|(IN)STABILITY TYPES DEFINED
METEOROLOGIST JEFF HABY
Here are the most common stability / instability terms you will run across and the definition.
Potential Instability-- Instability caused by mid-level
dry air being advected over the top of
PBL warm/moist air.
If the troposphere is forced to lift (i.e. by a front,
vorticity, etc.) the warm/moist air will initially
rise at the moist adiabatic lapse rate (avg. of 5 C/km) while the dry air will cool at the dry adiabatic lapse rate
(9.8 C/km). Over time, the temperature lapse rate (rate of cooling with height) increases.
Convective Instability-- The same as
Absolute Stability-- Any layer in the atmosphere where the rate of actual temperature decrease (or increase) with
height is less than the cooling rate of the moist adiabatic lapse rate. The most stable layers are
inversions (a.k.a. cap)
where temperature increases with height.
Absolute Instability-- Any layer in the atmosphere where the rate of actual temperature decrease with height is
greater than 9.8 C/km (rapid cooling with height). On a sounding these are termed
superadiabatic lapse rates. They
most commonly occur at the surface during strong solar surface heating.
Conditional Instability- Any layer in the atmosphere where the rate of actual temperature decrease with height
is between the moist and dry adiabatic lapse rate. The air is unstable if saturated but stable if unsaturated.
Saturated air cools less with height due to
latent heat release thus allowing the parcel to be warmer than the
environment if lifting occurs in a conditionally unstable environment.
Latent Instability-- Instability caused by the release of latent heat. The more latent heat that is released, the
more a parcel of air will warm. If the PBL is very moist and humid, the moist adiabatic lapse rate will cause cooling
with height of a rising parcel of air to be small (perhaps only 4 C/km) in the low levels of the troposphere. A low
pressure with an abundant amount of
moisture to lift will have more
latent instability than a low pressure that is
surrounded by dry air. Often low pressures will intensify once they get to the east of the Rockies because more
moisture becomes available to lift. A Nor-easter is a classic example of latent instability. Warm and moist air
from the Gulf Stream or Gulf of Mexico increases latent instability.
Notice that all the stability / instability terms deal in some way with the release (or lack of release) of latent heat.