A typical weather forecast will tell you three things.

  1. Will it rain or snow?
  2. Will it be hot or cold?
  3. Will it be clear or cloudy?

The precipitation part of the forecast is straightforward with the type of precipitation expected. The temperature part of the forecast is also easy to understand as it is expected to be a certain temperature in the time frame you are talking about.

Sky cover gets tricky when putting a forecast out. A forecaster must determine if it is going to be cloudy, mostly cloudy, partly cloudy or clear. There is a difference with each and it is related to the percentage of sky cover.

Here is how it works. The forecaster looks at computer models and develops their forecast. The forecast they develop will allow them to figure out what percentage of the sky they think will be covered in a certain period of time.

The sky condition is the average sky cover that is based on a percent of the sky that is covered by opaque (non-transparent) clouds. The sky condition is based on the amount of clouds expected.

Below is the technical definition for each sky condition:

Clear/Sunny… 1/8 or less = 0 to 5%
Mostly Clear/Mostly Sunny… 1/8 to 3/8 = 6 to 25%
Partly Cloudy/Partly Sunny… 3/8 to 5/8 = 26 to 50%
Mostly Cloudy… 5/8 to 7/8 = 70 to 87%
Cloudy… 7/8 to 8/8 = 88 to 100%

There is no difference between partly cloudy and partly sunny. They both represent the percentage of sky cover. It is up to the forecaster how they want to represent the forecast each day.