Fog, visibility and when it clears

Meteorologically, the boundary between fog and mist is set by visibility: below 1 kilometre it's called fog, between 1 and 10 kilometres it's mist. Both arise from the same thing – water vapour condensing into microscopic droplets in the air near the ground – but fog is dense enough to make it hard to see houses, trees or other cars at a reasonable distance.

The key to fog is the dew point: the temperature the air must be cooled to in order to become saturated with water vapour and release condensation. As the air temperature approaches the dew point, the relative humidity rises towards 100 percent, and eventually water vapour begins condensing into the tiny droplets that make the air hazy.

The most common types

Radiation fog is the most common type inland. It forms during clear, calm nights when the ground rapidly radiates its heat towards a cloud-free sky and cools down. The air closest to the ground cools with it, reaches the dew point, and a thin bank of fog settles in low-lying areas and valleys where cold, dense air gathers. It's often at its thickest just before sunrise and clears fairly quickly once the sun warms the ground.

Advection fog forms instead when relatively warm, moist air moves in over a colder surface, for example mild sea air sliding in over snow-covered ground, or warm air over cold water. It can occur at any time of day, regardless of wind, and tends to linger longer since it doesn't depend on nighttime radiation to form.

Sea smoke, sometimes seen on early cold mornings over open water, is actually the opposite of advection fog: very cold air moves out over comparatively warm water, the water evaporates quickly and the vapour condenses directly in the cold air above. Frontal fog forms along weather fronts where rain falls through colder air and moistens it until it becomes saturated.

Why clear, calm nights are fog's best friend

Cloud cover acts like a lid that traps the heat the ground would otherwise radiate out into space. A clear sky lets that radiation escape unhindered, which allows the ground temperature to drop several degrees below the air temperature at 2 metres in a single night. Wind, on the other hand, mixes the air in the lowest few metres and prevents the coldest air from settling still against the ground. That's why the thickest fog banks tend to form in exactly that combination: a cloud-free sky and almost no wind at all, ideally in a moist valley or near a watercourse.

How fog clears

The sun warming the ground is the most common reason radiation fog disappears. As soon as the ground becomes warmer than the air above it, the reason the fog formed disappears, and the tiny droplets evaporate back into invisible water vapour. The process is slower in autumn, when the sun sits lower and warms less, which explains why autumn fog sometimes lingers well into the morning or, in some cases, all day.

Wind can also clear fog, but the effect works both ways: moderate wind mixes in drier air from above and tears the fog bank apart, while very weak wind can instead spread the same moist air over a larger area without it disappearing. If the wind picks up strongly, the fog often turns into low-lying stratus cloud rather than clearing completely.

Visibility in practice

For a driver, dense fog with visibility under 200 metres means braking distance and reaction time are no longer sufficient at normal speed – which is why speed limits are effectively cut sharply in such weather, and fog lights front and possibly rear should be used. At visibility under 50 metres, which occurs in the very thickest fog banks, it's effectively dangerous to drive faster than walking pace.

At sea, fog is one of the most respected hazards, since ships and other obstacles can be impossible to see at several hundred metres' distance even though radar and AIS work perfectly well. Sound often carries noticeably differently in fog compared with clear air, making it harder to locate another boat's horn by hearing alone. Radar reflections, however, remain unaffected by fog, unlike visible light, which is one reason radar is standard equipment for longer voyages.

Why is it often foggy near lakes and rivers in the morning?

Moist air near watercourses cools easily to the dew point during clear nights, and the water adds extra moisture that makes condensation more likely right there.

Can it be foggy even though it doesn't feel particularly humid?

Yes. It's the relative humidity, meaning how close the air is to saturation at the current temperature, that decides, not the absolute amount of moisture. Cold air needs less water vapour to become saturated than warm air.

Why does fog clear faster in the city than in the countryside?

Cities are warmer at night due to heat stored in asphalt and buildings, the so-called urban heat island effect, meaning the ground temperature rarely drops as low as it does in open countryside.

Try comparing a clear, calm night with a windy, cloudy night in the visualiser and see how the conditions for fog differ. Also read about how wind and temperature interact in the temperature guide.