As an aid to weather forecasting to determine cloudiness The National Weather Service uses these terms to define these sky conditions:
| Sky Condition | Opaque Cloud Coverage |
| Clear/Sunny | 1/8 or less |
| Mostly Clear/Mostly Sunny | 1/8 to 3/8 |
| Partly Cloudy/Partly Sunny | 3/8 to 5/8 |
| Mostly Cloudy | 5/8 to 7/8 |
| Cloudy | 7/8 to 8/8 |
For more weather terms see Forecast Terms at the National Weather Service.