Superadiabatic Lapse Rate: when the environmental lapse rate exceeds the dry adiabatic rate; absolute instability such as this is usually limited to a very shallow layer near the ground on hot, sunny days; deep layers in the atmosphere are seldom, if ever, absolutely unstable; on rare occasions when the environmental lapse rate exceeds about 3.4 degrees C per 100m (autoconvective lapse rate), convection becomes spontaneous, resulting in the automatic overturning of the air
Unstable Atmosphere: ex: the environmental lapse rate is 11 degrees C per 1000 m; a rising parcel of unsaturated air will cool at the dry adiabatic rate; because the dry adiabatic rate is less than the environmental lapse rate, the parcel will be warmer than the surrounding air and will continue to rise, constantly moving upward away from its original position; a parcel of saturated air cooling at the lower moist adiabatic rate will be even warmer than the around it
Conditionally Unstable Atmosphere: occurs whenever the environmental lapse rate is between the moist adiabatic rate and the dry adiabatic rate; the average lapse rate in the troposphere is about 6.5 degrees C per 1000m; since this value is between the dry adiabatic rate & average moist rate, the atmosphere is ordinarily in a state of conditional instability
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Causes of Atmospheric Instability: becomes more unstable as the environmental lapse rate steepens or as the air temperature drops rapidly with increasing height; this circumstance may be brought on by either air aloft becoming colder or the surface air becoming warmer; the cooling of air aloft may be due to: 1) winds bringing in colder air (cold advection) 2) clouds or air emitting infrared radiation to space (radiational cooling)
5/26/15. Today was fairly active with a combination of somewhat steeper than normal mid level lapse rates (6.7C/km observed for 700-500 mb on the morning KTAE sounding) and the cold pool boundary from an eastward moving MCS. We'll see what tomorrow brings!
Neutral Stability: if the lapse rate is exactly equal to the dry adiabatic rate, rising or sinking unsaturated air will cool or warm at the same rate as the air around it; at each level, it would have the same temperature and density as the surrounding air; this air tends neither to continue rising nor sinking, so the atmosphere is neutrally stable; for saturated air, neutral stability exists when the environmental lapse rate is equal to the moist adiabatic rate
Atmospheric Mixing: the environmental lapse rate before mixing is less than the moist rate and the layer is stable; if the air in the layer is mixed either by convection or by wind-induced turbulent eddies, the air is cooled adiabatically as it is brought up from below and heated adiabatically as it is mixed downward