From: Subject: Wednesday Weather Topic in Greater Depth Date: Fri, 23 Oct 2009 10:14:06 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/learn/w_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Wednesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Wednesday, 21 = October=20 2009

PARTLY CLOUDY or PARTLY SUNNY?

The question is often raised by the public "What is the difference = between=20 partly cloudy and partly sunny?" At first glance, this question appears = to be=20 similar to describing a glass as being part empty or part full - in = other words,=20 the difference depends upon the eye of the beholder. The National = Weather=20 Service often uses the terms "partly cloudy" and "partly sunny" in the = wording=20 of a public forecast. While these terms may appear somewhat vague, many = of the=20 National Weather Service Offices use a particular set of criteria based = upon the=20 sky condition or cloud coverage when preparing the public forecast text. =

The sky cover represents the total fraction of the local sky = hemisphere (or=20 celestial dome) above a surface based observer that is covered or hidden = by=20 clouds. The evaluation is made with respect to the entire sky above the = local=20 unobstructed horizon. If distinct cloud layers are observed, the = fractional=20 amounts for each layer are added to form a total sky cover. Usually the = sky=20 cover is expressed to either the nearest tenths (1/10th) or the nearest = eighths=20 (1/8th or oktas).

The following descriptions are used in public forecasts:

SKY CONDITION (from NWS, = Media=20 Guide to National Weather Service Terminology, 1996)

Sky Condition

Cloud Coverage

Cloudy 
Mostly cloudy, or Considerable = cloudiness =20
Partly cloudy or Partly sunny 
Mostly clear = or=20 Mostly sunny 
Fair

9/10 to 10/10 of the sky covered by clouds 
7/10 to = 8/10 =20
3/10 to 6/10 
1/10 to 3/10 
Less than 4/10 = cloud=20 cover, no precipitation 
Generally pleasant weather=20 conditions

When communicating with other meteorologists and with = the=20 aviation industry, a slightly different code is used. This code is used = in=20 plotting of the station model on the surface weather maps. According to=20 observational practice, the following terminology and codes are used to = re port=20 the cloud cover.

SKY COVER CODE (as of=20 1 July 1996)

CLR
FEW
SCT
BKN
OVC

Clear 
Few 
Scattered 
Broken =20
Overcast

0/8 of sky covered by clouds 
1/8 through 2/8 of sky = covered=20 by clouds 
3/8  through 4/8 of sky is covered by=20 clouds 
5/8 through 7/8 of sky is cloud covered  =
8/8 of=20 sky is cloud covered 

Thus the term "fair" would cover cases of CLR, FEW, or SCT. Other = equivalents=20 would be approximately, "mostly clear" - FEW, "partly cloudy" - SCT, = "mostly=20 cloudy" - BKN, and "cloudy" - OVC.

You can make daily observations of the sky and determine the sky = cover for=20 yourself. No more sophisticated equipment is needed than your eyes. If = possible,=20 view the entire sky hemisphere above you from an unobstructed location. = Estimate=20 the total fraction of the entire sky above you that is covered by all = cloud=20 types and at all levels. If no clouds are present, the sky is clear. = With only a=20 little practice, you should be able to judge a broken cloud deck from = scattered=20 clouds, and ultimately, answer the question "Is it partly sunny or = partly=20 cloudy?"

REFERENCE:

Thompson, S. and A.F. Kapela, 1996: Media guide to National = Weather=20 Service Terminology. National Weather Service. NWS Forecast Office,=20 Milwaukee/Sullivan, WI. 21 pp.

Return to the Wednesday= Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu=20
=A9Copyright, 2009, The American Meteorological Society.=20