Fog and '˜haar' - what's the difference?

An image of the haar over north Edinburgh. Picture: TSPL

Visibility was so poor that Edinburgh Airport was forced to close for around half-an-hour just before 2pm, resulting in short delays for some passengers on departing flights.

But what is the difference between “haar” and the more commonly heard “fog”?

Haar refers specifically to the coastal fog which typically forms in Spring and Summer over eastern Scotland and England. It occurs when warm air moves over the cooler North Sea, causing moisture in the air to condense and form haar. The haar is then blown inland by the wind.

Variants of the term in Scots and northern English include har, hare, harl, harr and hoar.

Fog is a more general term referring to the weather phenomenon caused by tiny water droplets which become suspended in the air. It can form over bodies of land such as valleys and slopes, as well as over the sea.

The thickest fogs tend to occur in industrial areas where there are many pollution particles on which water droplets can grow.

There are several different types of fog, which are named according to the physical process which produces saturation or near-saturation of the air. The main types of fog include radiation, valley, advection and upslope fog.