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).

How Waffle House Helps Measure the Severity of a Natural Disaster

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iStock

There are a lot of ways the Federal Emergency Management Agency (FEMA) assesses and addresses the severity of a natural disaster. Meteorology can predict movement patterns, wind gusts, and precipitation. Resources are dispatched to areas hit hardest by torrential weather.

But when the agency needs an accurate, ground-level gauge for how a community is coping during a crisis, they turn to Waffle House.

Since 2004, FEMA has utilized what former administrator Craig Fugate called the “Waffle House Index.” Because the casual dining chain is open 24 hours a day, seven days a week, tracking to see if a location is closed or working with limited supplies can help inform the agency as to whether affected areas are ailing or taking steps toward normalcy.

“If a Waffle House is closed because there's a disaster, it's bad,” Fugate told NPR in 2011. “We call it red. If they're open but have a limited menu, that's yellow ... If they're green, we're good, keep going. You haven't found the bad stuff yet.”

For FEMA, the ability to order a plate of smothered and covered hash browns is an important analytic. If a Waffle House is having trouble getting stock, then transportation has been interrupted. If the menu is limited, then it’s possible they have some utilities but not others. If its locations have locked their doors, inclement weather has taken over. The chain’s locations would normally stay open even in severe conditions to help first responders.

The company has opened a Waffle House Storm Center to gather data in anticipation of Hurricane Florence, a Category 2 storm expected to touch down in the Carolinas this week. But not all locations are taking a wait-and-see approach. One Waffle House in Myrtle Beach, South Carolina has already closed due to the looming threat, making it the first red dot on the Index.

[h/t CNN]

See What Hurricane Florence Looks Like From Space

NASA via Getty Images
NASA via Getty Images

As Hurricane Florence continues to creep its way toward the Carolinas, it’s repeatedly being described as both "the storm of the century” and "the storm of a lifetime” for parts of the coastlines of North and South Carolina. While that may sound like hyperbole to some, Alexander Gerst—an astronaut with the European Space Agency—took to Twitter to prove otherwise with a few amazing photos, and issued a warning to “Watch out, America!”

According to the National Weather Service, “Hurricane Florence will be approaching the Carolina shores as the day progresses on Thursday. Although the exact timing, location, and eventual track of Florence isn't known, local impacts will likely begin in the afternoon hours and only worsen with time throughout the evening and overnight period.”

On Tuesday, Wilmington, North Carolina's National Weather Service took the warning even one step further, writing: "This will likely be the storm of a lifetime for portions of the Carolina coast, and that's saying a lot given the impacts we've seen from Hurricanes Diana, Hugo, Fran, Bonnie, Floyd, and Matthew. I can't emphasize enough the potential for unbelievable damage from wind, storm surge, and inland flooding with this storm.”

Gerst’s photos certainly drive that point home.

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