What’s the Difference Between “Mostly Sunny” and “Partly Cloudy”?

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iStock

Reader Marcus from Louisville wrote in to ask, “What’s the difference between ‘mostly sunny’ and ‘partly cloudy’ (or for that matter, ‘mostly cloudy’ and ‘partly sunny’) in weather forecasts? Are any of those even specifically defined terms?”

The short answer: about 1 to 4 oktas.

What’s an okta? That’s a unit of measurement that meteorologists use when they’re forecasting cloud conditions, equal to 1/8 of the sky (though sometimes 1/10 is used). When the forecast is delivered, the number of oktas covered by opaque clouds (meaning that you can’t see through them, and the sun/moon/stars/sky are hidden) is described using “mostly sunny,” “partly cloudy” and other terms we’re used to hearing. Each of these is defined by the National Oceanic and Atmospheric Administration and the National Weather Service in terms of oktas of cloud cover. The NWS Operations Manual breaks it down like this:

Daytime Forecast

Day or Nighttime Forecast

Cloud Conditions

Cloudy

Cloudy

8/8 opaque clouds

Mostly Cloudy 

Mostly Cloudy

6/8 - 7/8 opaque clouds

Partly Sunny

Partly Cloudy

3/8 - 5/8 opaque clouds

Mostly Sunny

Mostly Clear

1/8 - 2/8 opaque clouds

Sunny

Clear

0/8 opaque clouds

The difference in terms is a matter of how much cloud cover there is. Interestingly, partly sunny and partly cloudy mean the exact same thing—but partly cloudy is the correct term for nighttime conditions because you can’t see the sun. 

“Fair” skies is another cloud cover term that’s sometimes used. Technically, it means that less than three oktas are covered with opaque clouds, and that there’s “no precipitation, no extreme conditions of visibility, wind or temperature, and generally pleasant weather.” If you don’t know that definition, though, “fair” sounds pretty vague on its own, so the NWS discourages forecasters from using  it. 

A lot of terms used in precipitation forecasts are also precisely defined by the NWS and are less subjective than they sometimes sound. The qualifying terms that express uncertainty about rain and snow (like “chance of snow” or “scattered thunderstorms”) are laid out like this:

Chance of precipitation

Expression of uncertainty

Area Qualifier

0%

none

none

10%

Slight chance

Isolated, few

20%

Slight chance

Widely Scattered

30-50%

Chance

Scattered

60-70%

Likely

Numerous

80-100%

none

none

The area qualifiers are used when the chance of precipitation somewhere in the forecast area is very high, and correspond to the the expected coverage within the area (so “scattered thunderstorms” would mean that rain is very likely, but will affect only 30 to 50 percent of the area).

A Pile of Manure and a Heat Wave Combined to Cause Spain’s Worst Wildfire in 20 Years

Lumppini, iStock/Getty Images Plus
Lumppini, iStock/Getty Images Plus
A pile of manure in Spain ignited on June 27, sparking a wildfire that has torn through more than 10,000 acres of forest and other Catalonian landscapes, CNN reports. The manure combusted in part because of a scorching heat wave that has swept across Europe this week; temperatures in the affected region hit 104°F, and Germany, Poland, and the Czech Republic all reached record-breaking June temps. Catalan regional interior minister Miquel Buch reported that authorities believe the manure was improperly stored on a chicken farm in the village of La Torre de l’Espanyol, where it was exposed to the worst of the heat, according to NBC News. The wildfire—Spain’s most devastating in 20 years—is affecting the region just west of Tarragona, a port city that is best known for its Roman ruins, which is situated along the Balearic Sea about 60 miles southwest of Barcelona. [[{"fid":"315211","view_mode":"width-constrained-728","fields":{"format":"width-constrained-728","field_file_image_title_text[und][0][value]":false,"field_file_image_alt_text[und][0][value]":"Map of Catalonia, Spain","field_image_subhead[und][0][value]":"","field_image_subhead[und][0][format]":"unfiltered","field_caption[und][0][value]":"","field_caption[und][0][format]":"unfiltered","field_credits[und][0][value]":"PeterHermesFurian%2C%20iStock%2FGetty%20Images%20Plus","field_credits[und][0][format]":"unfiltered"},"type":"media","field_deltas":{"1":{"format":"width-constrained-728","field_file_image_title_text[und][0][value]":false,"field_file_image_alt_text[und][0][value]":"Map of Catalonia, Spain","field_image_subhead[und][0][value]":"","field_image_subhead[und][0][format]":"unfiltered","field_caption[und][0][value]":"","field_caption[und][0][format]":"unfiltered","field_credits[und][0][value]":"PeterHermesFurian%2C%20iStock%2FGetty%20Images%20Plus","field_credits[und][0][format]":"unfiltered"}},"link_text":false,"attributes":{"alt":"Map of Catalonia, Spain","height":728,"width":728,"class":"media-element file-width-constrained-728","data-delta":"1"}}]] The firefighting force includes about 350 firefighters, 12 fire engines, seven aircraft, two hydroplanes, and additional vehicles equipped with water tanks—all of which are techniques regularly used to fight massive blazes. Though high temperatures are expected to continue and the rough terrain itself is also a contributing factor in the rapid spread of the fire, the strong winds should soon abate; authorities have warned people to stay inside. Though more than 50 people have been evacuated from the area, no deaths have been reported thus far. While the spontaneous combustion of flammable materials like manure, hay, or compost is definitely possible, as evidenced by this situation, it’s not the most common way for wildfires to start. More common causes include lit cigarettes, unattended campfires, burning debris, and engine sparks. [h/t CNN]

Colorado Welcomed Summer With 2 Feet of Snow

Emmanuel Dunand/AFP/Getty Images
Emmanuel Dunand/AFP/Getty Images

While people in some parts of the country celebrated the first day of summer with barbecues and trips to the beach, residents of north and north-central Colorado had a snow day. Areas west of Denver—including Grand Lake, Aspen, and Steamboat Springs—experienced snow storms on summer solstice, with snowfall totaling nearly 2 feet at the highest elevations, CBS Denver reports.

Snow started falling in the Rocky Mountains the morning of Friday, June 21 and continued Sunday. Areas at ground level weren't cold enough to experience the unusual weather, but at elevations of 7000 feet and above, it looked like a winter wonderland. Steamboat Springs, a ski resort town in Colorado’s Yampa Valley, accumulated 20 inches of snow on the longest day of the year.

While Colorado mountain towns are used to seeing snow at odd times of year, the weekend's weather was still out of the ordinary. The average snowfall for Steamboat Springs in June is 0.1 inches. Prior to last Friday, it had been 91 years since a snowstorm hit the city in late June.

Snow has fallen in the Rockies later than average in six out of the past seven years. Though it feels like an extension of winter, the trend may actually be a product of the warming atmosphere. A warmer climate affects the jet stream, potentially pushing its course further south and leading to unusual weather patterns, such as unseasonable snowstorms in Colorado.

That means residents of some parts of the state will have to wait to have their summer hikes and picnics. The weather was serious enough to shut down one road in Rocky Mountain National Park.

[h/t CBS Denver]

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