Stability, Dewpoint Spread & Fog
~11 min read · Use temperature/dewpoint spread and stability to anticipate fog and cloud formation.
Stability decides the day's personality: smooth-but-murky or clear-but-bumpy. And the temperature-dewpoint spread is the fog alarm — when it closes to about 2°C, visible moisture is minutes-to-hours away. Both concepts are steady exam producers.
Stable vs unstable air
Stability is the atmosphere's resistance to vertical motion. Stable air resists lifting: expect smooth flying, poor visibility from trapped haze and pollutants, stratiform (layered) clouds, steady precipitation, and fog potential. Unstable air accelerates lifted parcels upward: good visibility (mixing carries haze away), cumuliform clouds, showery precipitation, turbulence, and thunderstorm potential. Temperature inversions — warm air over cold — are the ultimate stabilizer and haze trap.
- Stable: smooth + hazy, stratus, steady rain, fog
- Unstable: bumpy + clear, cumulus, showers, storms
- Inversion = stability lid; smog and fog collect beneath it
Temperature-dewpoint spread
Dewpoint is the temperature at which air saturates. The spread between temperature and dewpoint measures how close the air is to saturation: a closing spread (≈2°C / 4°F or less) with light wind signals fog or low cloud formation, especially in the cooling hours around sunset and dawn. Relative humidity rises as the spread narrows.
Fog types worth naming
Radiation fog: clear nights, light wind, ground cools the air from below — forms in valleys and burns off after sunrise. Advection fog: moist air flows over a cooler surface (coastal fog) — needs wind, can persist all day. Upslope fog: moist air cooled by riding terrain upward. Steam fog: cold air over warm water. Freezing fog deposits ice on structures — and on props. For a dawn drone operation, radiation fog is the routine adversary.
- Radiation: calm clear nights, burns off with morning sun
- Advection: wind-driven over cold surfaces, persistent (coasts)
- Upslope: terrain lifting; steam: cold air over warm water
Worked example
Evening brief for a sunrise bridge inspection over a river valley: skies clear, wind calm, 18°C with dewpoint 16°C at 2100 local. What do you forecast for launch at 0620, and what is your plan?
The setup is a radiation-fog textbook page: clear skies radiate heat away overnight, calm wind prevents mixing, the 2°C spread will close as surface temperature falls toward 16°C, and the river adds moisture in a valley that pools cold air. Expect fog or a low stratus deck at dawn, precisely at launch time. Plan: check the dawn METAR before departing, expect visibility below the 3-SM minimum early, and schedule the flight for mid-morning — radiation fog typically burns off within a few hours of sunrise as the spread reopens.
Common exam pitfalls
Equating smooth air with good flying weather.
Smooth usually means STABLE — which packages haze, low ceilings, and fog with the smoothness.
Watching temperature alone in the evening.
Watch the SPREAD. 18/16 is two degrees from saturation; overnight cooling closes it.
Expecting coastal advection fog to burn off like valley fog.
Advection fog is wind-fed and can persist all day; radiation fog is the one the morning sun erases.
Stable smooth and dirty, unstable rough and clean; spread under two, fog in view.
Recap
- Stable air: smooth, hazy, stratus, steady precip, fog-prone
- Unstable air: turbulent, clear, cumulus, showers, storm-prone
- Inversions trap haze and cap vertical development
- Temp-dewpoint spread ≤ ~2°C + light wind = fog watch
- Radiation fog: clear/calm nights, valleys, burns off after sunrise
- Advection fog: moist air over cool surface, persistent, needs wind
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