THERE are certain concepts in life that from time to time you think you understand, only to find a short time later that you have forgotten all you ever, knew. Examples might be the details of the geographical collage that makes up the former Yugoslavia, the intricacies of the CAO points system for getting into university, or the complex relationship between the Hutus and Tutsis in Rwanda. Let me add another to your list this morning: let me tell you all you might wish to know about the "dry adiabatic lapse rate" - aka DALR for short.
Let us first define the word "parcel" as the term is understood in weather parlance. It has nothing to do with "brown paper packages tied up with string": it describes a small, imaginary, and homogenous quality of air whose temperature, pressure and humidity can be inferred at any given time. If such a parcel of air in the atmosphere is warmer than the air ordinaire by which it is surrounded, it will be buoyant and will tend to rise. As this parcel ascends, the pressure on it from the surrounding air will steadily decrease - because pressure, as we know, always decreases with increasing height. The fall in pressure allows the parcel to expand, which in turn, according to the familiar law enunciated by Messrs Boyle and Charles, will bring about a fall in temperature.
Now let us return to the resonant phase we are trying to understand. Lapse rate simply means the rate or fall in temperature with height adiabatic describes a process where no heat is gained or lost from the surrounding; and "dry" in this context refers to air which is insufficiently laden with moisture for any condensation to take place. Voila! The "dry adiabatic lapse rate" is the rate at which the temperature of a parcel of dry air will decrease with height as it floats upwards through the atmosphere without being heated or cooled by its environment. It turns out to be about 3 Celsius per 1,000 ft.
You may wonder, of course, about the practical usefulness of this esoteric concept. Well suppose today's measurements show that the temperature in the atmosphere is decreasing with height at a rate that is greater than the DALR. Then, if a parcel of air in the vicinity is forced upwards by, say, a range of hills, it will find itself; at its new altitude, warmer than its new surroundings. Thereby endowed with a certain buoyancy, the parcel will accelerate upwards through an atmosphere that remains cooler at every level than the parcel itself; so a forecaster may deduce that the resulting convective current will give rise to cumulus clouds, to showers, or even thunderstorms.