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.

To see what I mean, suppose you baked a pie and decided to eat part of the pie while it was still warm. Unless you're a 300-pound offensive lineman or a heavyweight sumo wrestler, eating part of the pie means devouring less than half of it. Now let's apply this line of thinking to clouds and the sky. A day that is partly cloudy could mean that less than half the sky is covered by clouds. In other words, "partly cloudy" means that more than half of the sky must be cloudless, allowing plenty of unfiltered sunlight to illuminate the earth's surface.

Still sticking with my pie analogy, a partly sunny day means that less than half the sky allows unfiltered sunlight to reach the ground. For this to happen, more than half the sky must be covered by clouds. With this usage, a partly sunny day is actually cloudier than a partly cloudy day. Just keep in mind that in practice, "partly sunny" and "partly cloudy" sometimes get used interchangeably, but 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.

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 8 sections. Clear conditions (0/8 cloud coverage) constitute a perfectly sunny sky, while a "few" clouds (1/8 to 2/8 coverage) represent mostly sunny 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"). Picturing "overcast" conditions (8/8 coverage) is straightforward. When the sky is broken or overcast, weather observations will include the corresponding cloud ceiling. For the record, a ceiling 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 a low-level obstruction such as heavy fog, heavy rain, blowing snow, etc. In such cases when the observer cannot determine the sky coverage, the condition "sky obscured" is reported. The station model is thus marked with an "X" in the sky cover circle to designate that an obstruction prevents the weather observer from observing the rest of the sky. Note: Even if the observer is fairly confident that the sky is overcast, if the ceiling cannot be observed, the correct cloud entry should be "sky obscured".  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 surface obscuration (such as heavy fog, blowing snow, etc.) sufficiently limits vertical visibility as to prevent the weather observer from determining the ceiling of any layer of clouds that might be present.

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 add here that thick haze and smoke can also obscure the sky, preventing weather observers from assessing the specific fraction of cloud cover. In this striking photograph of Kuala Lumpur, thick haze (caused by fires in Indonesia and Malaysia in October 2006) obscures the sky.