Saturday, June 16, 2007

Adiabatic Lapse Rates

A few weeks ago I discussed expansion cooling, and the relationship of the stability of an air mass to its environmental lapse rate. You can pretend you remember that if the temperature decrease with altitude is less than the rate of cooling of a parcel of raised air, then the air is stable, but if the atmosphere decreases in temperature faster than raised air cools by expansion, then the air is unstable. I said that raised air always cools through expansion at three degrees celsius per thousand feet. But I hinted that I was leaving something out. I was.

I left out the consequences of condensation. Once air cools to its dewpoint, water vapour starts to condense into water droplets. These droplets make up clouds or mist, and as I explained in a different earlier post, the act of transforming from vapour to liquid actually releases heat. This release of heat partially cancels out the cooling by expansion, such that once visible moisture starts to form, raised air cools at an effective rate of one and a half degrees per thousand feet. You can think of it continuing to cool at 3 degrees/1000 ft, but then warming back up again by a degree and a half, for a net decrease of 1.5 degrees/1000 ft. Apply that knowledge to the idea of stability and instability and you see that moist air will be unstable with a shallower lapse rate than dry air.

Air that has an environmental lapse rate of more than three degrees per thousand feet is going to be unstable. As rising air is cooled to its dewpoint the temperature differential between raised air and its environment will increase even faster, and that's how giant cumulus clouds form. Air that has an environmental lapse rate of between one and a half and three degrees will be stable if the air is dry, but unstable if the air is saturated. so the air is termed conditionally stable. Or damn, maybe it's conditionally unstable. One of the two. But if the environmental lapse rate is less than one and a half degrees per thousand feet, the air is stable regardless.

Stable air is smoother to fly in and the clouds that form are flat and featureless. Unless flat is a feature. Rain falling from flat clouds is steady. Flat clouds are called stratus clouds, because everything is always cooler in Latin, so you get STable air with STratus clouds and STeady precipitation.

Unstable air is bumpy. And I suddenly remember an elementary school class in which Caroline, asked to provide an adjective to describe a cloud, supplied bouncy and refused to back down when the teacher rejected it. Caroline, you were right all along, and if I ever see that teacher I will take him for an airplane ride through a bouncy cumulus cloud. Cumulus is Latin for heap, I think. You can remember that the cloud acCUMULates upward. Precipitation falling from cumulus clouds is showery, meaning intermittent, not steady, but probably heavier than the precipitation from stratus clouds.