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9.11 Atmospheric Stability

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Adiabatic Process

The Adiabatic lapse rate is the rate of temperature change that occurs in an atmosphere as a function of altitude, assuming that air behaves adiabatically. This term is commonly used to describe the behaviour of air within the 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.

There are three lapse rates used in meteorology:

  1. 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. it is below its dew point. The term 'adiabatic' means that no heat will be gained or lost from outside the parcel. The DALR is a constant 3°C/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. Rising experiences a drop in ambient pressure, so it increases in volume loses energy, and its temperature drops.

    Air has poor thermal conductivity and the bodies of air involved are very large, so the transfer of heat by conduction is negligibly small. Therefore, the process may be approximated as adiabatic (perfectly insulated).

  2. When the air is saturated with water vapour and has reached its dew point, the saturated adiabatic lapse rate (SALR) applies. It varies with temperature and pressure, but is usually near 1 1/2°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. 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'.

  3. The environmental lapse rate (ELR) 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 2 °C/1000 ft from sea level.

    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.

Stability

As unsaturated air rises, its temperature drops at the dry adiabatic rate. The dew point also drops, but much more slowly. 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 condensation level. A cloud base will typically form at this altitude.

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 cumulonimbus clouds (thunderstorm clouds).

Stable: 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.

Conditionally Stable: 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.

Unstable: 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 possibility that air will rise. Charts of the environmental lapse rate are known as tephigrams.

Key items to remember:

  • When air rises, its temperature decreases
  • When air subsides, its temperature increases
  • When the temperature of a parcel of air decreases, its relative humidity increases
  • When the temperature of a parcel of air increases, its relative humidity decreases
  • The normal environmental lapse rate applies to still air
  • The dry adiabatic lapse rate applies to rising air, when the relative humidity is below 100%
  • The dry adiabatic lapse rate also applies to air that is subsiding, if there is no moisture present, and no evaporation is taking place
  • The saturated adiabatic lapse rate applies to rising air, when the relative humidity has reached 100%, and condensation is taking place (above dew point, in the cloud)
  • If the ELR is LESS than the SALR the parcel of air is Stable.
  • If the ELR is GREATER than the DALR the parcel of air is Unstable.
  • If the ELR is LESS than the DALR but GREATER than the SALR the parcel of air is conditional instability or conditional stability. (The terms conditional stability and conditional instability have the same meaning).