Stability

As discussed earlier, huge lumps of air roam freely over the surface of the earth. Some lumps are warmer, some are colder. Some are wetter, some are drier, and some are piled higher than others. And they are the way they are because of where they formed. Let an air mass sit over a warm ocean and you'll get a warm, moist air mass. Of course even an air mass that is a tropical thirty-five degrees at the surface is colder aloft, with the temperature decreasing by anywhere from about one to five degrees celsius for every thousand feet you go up. The rate of temperature decrease is called the lapse rate. There can be odd local variations in lapse rate, but by the time you reach the tropopause (the end of the first layer of air) at 30,000-60,000 feet, the temperature is -56C. At any one altitude within the same air mass, the temperature is about the same.

In addition to temperature and moisture, we are interested in the stability of an air mass. Stability is not with regards to lateral motion of the air mass, but rather to vertical motion within the air mass. If an air mass is stable then air displaced vertically tends to return to where it was, while in an unstable air mass, vertical displacement results in continued vertical motion. Kind of like a stable person who goes to Mexico for a vacation goes home and back to work, while an unstable one might get a new job as a llama herder and end up six months later calling you from Tierra del Fuego, asking you to wire money. Well maybe not much like that. But that's the terminology. I'll be using it in a few paragraphs.

Air within air masses is getting displaced all the time. As the air mass moves over uneven ground, some of the air is displaced upwards. An airplane flies by, swirling the air around. Some of the air is heated, becomes less dense and thus starts to rise above the denser air around it. There are lots of reasons for air to move.

As soon as some amount of air, some textbooks call it a "parcel," moves upward, it is in a new location. The air newly surrounding it is different than the air in its old neighbourhood. For starters, the pressure is lower. The only thing that was keeping the parcel of air at a higher pressure was the presence of air at that pressure all around it, so as it rises and the pressure around it drops, it is no longer as contained and it expands until its pressure matches the pressure around it. That expansion results in cooling, as I mentioned last time. Thus the raised air parcel has a lower pressure, a greater volume, and a lower temperature. The surrounding air hasn't changed as a result of the move, but the temperature of the surrounding air is going to be less than the temperature of the air that surrounded the parcel at its old altitude, simply because the atmosphere is colder at a higher altitude.

So which is colder, the parcel of air that has been raised, or the air that now surrounds it? They are both colder than the old temperature of the air parcel: the parcel of air cooled off as a result of expansion when it moved upward, and the surrounding air just happens to be colder than the air that surrounded the original parcel. The answer is, it depends on whether cooling by expansion was greater or less than the lapse rate, the change in temperature with altitude.

The trick is, cooling through expansion is predictable. A parcel of air that is raised one thousand feet will cool by three degrees. Done deal. So you need only look at the lapse rate of the surrounding air to predict whether the raised parcel will be warmer or cooler than the air in its new environment. If the lapse rate is steeper (i.e. greater) than three degrees per thousand feet, then the surrounding air will be cooler than the raised parcel. If the lapse rate is shallower than three degrees per thousand feet then the the raised parcel will be cooler than the surrounding air. (There's an exception to that last sentence, but I will explain it later).

Next question, why have I spent so many words wrangling with whether one bit of air is warmer or colder than another bit? Well what happens when a parcel of warm air is surrounded by colder air? (Hint: see the title of the last weather theory post). The warmer air rises. So if a parcel of air is disturbed in surrounding air that has a steep lapse rate, the parcel will continue to be warmer than the surrounding air and will continue to rise. If the lapse rate of the surrounding air is shallow, the parcel soon cools below the temperature of the surrounding air, and sinks back to its original level.

And now you can see that if the lapse rate of the surrounding air (known as the environmental lapse rate) is less than the rate of cooling with expansion of lifted air (known as the adiabatic lapse rate) then the air is stable. If the environmental lapse rate is greater than the adiabatic lapse rate, then the air is unstable.

And on that terribly technical-sounding but somewhat simplified sentence I will end this blog entry. If you know about the dry and saturated adiabatic lapse rate don't complain that I didn't mention them, I'm getting there, I promise.