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).

What Is a Bomb Cyclone?

Maddie Meyer/Getty Images
Maddie Meyer/Getty Images

The phrase bomb cyclone has re-entered the news this week as parts of the central U.S. face severe weather. Mountain and Midwestern states, including Colorado, Nebraska, Wyoming, and South Dakota, all fall in the path of a winter storm expected to deliver tornadoes, hail, heavy snow, flooding, and hurricane-force winds on Wednesday, March 13 into Thursday. It seems appropriate for a storm that strong to have bomb in its name, but the word actually refers to a meteorological phenomenon and not the cyclone's explosive intensity.

According to The Denver Post, the bomb in bomb cyclone stands for bombogenesis. Bombogenesis occurs when a non-tropical storm experiences at least a 24 millibar (the unit used to measure barometric pressure) drop within 24 hours. Low pressure makes for intense storms, so a bomb cyclone is a system that's built up a significant amount strength in a short length of time.

This type of storm usually depends on the ocean or another large body of water for its power. During the winter, the relatively warm air coming off the ocean and the cold air above land can collide to create a sharp drop in atmospheric pressure. Also known as a winter hurricane, this effect has produced some of the worst snowstorms to ever hit the U.S.

The fact that this latest bomb cyclone has formed nowhere near the coast makes it even more remarkable. Rather, a warm, subtropical air mass and a cold, Arctic air mass crossed paths, creating the perfect conditions for a rare bombogenesis over the Rockies and Great Plains states.

Central U.S. residents in the bomb cyclone's path have taken great precautions ahead of the storm. Over 1000 flights have been canceled for Wednesday and schools throughout Colorado have closed.

[h/t The Denver Post]

Watch a Rare ‘Ice Tsunami’ Slam Lake Erie

Clean Lakes Alliance, Flickr // CC BY 2.0
Clean Lakes Alliance, Flickr // CC BY 2.0

A combination of freezing cold temperatures and high winds is creating an unusual phenomenon along Lake Erie. As KDKA reports, ice tsunamis are toppling onto lake shores, and many locals have been asked to stay inside and even evacuate their homes.

On February 24, 2019, the National Weather Service in Buffalo, New York issued a warning about dangerous wind gusts in the Lake Erie area. The service urged citizens to seek shelter indoors and avoid traveling if possible. Winds peaked at 74 mph earlier this week, the level of a Category 1 hurricane, and tore down trees and power lines throughout the region.

People who got close to Lake Erie during the windstorm witnessed a rare event known as an ice tsunami. When wind pushed ice on the lake's surface toward the retaining wall, the sheet broke apart and dumped massive ice chunks on the shore. The video below captures the phenomenon.

In some areas, the ice piles grew so large that roadways had to be closed. Residents of Hamburg, New York's Hoover Beach area were asked to voluntarily evacuate due to the encroaching ice.

Ice tsunamis, or ice shoves, are rare, but in some cases they can be life-threatening. In 2013, waves of ice shards from a Minnesota lake destroyed people's homes.

[h/t KDKA]

SECTIONS

arrow
LIVE SMARTER