Lifting Condensation Level (LCL) -- the level at which the lofting air
parcel becomes saturated due to expansional cooling. Procedurally,
the elevation at which the mixing ratio line extended from the surface dew
point intersects the dry adiabat extended from the
surface temperature. Above this level, the air parcel, if lofted, must cool at
the wet adiabatic rate.
Level of Free Convection (LFC) -- the elevation above which a lofting
air parcel becomes warmer than the air surrounding it at the same elevation. As
long as the air parcel is warmer than its surroundings, it will accelerate
upwards, and that acceleration will be directly proportional to how warm the
air parcel is relative to its surroundings (usually shaded in red).
Convective Available Potential Area (CAPE) -- a direct
measure of how buoyant (unstable) the lofting air parcel will be above the LFC.
It really is a measure of how much warmer the air parcel is than its
surroundings for every level from the LFC to the spot at which the air parcel's
ascent curve recrosses the environmental lapse rate
(sounding) for the day (normally shaded in red).
Convective Inhibition Energy (CIN) -- a direct measure of how resistant
an air parcel will be to lofting and is related to how cold the air parcel is
relative to its surroundings. CIN has the same units as CAPE, but is negative.
If the atmosphere is conditionallly unstable, some mechanism
must be present to force lift the air parcel through the area of CIN (normally
shaded in blue).
Equivalent Potential Temperature (qe) – The temperature at
1000 mb (K) an air parcel would have if it were wet
adiabatically lifted from the LCL along a moist adiabat
until all the water vapor condenses out.
Procedurally, the equivalent potential temperature is obtained by
lifting an air parcel from its LCL along a moist adiabat to the point at which the moist adiabat becomes tangent to a dry adiabat
(isentrope) and then bringing the air parcel to 1000 mb dry adiabatically and recording its temperature (K).
Conditional Instability – Defined in a number of ways based upon a “condition”
being met: (a) the situation in which a
rising air parcel must be force lofted through an initially stable layer, but
if lifted moist adiabatically to an elevation (LFC) above which it is warmer
than the surrounding air; (b) a
situation in which the actual environmental lapse rate lies between the dry
adiabatic and moist adiabatic rates; (c)
a situation in which the equivalent potential temperature decreases with
height. For all three conditions the
saturation plays a role.
Absolute Instability – Defined in a number of ways: (a) the situation in which the LFC is
at the base of a layer so that any lofting of an air parcel at the bottom of
the layer brings it immediately to a situation in which it is warmer than the
air around it at the same elevation; (b)
the potential temperature decreases with height.
Absolute Stability– Defined in a number of ways: (a) the situation in which there is no
LFC at any level in the layer so that any lofting of an air parcel at the
bottom of the layer brings it immediately to a situation in which it is colder
than the air around it at the same elevation;
(b) the potential temperature and/or the equivalent potential
temperature increases with height.