What’s The Difference Between Partly Cloudy vs. Partly Sunny? Or Mostly Sunny vs. Mostly Cloudy?

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Ever wonder what the difference is between the weather being partly cloudy versus mostly sunny?

Or… mostly cloudy versus partly sunny?

See the difference between partly cloudy and partly sunny... and all other cloud coverage definitions.

 

The Short Story…

Most people have their own notions about what partly cloudy versus partly sunny means.

However, there are some very specific definitions from the National Weather Service — which weather forecasters use:

  • Partly Cloudy: About 30% to 70% of the sky is covered with clouds BUT usually refers to sky conditions at night.
  • Partly Sunny: About 30% to 70% of the sky is covered with clouds BUT refers only to sky conditions in the daytime.

Mostly cloudy skies and windmills.

 

The Long Version…

So, now you know.

Partly cloudy mostly refers to sky conditions at night, and partly sunny emphasizes sky conditions in the daytime — although it’s acceptable to use partly cloudy for sky condition observations in the day, too.

Here’s everything you need to know about cloudy skies terminology according to the National Weather Service:

  • Partly cloudy and partly sunny mean exactly the same thing — between 3/8 and 5/8 of the sky is covered by clouds. Sometimes, a “mix of sun and clouds” is used by some weather forecasters instead of “partly sunny” during the daytime hours, although that is not an official National Weather Service term.
  • Mostly cloudy means there are more clouds than sun (or stars, at night) — 3/4 to 7/8 of the sky is covered by clouds. This can also be referred to as “considerable cloudiness.”
  • Mostly sunny means there is more sun than clouds (or mostly clear, at night) — 1/8 to 1/4 of the sky is covered by clouds.
  • Sunny or clear means there are no clouds in the sky.
  • Cloudy means the entire sky is covered by clouds.

Is this partly cloudy or just cloudy, according to the National Weather Service.

 

Why Is Cloud Coverage Measured In Eighths?

The sky is technically observed using oktas:

  • 0 oktas means the sky is clear.
  • 4 oktas means half of the sky is cloudy.
  • 8 oktas means the sky is covered with clouds.

Here are some pictures to help you understand cloud coverage as measured by oktas.

 

Check out this video time lapse of a partly cloudy day:

Must read: What The Colors And Sizes Of Clouds Mean