How does fog form?

As we head into the latter half of the year, we'll likely begin to see seasonal features like fog.

While fog can form year-round, it’s most common during the fall and winter months, especially under high-pressure conditions. Weaker winds allow dense, persistent fog to develop, sometimes lingering for days.

In cooler months, clear night skies enable heat to escape from the ground quickly, causing temperatures to drop below the dew point. This rapid cooling fosters ideal conditions for fog formation. However, predicting fog can be tricky, as its development depends on multiple factors like cloud cover, wind speed, temperature, moisture, and daylight.

Different types of fog form in various ways. Valley fog occurs when cold air settles in low areas, while radiation fog develops on clear, calm nights as heat radiates from the ground. Advection fog, including sea fog, forms when warm, moist air moves over cooler surfaces, such as along the North Sea coast.

In sub-zero temperatures, freezing fog can develop. The water droplets in the fog remain in liquid form despite the freezing air, becoming supercooled. When these droplets touch surfaces, they freeze instantly, forming a frosty layer of ice crystals known as rime, creating stunning winter landscapes.

To learn more about exactly how and why fog forms, check out our Weather Explained video below: