June 2, 2016

Fog: all you need to know

I DON’T KNOW anybody who has seen a fogbow, and I’ve never seen one myself, but I’m prepared to believe they exist because fogbow is one of the terms that the National Oceanic and Atmospheric Administration (NOAA) uses when describing different types of fog.

A fogbow, according to NOAA, is a rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside. I think you’d know it if you’d seen one.

NOAA seems to be well informed about fog in general. Here are some other terms they use, and their descriptions:

Ø Advection fog — Fog that results from the advection of moist air over a cold surface, and the cooling of the air to its dew point. The most common type of fog in coastal regions.

Ø Dense fog — Fog in which visibility is less than one-quarter mile.

Ø Fog — Water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than 3,300 feet. (Why 3,300 feet, you ask. Sorry, I just don’t NOAA.)

Ø Ground fog — Fog that’s less than 20 feet deep. It’s produced by the cooling of the lower atmosphere coming into contact with the ground (or, presumably, cold water). It’s also called radiation fog.

Ø Ice fog — It’s composed of minute ice crystals suspended in the air, or water droplets at temperatures below 0°C, down at surface level and restricting horizontal visibility. It’s also known as freezing fog, and usually occurs at minus 20°F and below.

Ø Mist — Mist is very much like fog, being composed of tiny water droplets suspended in the air. The difference is that it doesn’t reduce visibility as much as fog does.

Ø Overrunning — This is a combination of low clouds, fog, and steady light rain. It comes about when a comparatively warm air mass moves up and over colder, denser air on the surface.

Ø Shallow fog — Fog in which visibility is 5/8 of a mile or more, viewed from 6 feet above ground (or sea) level.

Ø Steam fog — That is what happens when very cold air drifts over comparatively warm water. It’s also known as sea smoke.

Today’s Thought
The fog comes
on little cat feet.
It sits looking
over the harbor and city
on silent haunches
and then, moves on.
— Carl Sandburg, Fog

Tailpiece
As I was laying on the green,
A small English book I seen.
Carlyle’s Essay on Burns was the name of the edition,
So I left it laying in exactly the same position.

(Drop by every Monday, Wednesday, Friday for another  Mainly about Boats column.)

4 comments:

Patrick Hay said...

I think it's 3,300 ft because that's the equivalent of 1,000 metres (1Km in the SI system), which itself is the nearest SI equivalence to half-a-cable in proper nautical measurement.

Hudson Barton said...

Fog be damned. Full speed ahead.

John Vigor said...

Patrick, that's probably the answer. Sounds reasonable to me.

Cheers,

John V.

John Vigor said...

Hudson, I seem to have heard that before somewhere!
Can't remember where.

John V.