ByMelissa
If you ever thought it was difficult to tell the difference between partly cloudy and mostly sunny, youre 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 cant 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 theprobability 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 providesa chartthat 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 cant evaporate, you cant cool down so easily, and so the apparent temperature feels hotter than it actually is.
On that note, the NWS provides aheat index chartas 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 its soupy outside, say 95% RH, then the heat index shoots up to 127F.
This article was originally published on Today I Found Out. Follow them on their Youtube channel here.
Featured Image Courtesy: Pixabay
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