Which fog is most likely to present hazards due to low-level turbulence and potential icing?

• A. Steam fog
• B. Radiation fog
• C. Advection fog
• D. Upslope fog

Answer: (A)

Fog hazards come from how moist air near the surface moves and how many droplets are present. Steam fog forms when warm water keeps evaporating into cold air, creating a dense, moisture-rich layer just above the water. The temperature difference at the surface creates instability and small-scale convection, so there’s more low-level turbulence inside the fog. At the same time, the droplets inside steam fog can be close to freezing or even supercooled, so if an aircraft penetrates it, icing can form on exposed surfaces.

Radiation fog tends to form under calm, clear nights with stable air, so it’s usually more quiescent and not as turbulent, and icing potential is lower. Advection fog arises from moist air moving over a cooler surface; while it can be dense, the mixing is typically steadier than the turbulent edge of steam fog. Upslope fog forms along terrain where lifting air condenses, which can produce some turbulence, but it’s tied to terrain and wind direction rather than the near-water convection that drives steam fog.

So steam fog is the type most likely to bring noticeable low-level turbulence and a real icing threat due to the combination of surface-based convection and droplet content near freezing.