Does fog form when cold air moves over warm water?

1. The straightforward answer and the named phenomenon

The type of fog that forms when cold air passes over warm water is called steam fog, sea smoke or evaporation fog; authoritative sources explicitly define steam/evaporation fog as forming when cold air moves over warmer water or moist land (National Weather Service; Wikipedia; Met Office) ^{[1] [2] [3]}. This is not an obscure edge case: weather handbooks, training guides and public meteorology pages list steam fog alongside other common fog types and explain it as a routine physical outcome of evaporation and mixing ^{[4] [5]}.

2. How it forms — the physical mechanism in plain language

Warm water gives off vapor that raises humidity in the thin layer of air just above the surface; when that layer mixes with an incoming colder, drier air mass the combined air can cool to saturation so water vapor condenses into droplets and becomes fog — visually like wisps of steam rising from the surface ^{[1] [6]}. Several accounts emphasize the same two-step process: evaporation from the warm surface followed by mixing and cooling of the moist air until relative humidity reaches 100 percent and fog forms ^{[1] [7]}.

3. Conditions that control whether fog actually appears

Steam fog is most common where water remains relatively warm while air masses drop in temperature — for example lakes or coasts in autumn and early winter — because the contrast and available moisture favor saturation ^{[4] [8]}. Calm or light winds, large temperature differences, and available evaporation all influence development; conversely, strong mixing or different wind directions can disperse moisture or shift the air mass, preventing formation ^{[4] [3]}. Sources describe sea fog forming more persistently in specific synoptic setups and coastal currents where temperature gradients are steady ^{[4] [7]}.

4. Variations and hazards — freezing fog, steam devils and aviation impacts

When surface temperatures are well below freezing, steam fog droplets can freeze on contact producing freezing fog or hoar frost, and under some conditions steam devils (whirling columns of steam) can form above water bodies ^{[2] [4] [6]}. Meteorological guides flag evaporation and advection fogs as important visibility hazards for shipping, road travel and aviation; pilots’ manuals and training materials therefore include steam fog among the key fog types to anticipate ^{[9] [10]}.

5. Where nuance matters and what reporting sometimes misses

Popular explanations sometimes conflate different fog types (advection vs. evaporation vs. radiation fog) or reverse cause-and-effect; authoritative sources distinguish steam/evaporation fog — cold air over warm water — from advection fog, which is typically warm, moist air cooled over a colder surface ^{[1] [11]}. The literature also makes clear that fog formation is not automatic: it requires sufficient evaporation, mixing, and atmospheric stability — factors often glossed over in short summaries ^{[4] [5]}. The sources provided cover mechanism and typical settings, but do not supply specific numeric thresholds for temperature difference or wind speed at which fog will or will not form, so precise trigger values cannot be asserted from this reporting ^{[1] [4]}.

6. Bottom line — what can be stated with confidence

Based on multiple meteorological and educational sources, cold air moving over warm water commonly produces steam or evaporation fog through evaporation and mixing that leads to saturation; this is a standard, well-observed process with predictable variants and situational caveats ^{[1] [2] [3]}. Where details matter — forecasting, safety planning, or modelling — practitioners rely on synoptic patterns, temperature contrasts and wind behavior rather than the simple statement alone ^{[4] [9]}.