Vertical Stability of the Atmosphere

AVIATION REFERENCE MATERIAL Vertical Stability of the Atmosphere

Bureau of Meteorology › Aviation Meteorological Services

Vertical motion in the atmosphere is The DALR is the rate at which the largely responsible for turbulence and temperature of a dry (unsaturated) cloud formation. air parcel changes as it ascends or descends through the atmosphere.It The strength of vertical motion is is approximately 3°C per 1000 feet. mostly determined by the vertical stability of the atmosphere. A stable The SALR is the rate at which the atmosphere will tend to resist vertical temperature of a moist (saturated) motion, while an unstable atmosphere air parcel changes as it ascends or will assist it. When the atmosphere descends through the atmosphere. neither resists nor assists vertical The SALR is often taken as 1.5°C per motion it is said to have neutral 1000 feet, although the actual figure stability. varies according to the amount of water vapour present and also the temperature of the air parcel (an air Adiabatic Processes and parcel at a higher temperature can Lapse Rates contain more water vapour than when To explain the concept of stability in at a lower temperature). the atmosphere, it is useful to consider Vertical motion of air is what happens to an imaginary The SALR is less than the DALR an important driver of parcel of air displaced vertically from because as a parcel of saturated air weather. Sometimes rising one level to another. During such ascends and cools, water vapour air is made visible by the displacement it is assumed that condenses into water droplets, development of clouds or the parcel undergoes an adiabatic releasing latent heat into the parcel, which slows the cooling (and helps to by the rising dust in dust process, i.e. no heat from the external environment is added to, or subtracted maintain the air parcel’s bouyancy). devils. Violent vertical motion from, the parcel (adiabatic heating is Conversely, if a saturated parcel can be seen in tornadoes. demonstrated when using a bicycle descends and warms, water droplets At other times rising air pump - compression heats the air and will evaporate causing heat (latent heat may occur in the absence of thus the outer casing of the pump; of evaporaration) to be absorbed from any visual clue. Subsiding the reverse occurs when air escapes the parcel, thus reducing the rate of warming (generally termed evaporative air is normally relatively from a tyre - it cools due to rapid expansion). Some mixing with the air cooling). gentle and associated with outside the parcel normally occurs, and clear conditions (except in heat may also be lost or gained through Determining Stability association with mountain radiation, however assuming adiabatic Air rises if it is warmer than its waves and downburst processes is useful in explaining how surroundings. Technically air doesn’t activity from convective the atmosphere behaves. rise because it is warmer; it rises clouds). because warm air is less dense than An air parcel will expand and become cold air, and thus more buoyant. less dense and will thus cool when it moves to lower pressure (higher The vertical temperature profile of altitude); and will contract and become the atmosphere changes as different more dense and will thus warm when air masses dominate a region. If the it moves to higher pressure (lower temperature profile is known, the rate altitude). of change of temperature with height (the environmental lapse rate), and The rate of change of temperature for thus the stability of the atmosphere, at a vertically displaced parcel of air is that point in time can be determined. termed the adiabatic lapse rate. Two Temperature profiles are plotted on different rates apply - the dry adiabatic aerological diagrams to determine the lapse rate (DALR) and the saturated stabilty of the atmosphere or layers adiabatic lapse rate (SALR). thereof. 20 000 A simplified aerological diagram, on which Actual lapse rate the temperature profile (and thus the environmental lapse rate) of the air mass E is displayed by the orange line. The temperature decreases with height S except for the inversion layer D to E. In the A L R layer A to B, the temperature lapse rate is superadiabatic, i.e. it is greater than the D A DALR. LR Stable for both dry and 10 000 saturated air parcels The DALR and SALR are depicted by the white dashed lines. D The yellow dashed line depicts the cooling Conditionally of a saturated air parcel displaced from unstable Unstable for a saturated parcel but stable for point C and rising to E along the SALR. unsaturated It would stop rising at E because its C ) Neutral stability temperature at this point is the same as for dry air parcels the environmental temperature and thus B the air parcel loses its bouyancy. Stable for both dry and saturated air parcels

Height (feet 0 A 0 10 20 30 Temperature (˚C)

An air mass is considered to Referring to the diagram above, parcel forced to rise would cool at be stable, unstable, neutral or stable, unstable, neutral and the DALR and be cooler than the conditionally unstable as follows: conditionally unstable layers in environment, and therefore sink • if a lifted parcel of air is cooler, the air mass can be determined by once any forcing had been removed; and therefore denser, than the comparing the temperature lapse • like the layer from A to B, all surrounding atmosphere, the rates (orange line) of the air mass parcels, saturated or unsaturated parcel will tend to sink once the with the DALR and the SALR: rising between D and E would only lifting mechanism ceases. Such an • any parcel (dry or saturated) forced continue to rise if they were forced environment is defined as being to rise (by, for example, convection) upward, since the layer is stable. STABLE; between A and B, and cooling at In general, when the lapse rate of • if a lifted parcel is warmer and either the DALR or SALR, will remain the air mass: less dense than the surrounding cooler than the environmental • is between the DALR and the SALR, atmosphere, the lifted parcel will temperature and would therefore sink the atmosphere is considered to be continue to rise once the lifting once forcing had ceased. The layer is conditionally unstable; mechanism ceases. In this case the therefore stable, as is the inversion • is steeper than the SALR, the environment is defined as being layer above D; atmosphere is considered to be UNSTABLE; • a dry parcel forced to rise through the absolutely stable; • if a lifted parcel is the same neutrally stable layer between B and • is the same as the DALR, the temperature as the surrounding C will continue to rise only if forcing atmosphere is considered to be air, the conditions are said to be continues, because the parcel would neutrally stable; NEUTRAL; be neither warmer or cooler than the • is less steep than the DALR, the • in some situations the atmosphere environment; atmosphere is considered to be is stable for unsaturated parcels of • a saturated parcel rising through the absolutely unstable. air but unstable if saturated. This is conditionally unstable layer from C For more information on lapse rates called CONDITIONAL INSTABILITY. to D, and cooling at the SALR, would and atmospheric temperature profiles, be warmer than the environment readers can refer to the companion and thus continue to rise unaided. brochure in this series, The Skew T - On the other hand, an unsaturated Log P Aerological Diagram.

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