CLOSE
iStock
iStock

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

iStock
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).

nextArticle.image_alt|e
Henrik Djärv, Flickr // CC BY-NC 2.0
arrow
Weather Watch
It's So Cold In One Part of Russia That People's Eyelashes Are Freezing
Henrik Djärv, Flickr // CC BY-NC 2.0
Henrik Djärv, Flickr // CC BY-NC 2.0

Oymyakon, a rural village in the eastern Russian region of Yakutia, is one of the coldest inhabited spots in the world. While some schools in the U.S. cancel classes as temperatures approach zero, schools in Oymyakon remain open in -40°F weather. But recently temperatures in the region have dropped too low even for seasoned locals to handle. As AP reports, the chill, which hit -88.6°F on January 16, is cold enough to break thermometers and freeze eyelashes.

Photos shared by residents on social media show the mercury in thermometers hovering at -70°F, the lowest temperature some are built to measure. When thermometers fail, people in Oymyakon have other ways of gauging the cold. Their uncovered eyelashes can freeze upon stepping outside. Hot water tossed in the air will also turn to snow before hitting the ground.

To Oymyakon's 500-odd citizens, the most recent cold snap is nothing out of the ordinary. Temperatures are perpetually below freezing there from late October to mid-May, and average temperatures for the winter months frequently reach −58 °F. On Tuesday, residents were advised to stay inside and stay as warm as possible. Of course, that directive wasn't enough to stop some adventurous locals from sneaking outside for selfies.

[h/t AP]

nextArticle.image_alt|e
Amazon
arrow
Weather Watch
Heated Mats Keep Steps Ice-Free in the Winter
Amazon
Amazon

The first snow of the season is always exciting, but the magic can quickly run out when you remember all the hazards that come with icy conditions. Along with heating bills, frosted cars, and other pains, the ground develops a coat of ice that can be dangerous for pedestrians and drivers alike. Outdoor steps become particularly treacherous and many people find themselves clutching their railings for fear of making it to the bottom headfirst. Instead of putting salt down the next time it snows, consider a less messy approach: heated mats that quickly melt the ice away.

The handy devices are made with a thermoplastic material and can melt two inches of snow per hour. They're designed to be left outside, so you can keep them ready to go for the whole winter. The 10-by-30-inch mats fit on most standard steps and come with grips to help prevent slipping. A waterproof connector cable connects to additional mats so up to 15 steps can be covered.

Unfortunately, this convenience comes at a price: You need to buy a 120-volt power unit for them to work, and each mat is sold separately. Running at $60 a mat, the price can add up pretty quickly. Still, if you live in a colder place where it's pretty much always snowing, it might be worth it.

SECTIONS

arrow
LIVE SMARTER
More from mental floss studios