Atmospheric Stability - Thunderstorms

[JD Jaya]

When the adiabatic lapse rate becomes warmer than the environmental lapse rate at, say, 5000 feet, is that the point where an uplifting force is no longer necessary? My understanding is that below 5000 feet, the air remains stable, so an uplifting force is needed to reach that altitude. Once at 5000 feet, however, the adiabatic air would be warmer and less dense than the surrounding air, causing it to rise on its own.

In addition, In an absolutely unstable atmosphere, where the adiabatic lapse rate consistently exceeds the environmental lapse rate, at what point do clouds stop forming? Do they stop once they reach stable air in the stratosphere/tropopause?

 

[Dan Thomas]

The lapse rate has to do with the temperature decease with altitude, from the ground up. Normally, it's about 2°C per thousand feet. At this rate, or less, the air is stable and won't support thermal lift. But if that lapse rate gets a bit steeper, more than 2°C per thousand feet, air that rises will cool at 2°C per 1000 as its pressure drops, so it ends up warmer than the surrounding air at any altitude and is less dense, so less heavy, and it continues to rise. If that air has enough water vapor in it, that water condenses as the pressure and temperature fall, releasing MORE heat into the air and making it rise even more aggressively. That heat was the energy it took to turn the water into water vapor in the first place. This is where thunderstorms develop.

When that rising air hits the tropopause, the meeting place of the troposphere and stratosphere, the rise usually slows or stops. That's where you see the classic "anvil" head of the storm. At the tropopause, the lapse rate reverses and the air begins to get warmer with more altitude, so it won't support a bunch more lift. But if the thunderstorm was enthusiastic enough, the momentum of that rapidly rising air will punch through the tropopause and go thousands of feet into the stratosphere, making some really damaging thunderstorms.