Nature / Earth / Everyday water physics
Why fog forms overnight—and sometimes arrives with the wind
A still, misty morning and a bank of fog moving inland can begin in different ways. The key is how moist air cools to saturation.
Fog often feels like a still-morning phenomenon: pale ground, softened trees, a horizon that seems to have moved closer. Yet fog can also travel across a landscape with the wind. The difference lies in how the air becomes cool enough for droplets to form.
A cloud at ground level
The Met Office describes fog as tiny suspended water droplets close to the ground. Mist is related, with better visibility; haze is different because its obscuring particles are dry.
For understanding formation, the useful question is what brings moist air to saturation. Two common answers are cooling overnight near the ground and moving across a colder surface. These lead to the names radiation fog and advection fog.
When the cooling happens here
In radiation fog, air close to the ground cools to saturation. The National Weather Service describes a process that commonly begins overnight, with fog developing at the surface and deepening as more air cools.
Sheltered places with light winds are favorable, including valleys and areas near water. The fog can be patchy and fairly stationary. Warming the following day often helps it clear, which explains the familiar transition from a foggy morning to a clearer afternoon. “Often” matters: it is a typical development, not a timetable that every fog bank follows.
When the air arrives over something colder
Advection fog involves movement. Warm, moist air passes over colder ground and cools to saturation. The National Weather Service notes that this can happen under cloudy skies and with moderate or strong winds. The fog can be carried over the landscape and may persist for days.
Wind therefore does not automatically rule out fog. In this case, moving air is part of the explanation. The Met Office also describes the sea-fog example: warm, moist air crossing cooler water.
A comparison to keep in mind
| Question | Radiation fog | Advection fog |
|---|---|---|
| What is doing the cooling? | Overnight cooling near the ground | A colder surface beneath arriving air |
| What does wind tell us? | Sheltered, light-wind conditions are favorable | Moving air can help bring the fog in |
| Must it clear after sunrise? | Daytime warming often helps | It can persist much longer |
The comparison summarizes the linked weather-service explanations. It is not a way to classify every foggy photograph. Our cover scene shows what low fog can look like; a picture alone does not provide the preceding temperature, moisture and wind history.
Continue with another everyday water question: why falling raindrops do not have the familiar teardrop shape.
References
- Met Office. (n.d.). What is fog? https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/fog
- National Weather Service. (n.d.). Advection Fog. https://www.weather.gov/safety/fog-advection
- National Weather Service. (n.d.). Radiation Fog. https://www.weather.gov/safety/fog-radiation
Reading notes and scope
Checked 28 September 2026. These are introductory explanations from national weather services, not a local forecast or a new measurement. Other fog mechanisms exist; this article compares two common routes. The National Weather Service pages share one publisher.