Weather forecasts often use simple, descriptive phrases like “mostly cloudy” that can feel vague to the average person. These common terms are actually shorthand for precise scientific measurements used by meteorologists. To understand the specific, quantified standards that define the sky’s condition, this article provides a precise look at what the term “mostly cloudy” means in a professional forecast.
The Technical Definition of Mostly Cloudy
The term “mostly cloudy” is a specific meteorological designation that quantifies the fraction of the sky covered by opaque clouds. According to the National Weather Service, this condition occurs when six-eighths to seven-eighths of the sky dome is obscured, translating to a cloud coverage of approximately 75% to 87.5%. This designation is also frequently associated with the term “Broken,” which technically covers a slightly broader range of cloud coverage.
This level of cloudiness means that the majority of the sky is blanketed, leaving only small, distinct patches of blue visible. While the sun may be obscured, there is typically still a significant amount of diffuse illumination reaching the ground.
The Full Spectrum of Cloud Cover Terms
To fully appreciate the precision of “mostly cloudy,” it is helpful to look at the entire scale of cloud cover terminology used by forecasters:
- Sky Clear: Zero clouds present in the sky.
- Few: One-eighth to two-eighths of the sky is covered by clouds.
- Scattered (Partly Cloudy): Three-eighths or four-eighths of the sky is covered.
- Broken: Five-eighths to seven-eighths of the sky is covered, containing the “mostly cloudy” forecast.
- Overcast: The entire sky (eight-eighths) is covered, representing 100% cloud coverage.
The distinction between “Scattered” and “Broken” is important, as it marks the transition from a day where clear sky dominates to one where cloud cover is the prevailing condition.
Measuring Cloud Cover: Oktas and Percentages
The precise fractions used in cloud cover terminology are derived from the Okta system, the standard unit of measurement in meteorology. The Okta system divides the sky into eight equal parts, or eighths, allowing trained observers and automated instruments to quantify the sky’s condition with consistency. A reading of six oktas, for instance, means that six of the eight sections of the sky are obscured by clouds.
While the Okta system remains the foundation of official weather reporting, the data is often translated into percentages for the public. Observations are made either visually by a trained observer or by automated sensors, such as ceilometers, which use laser technology to measure the cloud base and amount directly overhead.