Considering Clouds (and Slicing Pie)

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Let me start with the age-old question: "Which phrase do you think describes a cloudier sky -- partly sunny or partly cloudy?" The answer to that question might depend on who you ask. The National Weather Service defines partly sunny and partly cloudy as essentially the same, with the caveat that we wouldn't use "partly sunny" at night, of course. But, in practice, some forecasters use these terms differently because the word "partly" is somewhat vague, so it's not clear-cut. Some folks use "partly sunny" to emphasize that there will be a bit more clouds than sun, and use "partly cloudy" to emphasize that there will be a bit more sun than clouds. With this usage, a partly sunny day is actually cloudier than a partly cloudy day.

Most weather forecasters don't want to get drawn into such an argument of semantics, so when it comes to quantifying the coverage of the sky by clouds, they rely on a specific "pie-chart" system that leaves little room for debate (see table below). The "pie" that makes up the sky coverage observation is divided into eight sections. Clear conditions (0/8 cloud coverage) constitute a perfectly sunny sky, while "overcast" conditions (8/8 coverage) constitute a completely cloudy sky. Those two are pretty straightforward. In between those two extremes, a "few" clouds (1/8 to 2/8 coverage) represent mostly sunny (or mostly clear) conditions. "Scattered" clouds (3/8 to 4/8 cloud coverage) correspond to a partly cloudy or partly sunny sky, with "broken" clouds (5/8 to 7/8 cloud coverage) describing a partly cloudy or partly sunny (5/8 coverage) to mostly cloudy (6/8 to 7/8 coverage) sky. When the sky is nearly overcast except for a few breaks, forecasters refer to the cloud coverage as breaks in the overcast (abbreviated as "BINOVC"). This photograph shows an example of BINOVC conditions (note the patches of blue sky toward the bottom left of the photo in an otherwise overcast sky). When the sky is broken or overcast, weather observations will include the corresponding cloud ceiling, which is simply the height of the base of a broken or overcast layer of clouds.

On occasion, the sky cover cannot be seen due to near-surface conditions such as dense fog, heavy rain, blowing snow, etc. For example, check out this webcam shot of Penn State's Beaver Stadium in dense fog. You can't really see the stadium, and you can't really see the sky, either! In such cases when the observer cannot determine the sky coverage, the condition "sky obscured" is reported. Note: Even if the observer is fairly confident that the sky is overcast, if the ceiling cannot be observed, "sky obscured" would still be reported (also note that the observation is "sky obscured," NOT "sky obstructed" -- a common mistake). Also, when sky obscured conditions exist and vertical visibility is very low, you'll sometimes see references to an indefinite ceiling. This simply means that the near-surface conditions (such as dense fog, blowing snow, etc.) have limited the vertical visibility to the point that the cloud ceiling can't be determined.

A massive sandstorm struck a military base near Al Asad, Iraq, on April 28, 2005. If you were taking a weather observation within the wedge of dust at this time, you would not have been able to determine the cloud ceiling because airborne sand would have obscured the sky. In this case, you would have reported sky obscured with an indefinite ceiling (very low vertical visibility).

Credit: U.S. Army

I should add that thick haze and smoke can also obscure the sky, preventing weather observers from assessing the specific fraction of cloud cover. Thick smoke, for example, often obscures the sky in the vicinity of major wildfires, such as in this striking photograph of the Pine Gulch Fire (Credit: Public Domain) in Colorado in 2020. Now that you know the conventions for reporting sky coverage, let's take a look at how to identify and interpret sky coverage on a station model in the Key Skill section below.