists use radiosondes to measure the environmental lapse rate and compare it to the predicted adiabatic lapse rate to forecast the likelihood that air will rise. Charts of the environmental lapse rate are known as tephigrams. (See also Thermals).

External links

References

www.air-dispersion.com

Atmospheric thermodynamics | Fluid mechanics

Adiabatisk proces | Temperaturgradient (Meteorologie) | Gradient thermique adiabatique | Gradient adiabatyczny

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