Remember you
determine the stability or instability of the air by comparing the DALR or SALR
to the ELR. The DALR and the SALR will apply to a parcel of air that is forced
to rise by some trigger action.
The DALR is always 3 degrees/1000ft or 0.98 degrees/100m (use 1 degree for
practical calculations) and in our latitudes the SALR is 1.8 degrees/1000ft or
0.6 degrees/100m.
The variable is the ELR. It can be any temperature, but if it is an ISA day
then it will be 1.98 degrees/1000ft or 0.65 degrees/100m.
If the ELR is less than 1.8 degrees/1000ft then the air is stable, because a
parcel of air that is forced to rise will cool at either at the DALR or the
SALR (from the cloud base) so it is cooling faster than the environment, so the
minute the trigger action stops, the parcel will sink back to its original
level.
If the ELR is more than 3 degrees/1000ft then the air is unstable, because a
parcel of air that is forced to rise, will always be warmer than the
environment, so it no longer needs a trigger, it is like a hot air balloon.
If the ELR is between 3 and 1.8 degrees/1000ft the the
air is conditionally unstable. The condition is that the trigger must be strong
enough to take it to the height where it will become unstable. The reason is
that if it is cooling at the DALR, initially it will be stable, but from the
cloud base it will now cool at the SALR so it will slowly catch up to the ELR,
and if the trigger is strong enough, to lift it enough, it will eventually get
warmer than the environment, and therefore become unstable.
In this example the environment is cooling at more than 1 degree/100m (more
than 3 degrees/1000ft) so the parcel, regardless of whether it is dry or
saturated, will always be warmer than the environment, so it will be a hot air
balloon, and no longer needs a trigger. That is absolutely unstable.