
If you ever thought it was difficult to tell the difference between âpartly cloudyâ and âmostly sunny,â youâre 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 âopaque cloud coverageâ (OCC) is expected that day. While the NWS has apparently not defined âopaque clouds,â they are presumed to be those that canât be seen through, or more technically, those that are âopaque to terrestrial radiation.â
To qualify as âSunny,â there can be no more than 25% OCC. âClear,â on the other hand is sometimes used as synonym for âSunny,â but is only applied when there is no more than 5% OCC. âMostly Clear,â which is also a synonym for âSunny,â is used when there is between 6% and 25% OCC.
âMostly Sunnyâ and âPartly Cloudyâ are apparently interchangeable, and apply when the OCC is between 26% and 50%.âPartly Sunnyâ and âMostly Cloudyâ can also be synonyms, when the OCC is between 51% and 69%, although âMostly Cloudyâ can be applied for OCC up to 87%.
At an OCC of 88% and above, the sky is considered âCloudyâ or âOvercast.â
Note that when there is a âhigh probabilityâ of precipitation (60% or more), many weather folks skip the sky condition forecast, since it may be inferred to be âCloudy.â
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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 âchance of rainâ and âlikely,â as well as âisolatedâ and âscattered,â are considered either âexpressions of uncertaintyâ or âqualifiersâ (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 âuncertaintyâ is low and so the forecast may often include the word âlikely,â while when the POP is only 20%, the âuncertaintyâ is higher, so the phrase âslight chance,â may be used.
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âIsolatedâ is used when the POP is between 10% and 29%, while âscatteredâ is used when the POP is between 30% and 59%. âOccasional,â âintermittent,â and âperiods of,â denote a POP of greater than 79%, but also that the precipitation will be âon and off.â
When the forecast temperature is given in a range, it has a particular meaning, as well. For example, ânear 40âł means the temperature is expected to be anywhere from 38F to 42F, âlower 40sâ denotes anywhere from 40F to 44F, âmid 40sâ from 43F to 47F and âupper 40sâ from 46F to 49F.
Wind terms are tied to specific ranges too, all related to âsustained wind speedâ (SWS), and they can overlap. âSustained windâ is defined as the average of observed wind speeds over a two-minute period.
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âHigh,â âstrongâ and âdamagingâ winds are those expected to have SWS of at least 40 miles per hour (mph). âVery windyâ denotes when SWS is between 30 and 40 mph, and âwindyâ between anywhere from 20 to 35 mph.
When the SWS is between 15 and 25 mph, âbreezyâ is used when the weather is mild, and âbriskâ or âblusteryâ are used when it is cold. âCalmâ and âlightâ 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.
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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 canât evaporate, you canât 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 itâs soupy outside, say 95% RH, then the heat index shoots up to 127F.
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If you liked this article, you might also enjoy:
- Can Bad Weather Cause Joints to Ache?
- What Causes the Smell After Rain
- Why We Call the Seasons Summer, Autumn, Winter, and Spring
- What Causes Dew?
- The Tale of the Man Who Nearly Drowned While Falling from the Sky
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 âmoderateâ (91F to 103F) to âhighâ (103F to 115F) to âextremeâ (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 âhigh,â OSHA also recommends limiting physical exertion and becoming vigilant for heat related illness. Once the heat index moves into âextreme,â OSHA recommends only essential tasks be performed, and then only during the coolest time possible.
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Daven Hiskey writes for the wildly popular interesting fact website TodayIFoundOut.com. To subscribe to Today I Found Outâs âDaily Knowledgeâ newsletter, click here or like them on Facebook here.
This post has been republished with permission from TodayIFoundOut.com. Image by Wendell under Creative Commons license.