Published: 2008-06-23 10:09:59
Updated: 2008-06-23 10:09:59
Posted June 23, 2008
MIKE MOSS SAYS: Sravan, The principle definition of "partly cloudy" is opaque cloud covering 3-5 eighths (or oktas) of the sky as seen from a specific location. This can fall into either of your latter two definitions, as a snapshot in which one looks up and sees the sky 35-60 percent covered with opaque clouds at a given moment (like your number 3) or, if one totals up the amount of cloudiness over the course of a day and on average the amount of cloud cover is 3-5 oktas (like your number 2).
The "time average" version can be a little misleading of course, as you could theoretically have a partly cloudy day which is overcast the first half of the day and then clear the second half, for an average of 4 oktas coverage even though for most of the day you would look up and see a "cloudy" or "sunny" sky. If we expect conditions like that, we will usually forecast "decreasing clouds" or "morning clouds followed by afternoon sun," etc.
Likewise, if it is apparent that different parts of our viewing area will have significantly different cloud cover for most of the day, we will try to incorporate that into the wording of our on-air and online forcasts, such as "partly cloudy east of I-95, mostly sunny farther west" or "sunny for the northern half of the viewing area, partly cloudy south," or something like that.
Sometimes it is apparent that from one location to another across the viewing area there may be large differences in cloud cover that change erratically with time through the day, but not necessarily an overall increase or decrease or a consistent bias from one region to another. In these cases, we'll often go with "variably cloudy" or "mix of clouds and sun" to attempt a reasonable description.
Hope that explanation was at least "partly clear!"