Andrew Sarangan wrote:

On a related question, where does the concept of 'average' lapse rate
(2C/1000') come from? I always took this to mean 50% RH air, but it
took me a long time to learn that that was not the case. The air is
saturated or it is unsaturated. How can there be an average between
saturated and unsaturated?

Just realized that my last response didn't fully answer your question.
Lapse rate and moisture content are only loosely related--they are not
inextricably linked. It is possible to have moist air (at or near 100%
relative humidity) in one location with a steeper lapse rate than dry
air at another location. An inversion in dry conditions is just such a
situation.

One thing that CAN be said about the relation between lapse rate and
moisture content: unsaturated air can accomodate a steeper lapse rate
(up to the dry adiabatic rate of around 3C/1000') than can saturated
air. Saturated lapse rates can only reach saturated adiabatic values,
which are less than 3C/1000'. Actual saturated adiabatic lapse rates
are nonlinear functions of temperature, with lower lapse rates for
higher temperatures. This is because warm air can hold more moisture
than cold air.

Hope this helps.

Jim Rosinski