Fog
Fog is a low cloud at the ground, made of tiny water droplets or ice crystals that can sharply reduce visibility and disrupt travel.
What fog is
Fog is a cloud at ground level. It forms when water vapor near the surface condenses into tiny liquid droplets or, in very cold air, changes into ice crystals. The particles scatter light and hide distant objects, so the most noticeable effect of fog is reduced visibility.
How fog forms
Fog usually forms when air near the ground becomes saturated. That can happen when the air cools to its dew point, when moisture is added to the air, or when moist air moves over a colder surface. Like other clouds, fog often needs tiny particles such as dust, smoke, sea salt, or pollen for droplets to form around.
Radiation fog
Radiation fog commonly forms overnight when clear skies, light winds, and moist air allow the ground to lose heat. The ground cools the air just above it until saturation is reached. This fog often starts near the surface, can deepen overnight, and usually thins after sunlight warms the ground.
Advection fog
Advection fog forms when warmer, moist air moves over a colder surface and cools from below. It can occur over cold ocean water, snow cover, or chilly land. Unlike radiation fog, it may form under cloudy skies and with stronger winds, and it can move across the landscape with low-level air flow.
Other fog types
Valley fog is often a form of radiation fog that pools in low terrain as cool, dense air drains downhill. Evaporation or steam fog can appear when cold air moves over warmer water. Freezing fog contains supercooled droplets that freeze on contact with roads, trees, wires, and vehicles, creating icy hazards.
Visibility and travel
Fog becomes especially dangerous when visibility falls quickly or varies across short distances. Drivers may overestimate how far they can see, approach stopped traffic too fast, or miss curves and intersections. Aviation, shipping, rail, emergency response, and school transportation can also be delayed when fog is dense or persistent.
Satellites and forecasts
Forecasters use surface observations, temperature and dew point, wind, terrain, recent rainfall, model guidance, and satellite imagery to monitor fog. Satellites are useful because fog can cover large areas, especially at night or over water, but low clouds and fog can be hard to separate without the right sensor data.
Why it matters
Fog matters because a thin layer of droplets can turn ordinary roads, airports, harbors, and valleys into low-visibility environments. It also links everyday experience to cloud physics: cooling, saturation, condensation, terrain, and tiny particles all meet in the air just above the ground.