050 Meteorology topic guide
Fog Types: Radiation, Advection and More
Fog is simply cloud sitting on the ground, and every type forms by pushing air to saturation in one of two ways: cooling it until its temperature meets the dew point, or adding moisture until the dew point rises to meet the temperature. Which mechanism is at work, and what is supplying it, is what actually distinguishes one fog type from another, far more than any description of how thick or how long it lasts.
Radiation and advection fog cover most of the exam questions, and telling them apart by their formation conditions, especially the wind, is the single most examined skill on this topic. Steam fog, frontal fog and hill fog then round out the syllabus with less common but still testable mechanisms.
Cooling fogs: radiation and hill fog
Radiation fog forms overnight under a clear sky, which allows the surface to cool by radiating heat away unobstructed, combined with a light wind, enough to mix a shallow layer of air so the cooling reaches slightly above the ground, but not so much that it stirs the cooled air away entirely. Given a moist airmass, these conditions build a shallow fog layer that typically clears, or burns off, after sunrise, once solar heating raises the surface temperature back above the dew point and mixes the shallow layer away, though it can persist longer under overcast skies or in a sheltered valley.
Hill, or upslope, fog is close cousin to radiation fog: it forms either when moist air is forced up rising terrain and cools adiabatically to saturation, or when radiational cooling on a slope drains cold air downhill until it pools and saturates in a valley. It behaves similarly to radiation fog in how sensitive it is to wind strength and daytime heating.
Advection fog: why it survives the wind that kills radiation fog
Advection fog forms when relatively warm, moist air moves horizontally over a colder surface, sea or land, and is cooled from below until it saturates. Unlike radiation fog, advection fog needs wind to keep carrying warm, moist air across the cold surface, and it can persist, or even thicken, in wind speeds that would have dispersed a radiation fog entirely. It can also cover a much larger area and last through the day, since heating from the sun above does little to warm air that is being continuously cooled from underneath.
Sea fog is the classic advection case, and the same mechanism appears inland whenever moist maritime air moves onto a colder landmass.
Steam fog and frontal fog
Steam fog, sometimes called sea smoke, has the opposite temperature arrangement to advection fog: very cold air moves over much warmer water, the warm water evaporates rapidly into the cold air, and that air saturates almost immediately at the surface, producing rising wisps that look like smoke. It is a moisture-addition mechanism, not a cooling one.
Frontal fog forms when rain, usually ahead of a warm front, falls into cooler air below, evaporates, and raises that air's dew point until it saturates. This too is a moisture-addition mechanism, tied to precipitation near a front rather than to overnight surface cooling. Because cooling fogs and moisture-addition fogs clear by different means, cooling fogs disperse once heating or mixing overcomes the process that made them, while moisture-addition fogs disperse only when the moisture source is removed or the airmass changes, which is exactly why waiting for sunrise does not reliably clear an advection or a frontal fog the way it clears a radiation fog.
Worked example
Worked example: identifying a persistent coastal fog
Not quite. The correct answer is B.
Correct answer: B
- A. Radiation fog needs a clear night with only a light wind. A steady 12 kt wind would normally have prevented or mixed away a radiation fog rather than sustaining it into the afternoon.
- B. Correct. The onshore wind is carrying warm, moist air over a colder surface, cooling it continuously from below, so neither the wind nor daytime sunshine disperses it the way they would a radiation fog.
- C. Steam fog needs very cold air moving over much warmer water, the reverse temperature arrangement to the one described here, and it produces rising wisps rather than a persistent, wind-sustained fog bank.
- D. Hill fog depends on terrain lifting moist air, not on a sustained onshore wind over flat coastal ground, and offers no mechanism of its own to explain resistance to full sunshine.
Step by step
- Check the wind: 12 kt is too strong for radiation fog, which needs only a light wind to form a shallow, quiet, radiatively cooled layer.
- Check persistence: radiation fog reliably burns off once the sun heats the surface above the dew point, so a fog resisting full sunshine is not primarily a radiation-cooling case.
- Advection fog is created by a continuous, wind-borne supply of warm, moist air over a colder surface, so its cooling mechanism keeps running regardless of sunshine from above.
- The coastal setting with a steady onshore wind matches the classic sea fog case of advection fog exactly.
- Since the source, a continuous cold surface and an onshore supply of moist air, has not changed, the fog has no reason to clear on its own.
Common mistakes
Believing fog always needs calm conditions
Only radiation fog needs a light wind. Advection fog specifically requires wind to keep supplying warm, moist air, so a stem quoting a stronger wind usually points away from radiation fog rather than ruling out fog altogether.
Expecting sunrise to clear every fog
Burn-off by daytime heating is a radiation-fog behaviour. Advection, steam and frontal fog are sustained by mechanisms sunshine does not remove, so predicting clearance at dawn for the wrong fog type costs the mark.
Confusing the direction of the temperature difference in steam fog
Steam fog needs cold air over warm water, the opposite arrangement to advection fog's warm air over a cold surface. Swapping the two collapses the distinction the question is testing.
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Last reviewed July 2026