What Determines “Partly Cloudy” vs. “Mostly Sunny”? (And Other Weather Terms Explained)

Brett C. asks: How do they decide the cuttoff between partly cloudy and mostly sunny?

weatherIf you ever thought it was difficult to tell the difference between “partly cloudy” and “mostly sunny,” you’re not alone. Established by the National Weather Service (NWS) according to a loose set of rules, the criteria used to describe different elements of your forecast can be pretty vague.

Sky conditions are classified according to how much “opaque cloud coverage” (OCC) is expected that day. While the NWS has apparently not defined “opaque clouds,” they are presumed to be those that can’t be seen through, or more technically, those that are “opaque to terrestrial radiation.”

To qualify as “Sunny,” there can be no more than 25% OCC. “Clear,” on the other hand is sometimes used as synonym for “Sunny,” but is only applied when there is no more than 5% OCC. “Mostly Clear,” which is also a synonym for “Sunny,” is used when there is between 6% and 25% OCC.

“Mostly Sunny” and “Partly Cloudy” are apparently interchangeable, and apply when the OCC is between 26% and 50%.”Partly Sunny” and “Mostly Cloudy” can also be synonyms, when the OCC is between 51% and 69%, although “Mostly Cloudy” can be applied for OCC up to 87%.

At an OCC of 88% and above, the sky is considered “Cloudy” or “Overcast.”

Note that when there is a “high probability” of precipitation (60% or more), many weather folks skip the sky condition forecast, since it may be inferred to be “Cloudy.”

When forecasting the chance of precipitation, the NWS considers the likelihood that there will be at least 0.01 inches of precipitation at one place in the forecast area within (usually) a 12-hour period (called the probability of precipitation or POP).

Words used in the forecast, such as “chance of rain” and “likely,” as well as “isolated” and “scattered,” are considered either “expressions of uncertainty” or “qualifiers” (the last two denote that the entire area will not be affected), and they are tied to ranges of POPs.

So, when the probability of precipitation (POP) is between 60% and 70%, the “uncertainty” is low and so the forecast may often include the word “likely,” while when the POP is only 20%, the “uncertainty” is higher, so the phrase “slight chance,” may be used.

“Isolated” is used when the POP is between 10% and 29%, while “scattered” is used when the POP is between 30% and 59%. “Occasional,” “intermittent,” and “periods of,” denote a POP of greater than 79%, but also that the precipitation will be “on and off.”

When the forecast temperature is given in a range, it has a particular meaning, as well. For example, “near 40” means the temperature is expected to be anywhere from 38F to 42F, “lower 40s” denotes anywhere from 40F to 44F, “mid 40s” from 43F to 47F and “upper 40s” from 46F to 49F.

Wind terms are tied to specific ranges too, all related to “sustained wind speed” (SWS), and they can overlap. “Sustained wind” is defined as the average of observed wind speeds over a two-minute period.

“High,” “strong” and “damaging” winds are those expected to have SWS of at least 40 miles per hour (mph). “Very windy” denotes when SWS is between 30 and 40 mph, and “windy” between anywhere from 20 to 35 mph.

When the SWS is between 15 and 25 mph, “breezy” is used when the weather is mild, and “brisk” or “blustery” are used when it is cold. “Calm” and “light” are used to denote SWS of 5 mph or less.

Wind chill incorporates considerations of how much heat a human body will lose to the environment on a cold or windy day. Calculations are estimated at weather conditions at 5 feet above ground level (said to be the typical height of a human face), and begin when SWS reach 3 mph.

The NWS provides a chart that shows wind chill for any temperature between 40F and -45F with winds between 5 mph and 60 mph, and it reveals that even a slight wind, with cold temperatures, can have a big effect on wind chill. For example, at 0F with only calm winds of 5 mph, the wind chill is -11F. Likewise, even when temperatures are relatively mild, say at 35F, if the winds are high, say 60 mph, it can make it feel about half the temperature it really is (17F).

On the other hand, the heat index reflects the fact that when the humidity reaches a certain point, the perspiration on your skin can’t evaporate, you can’t cool down so easily, and so the apparent temperature feels hotter than it actually is.

On that note, the NWS provides a heat index chart as well, which shows temperatures between 80F and 110F and relative humidity (RH) between 40% and 100%. Just as with wind chill, slight changes in a single variable can have a dramatic effect, and when both are high, the heat index becomes dangerous to human health. For example, at 90F and 40% RH, the heat index is only 91F, but if it’s soupy outside, say 95% RH, then the heat index shoots up to 127F.

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Bonus Fact:

  • The Occupational Health and Safety Administration (OSHA) has guidance for using the heat index to protect workers, and calls for escalating measures as the heat index increases from “moderate” (91F to 103F) to “high” (103F to 115F) to “extreme” (116F or greater). At all three levels, OSHA recommends for those in the heat: drinking about 4 cups of water per hour, being prepared for heat illnesses, acclimatizing, wearing sunscreen and taking frequent, shady breaks. As the heat index moves into “high,” OSHA also recommends limiting physical exertion and becoming vigilant for heat related illness. Once the heat index moves into “extreme,” OSHA recommends only essential tasks be performed, and then only during the coolest time possible.
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  • They are picking on your over at Giz for “Party Cloudy”

  • John Fembup

    There is no wind sear factor for a hot, windy day.

    Why not?