Cloud cover, or cloud amount, is a measurement in meteorology that describes the extent to which the sky is obscured by clouds. It is a standardized quantification used by observers and automated systems worldwide to communicate atmospheric conditions and aid in weather forecasting. Determining the fraction of the sky covered is essential for understanding atmospheric processes.
Defining and Measuring Cloud Coverage
Meteorologists quantify cloud coverage using the okta, a unit representing one-eighth of the sky’s dome. This scale ranges from zero to eight, allowing for a precise assessment of the total cloud amount visible from the ground. The measurement estimates the fraction of the sky covered by any type of cloud. A clear sky is 0 oktas, and a sky completely covered is 8 oktas, or overcast. The okta measurement focuses only on the area obscured, disregarding cloud type, altitude, or thickness.
Standardized Reporting Terminology
While the okta system is the technical standard for measurement, public weather reports translate these fractional values into more descriptive terms. The World Meteorological Organization provides a standardized vocabulary to bridge this gap.
- 1 to 2 oktas is described as “Few” clouds.
- 3 or 4 oktas is termed “Scattered,” meaning approximately half the sky remains clear.
- 5, 6, or 7 oktas is referred to as “Broken,” signifying that more than half of the visible sky is covered.
- 8 oktas is reported as “Overcast.”
The Impact on Surface Temperature
Cloud cover significantly influences the Earth’s surface temperature by modulating the flow of both incoming and outgoing energy through the atmosphere. This modulation occurs through two distinct processes involving shortwave and longwave radiation.
During the day, clouds primarily affect the amount of solar energy, or shortwave radiation, that reaches the ground. Clouds have a higher albedo, or reflectivity, than the surface below them, meaning they reflect a substantial portion of the sun’s energy back into space. This reflective property, often called cloud albedo forcing, acts to cool the planet’s surface during daylight hours. Thicker, lower-altitude clouds, which contain more liquid water, are effective at reflecting shortwave radiation and reducing daytime heating.
At night, the influence of clouds shifts to the Earth’s thermal energy, which is emitted as longwave radiation. The ground continuously radiates heat upward, but clouds absorb this outgoing energy and then re-radiate some of it back toward the surface. This process, known as cloud greenhouse forcing, effectively traps heat near the ground, preventing rapid nocturnal cooling. Consequently, a cloudy night is warmer than a clear night because the clouds act as a thermal blanket, slowing the loss of heat to space. The overall effect on temperature is a balance between these two opposing forces, with the net outcome depending on the time of day, the cloud’s altitude, and its physical properties.