How and why do we have fog?

Autumn is here and although the sun is shining this week, fog may be filling our mornings first.

Fog can form year-round, however it’s most common during the fall and winter months, especially under high-pressure conditions, as we are currently experiencing.

Weaker winds allow dense, persistent fog to develop, sometimes lingering for days. There are different types of fog that form in various ways.

Valley fog occurs when cold, dense 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. It is locally known as haar or sea fret in some areas.

Despite fog being somewhat regular during cooler months, it is harder to forecast than you might think. Its development depends on multiple factors like cloud cover, wind speed, temperature, moisture, and daylight.

When air temperatures are sub-zero, another type can also occur: freezing fog. This is more likely in the depths of winter, rather than autumn though.

The water droplets from the drizzle in the fog become supercooled, still remaining in liquid form in the sub-zero air. They freeze instantly upon contact with an object or surface creating rime, a layer of tiny ice crystals.