Fog is simply a cloud that has formed at ground level instead of high above it.

The longer version

Meteorologists classify fog by how the cooling happened, but all types share one feature: the condensation level lies at or within a few metres of the ground. Radiation fog forms on clear nights, advection fog when moist air drifts over cold ground or water, and upslope fog when air is forced up a hillside.

Because the droplets sit at eye level, they scatter light in all directions and sharply reduce visibility. Fog lifts when sunshine warms the surface or when stronger wind mixes in drier air from above, and it can thicken further if cooling continues.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.