28 November 2006

Listening to NOAA weather radio I have been noticing that they don’t announce cloud cover the way I had previously found it listed in Jetstream so I had to go back to that source to see how they define the terms they use in the announcement.

Clear Skies

Skies are clear when no clouds or obscurations are observed or detected from the point of observation.

Cloudy Skies

When the predominant/average sky condition is covered completely by opaque (not transparent) clouds. In other words, 8 octants of the sky is covered by opaque clouds.

Considerable Cloudiness

When the predominant/average sky condition is covered by more than half, but not completely covered by opaque (not transparent) clouds. In other words, 5/8 to 7/8 of the sky is covered by opaque clouds. Same as Mostly Cloudy

Fog

A visible aggregate of minute water particle (droplets) which are based at the Earth's surface and reduces horizontal visibility to less than 5/8 statue mile, and unlike drizzle, it does not fall to the ground. It occurs most frequently in coastal regions because of the great water vapor content of the air. However, it can occur anywhere. The rapidity with which fog can form makes it especially hazardous. It forms by any atmospheric process that does one of the following:

• Cools the air to its dew point, or
• Raises the dew point to the air temperature.

Names given to fog types identify their methods of formation. The principle types are radiation fog, ice fog, advection fog, upslope fog, rain induced fog, and steam fog. These types of fog are called "dense" when the surface visibility is equal to or less than 1/4 miles. A Dense Fog Advisory will be issued when the dense fog becomes widespread.

Haze

A concentration of salt particles or other dry particles not readily classified as dust or other phenomenon. Occurs in stable air usually only a few thousand feet thick, but may extend as high as 15,000 feet. Haze layers often have definite tops above which the visibilities are good. However, the visibility in the haze layer can be very poor.

Mostly Clear

When the predominant/average sky condition is covered 1/8 to 2/8 with opaque (not transparent) clouds. Sometime called Mostly Sunny if it is during the day.

Mostly Cloudy

When the predominant/average sky condition is covered by more than half, but not completely covered by opaque (not transparent) clouds. In other words, 5/8 to 7/8 of the sky is covered by opaque clouds. Same as considerable cloudiness

Mostly Sunny

When the predominant/average sky condition is covered 1/8 to 2/8 with opaque (not transparent) clouds. Same as Mostly Clear.

Partly Cloudy

When the predominant/average sky condition is covered 3/8 to 4/8 with opaque (not transparent) clouds. Same as Partly Sunny

Partly Sunny

When the predominant/average sky condition is covered 3/8 to 4/8 with opaque (not transparent) clouds. Same as Partly Cloudy

Smog

Originally smog meant a mixture of smoke and fog. Now, it means air that has restricted visibility due to pollution or pollution formed in the presence of sunlight--photochemical smog.

Comparing these terms with the ones that I previously found I find that there is no direct translation between them.

1/8	2/8	3/8	4/8	5/8	6/8	7/8	8/8
Clear	Mostly Clear Mostly Sunny		Partly Clear Partly Cloudy		Mostly Cloudy Considerable Cloudiness		Cloudy

Clear	Isolated Clouds	Scattered Clouds	Broken Clouds	Overcast
10%	25%	50%	90%	100%