Fog, Stability and Visibility
How stable air and moisture combine to form fog and low stratus — and why mornings catch pilots out.
Whether the atmosphere is stable or unstable shapes the weather you'll fly in. Stable air resists vertical motion: it produces stratus cloud, smooth flying, poor visibility, and fog. Unstable air rises freely: cumulus, good visibility, showers and turbulence.
Types of fog
- Radiation fog — forms on clear, calm nights as the ground cools; common at dawn in valleys and over damp ground, often burning off after sunrise.
- Advection fog — warm, moist air flows over a cooler surface (classic at coastlines); can persist in wind.
- Upslope fog — moist air forced up rising terrain cools to saturation.
- Steam fog — cold air over warmer water.
- Precipitation/freezing fog — rain saturates cool air; freezing fog also deposits rime.
The practical lesson
Fog forms when the temperature and dew point converge (within ~2–3 °C with light wind). A clear, calm night with a small temperature–dew point spread is a strong radiation-fog signal for the morning — exactly when a VFR pilot plans an early departure. Check the trend in the TAF and the spread in the METAR before committing.
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*Reference and training only — not for navigation. Always consult the current AIM, FAR/AIM, your aircraft’s POH/AFM, official weather products and a certificated flight instructor.*