Coastal fog refers to the occurrence of fog over coastal regions, usually as a result of advection fog formation.
Coastal fog is a regular occurrence along the eastern coast of the UK and is most common during Spring and Summer. In Eastern Scotland, it is known locally as Haar whilst in in Eastern Scotland and Eastern England the coastal fog is referred to as Fret.
How does coastal fog form?
Coastal fog is usually a result of advection fog which forms when relatively warm, moist air passes over a cool surface. In the UK, the most common occurrence of coastal fog is when warm air moves over the cool surface of the North Sea towards the east coast of the UK.
When this happens, the cold air just above the sea's surface cools the warm air above it until it can no longer hold its moisture and so forces it to condense forming tiny particles of water which forms the fog that we see.
Coastal fog usually occurs in the spring and summer months when conditions begin to warm up but the sea (which warms more slowly) stays relatively cold.
The impact, location and movement of coastal fog depends upon a number of conditions, including wind strength, wind direction and land temperature. If, as is common along the UK's east coast, the winds blow in from the east, the fog will often rapidly cover the coast in a blanket of fog. If the land temperature is warm the fog can quickly dissipate as the parcel of air warms, however if the land temperature is cooler, the fog can linger for a longer time.
Coastal fog might also refer to pre-existing fog which is transferred from a distant source and is simply moved to the coast by prevailing weather patterns.
The sudden onset of coastal fog can sometimes be dangerous causing disorientation as it dramatically reduces visibility. It can also affect industries such as shipping and oil platforms where it has been known for stubborn coastal fog to disrupt productivity for long periods.
Find out more about the other types of and how they form on our What is fog? page.