Unit 2 of 12 · 4 of 4 lessons published
Stability and instability
Whether air rises on its own decides most of what the sky does: towering storms or flat stratus, a gusty afternoon or a calm, hazy one, fog at dawn or none. Stability is the atmosphere's answer to a simple test. Push a sample of air upward, and it either sinks back, stays put, or keeps going.
This unit works through that test step by step. It starts with the air parcel and why rising air cools, compares that cooling with the measured temperature profile to define stable and unstable air, measures the result as energy with CAPE and CIN, and ends with the inversions that hold storms back and the cap that decides when they break loose. The examples are observed soundings from real days.
The unit assumes the first unit's lessons on temperature and on dew point. It leads directly into the skew-T lesson, which reads the same ideas off the chart forecasters use.
- Air parcels and adiabatic cooling
Why rising air cools, at the dry rate and then the moist rate.
- Lapse rates and stability
Stable, unstable and conditionally unstable air, read from the temperature profile.
- CAPE, CIN and instability
The energy for an updraft, the energy against one, and the three ways to lift a parcel.
- Inversions and the cap
Radiation, subsidence and frontal inversions, and the elevated mixed layer.