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
iStock
Britain Is in the Midst of a Rare ‘Wind Drought’
iStock
iStock

Generating renewable energy in Britain is a little less than a breeze these days: A “wind drought” is halting the country’s wind turbines.

This month’s wind energy output is down 40 percent from the same time last year, New Scientist reports. On average, about 15 percent of Britain’s electricity comes from wind power. Data starting from July 1 of this year put the monthly average closer to 6.9 percent. Last month, turbines were producing less than 2 percent of Britain’s electricity—the lowest output in two years.

That’s with even more wind turbines being installed over the course of the past year, New Scientist says. The data aren’t entirely surprising, though. The jet stream tends to make the UK’s weather drier and calmer during the summer and wetter and stormier during the winter. But the high pressure the jet stream has brought with it this year has been unusually prolonged, scientists say.

“It’s like a lid, it keeps everything still,” UK Met Office spokesperson Grahame Madge told New Scientist. “From the forecast looking out over the next couple of weeks, there doesn’t seem to be any significant change on the way.”

The wind drought shouldn’t cause too many problems in the short term. Electricity demand is low during the summer (very few British homes have air conditioning), and the country’s been able to compensate for the lack of wind by burning more natural gas. If the wind drought continues to persist, though, UK residents may begin to see an increase in utility fees. Natural gas prices have already risen with the increased demand.

“As we continue to transition to a low-carbon energy system, managing the intermittency of renewable power an important role in balancing supply and demand,” a National Grid spokesperson told New Scientist. “However, we have planned for these changes and [are] ready to play our part.”

The wind drought comes about eight years after British politicians vowed to reduce the UK's dependence on fossil fuels. Last year was the first year that electricity generated from low-carbon energy sources like solar power, wind power, and nuclear power outpaced high-carbon energy sources like coal and natural gas. This summer’s wind drought may make it difficult to improve on last year’s numbers.

[h/t New Scientist]

nextArticle.image_alt|e
iStock
Why Does the Sky Look Green Before a Tornado?
iStock
iStock

A common bit of folklore from tornado-prone parts of the U.S. says that when the skies start taking on an emerald hue, it's time to run inside. But why do tornadoes tend to spawn green skies in the first place? As SciShow's Michael Aranda explains, the answer has to do with the way water droplets reflect the colors of the light spectrum.

During the day, the sky is usually blue because the shorter, bluer end of the light spectrum bounces off air molecules better than than redder, longer-wavelength light. Conditions change during the sunset (and sunrise), when sunlight has to travel through more air, and when storms are forming, which means there are more water droplets around.

Tornadoes forming later in the day, around sunset, do a great job of reflecting the green part of the light spectrum that's usually hidden in a sunset because of the water droplets in the clouds, which bounce green light into our eyes. But that doesn't necessarily mean a twister is coming—it could just mean a lot of rain is in the forecast. Either way, heading inside is probably a good idea.

For the full details on how water and light conspire to turn the sky green before a storm, check out the SciShow video below.

SECTIONS

arrow
LIVE SMARTER
More from mental floss studios