What causes fog?

Fog is a cloud that happens to form at ground level, made of suspended water droplets that condense once air cools to its dew point, the temperature at which it can no longer hold its water as vapor. Condensation needs surfaces, so the droplets form on microscopic particles of dust, salt, and pollution. The most familiar variety is radiation fog, which forms overnight when clear skies and calm winds let the ground radiate its heat into space, chilling the air just above it. That is why fog often sits in valleys at dawn and burns off within a couple of hours of sunrise as the ground reheats.

Other types form by different routes. Advection fog occurs when warm moist air moves over a cold surface, which is what blankets San Francisco when Pacific air crosses the cold California Current, and what produces sea fog. Upslope fog forms as air is pushed up terrain and cools by expanding. Freezing fog happens when droplets stay liquid below freezing and then flash-freeze onto whatever they touch, coating roads and branches in rime. Visibility below a quarter mile is the threshold that triggers dense fog advisories, and fog contributes to hundreds of traffic deaths annually in the United States, which is why the correct response is low beams rather than high beams, since high beams reflect off the droplets and make it worse.