Not a bad effort after 25 years, Chimbu, but from my hazy recollection of this esoteric subject, wrong nonetheless.
The Dry Adiabatic Lapse Rate (DALR) is actually 3 degC per 1,000 feet, and is the rate at which a parcel of unsaturated air will change temperature (solely due to change of pressure) when moved vertically through the atmosphere.
It is the Environmental Lapse Rate (ELR) that, in the International Standard Atmosphere, is 2 degC (well, actually 1.98 degC) per 1,000 feet. This is the variation in temperature with height within a non-moving column of air ... and in real life as opposed to the ISA may be greater or less than the DALR. Air forced to rise where the air mass ELR is greater than the DALR will continue to rise (unstable air mass).
The Saturated Adiabatic Lapse Rate (SALR) is the rate at which a parcel of air that has become saturated (i.e. visible moisture) will change temperature if it is forced to move vertically. Its value is half that of the DALR since latent heat is released as condensation occurs. The SALR has no bearing, as far as I'm aware, on the height of the cloud base.
But as with its temperature, the dew point of the air also normally drops with increased height, since its capacity to hold water vapour at a given temperature increases as its pressure is reduced. In a parcel of unsaturated air forced to rise, the dew point decreases by 0.5 degC per 1,000 feet.
So, on the day in question, cloud formed by adiabatic cooling would have a predicted base (from a tephigram) given by the convergence of temperature and dew point, where temperature starts at +3 degC and decreases by 3 degC per 1,000 feet, and dew point starts at +2 degC and decreases by 0.5 degC. This would suggest a low cloud base of circa 400 feet with an OAT of circa 1.8 degC. Markedly different from what was actually observed.
Well, that's as I remember the theory. Hopefully, a meteorologist will be along shortly to set us straight.
