Stability, Fronts, and Air Masses
Key instrument-meteorology concepts linking atmospheric stability and air-mass movement to ceilings, visibility, icing, and turbulence.
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Questions Covered in This Set
12 cards to master
Dry adiabatic lapse rate
About 3°C per 1,000 ft — the cooling rate of a rising unsaturated parcel.
Saturated (moist) adiabatic lapse rate
About 1.5–2°C per 1,000 ft; slower because condensation releases latent heat.
When is the atmosphere unstable?
When the environmental lapse rate is steeper than the parcel's lapse rate (e.g., 3.5°C/1,000 ft) — the rising parcel stays warmer and buoyant and keeps rising.
What does an inversion tell you about stability?
Temperature increasing with height means very stable air — displaced parcels sink back; expect haze, stratus, fog, smooth air, and possible low-level wind shear at the inversion.
Rule of thumb for cloud bases from a METAR
Bases (ft AGL) ≈ (T − Td in °C) × 400. Example: 20°C/16°C gives bases near 1,600 ft.
Temperature/dew point spread of 2°C or less in stable, cooling air suggests what?
Fog or a very low ceiling.
Stable air: clouds, precip, visibility, turbulence, icing
Stratiform layers, steady widespread drizzle/rain, poor visibility (haze/smoke/fog), smooth ride, rime icing over long horizontal stretches.
Unstable air: clouds, precip, visibility, turbulence, icing
Cumuliform buildups, brief heavy showers, good visibility between showers, moderate–severe turbulence, clear/mixed icing that is intense but short.
Air-mass modification rule
Warm air over a cold surface stabilizes (fog, stratus, rime, poor visibility); cold air over a warm surface destabilizes (showers, turbulence, good visibility).
Warm front structure and hazard
Shallow ~1:200 slope; warm moist air glides over retreating cold air giving a 500–700 nm shield of stratiform cloud and steady precipitation — with rain falling through subfreezing air (freezing rain/ice) as the main hazard.
cP air mass characteristics
Continental polar from Canada: cold and dry, stable at the source but destabilized when heated from below (lake-effect snow showers).
mT air mass characteristics
Maritime tropical off the Gulf: warm and moist — source of summer thunderstorms and winter stratus/fog.
