The definition of fog is when visual range is reduced to less than 1000 meters by the presence of airborne suspended water droplets in the direction of lowest visibility as viewed by the observer. If the lowest visible range is greater than 1000 meters due to etc. etc. then it is termed mist.

The droplets can be any temperature however if the droplets are observed to freeze onto a structure then the current weather is freezing fog. A dry bulb temperature below zero deg C is usually sufficient grounds to declare freezing fog. I could make a bad joke about hoar frost but that's for another day.

Freezing rain is extremely nasty as water droplets (either supercooled or > zero deg C) coming into contact with the ground or a structure at a temperature below zero will freeze on impact quickly covering the runway / taxiway / road home with an instant sheet of ice.

One surprising aspect of overnight radiation fog is it often thickens shortly after sunrise as a bit of thermal mixing stirs up the cold and moist layers.

Lovely stuff meteorology but the pay is absolutely :yuk:

The US defintion of fog is visibility below 5/8 statute mile, above that it is defined as mist.

If it helps anyone else, a mile is 1,609.344 mtrs (roughly), so I guess 5/8 is ..............................around 1000m.

'Two countries separated by a common language' as someone famous once said:D

thanks for the replies.

Astra Driver:
is this an official definition, what is the source, FAA definition?

thanks

stu

"Freezing Fog

A suspension of numerous minute water droplets which freezes upon impact with ground or other exposed objects, generally reducing the horizontal visibility at the earth's surface to less than 1 km (5/8th mile)."

Published in: AEA 'Recommendations for De-icing / Anti-icing of Aircraft on the Ground' (19th Edition September 04), JAA ACJ OPS 1.345, CAA AIC 105/2003 and SAE ARP4737 Revision D (this is not the latest revision of the ARP).