Cloud cover is the fraction of the sky hidden by clouds when you look up from a given location. Meteorologists express it as a percentage or a fraction of the total visible sky, and at any given moment, roughly 67 percent of Earth’s surface is covered by clouds. The concept sounds simple, but the way cloud cover is measured, reported, and used varies depending on whether you’re checking a weather forecast, flying a plane, or installing solar panels.
How Cloud Cover Is Measured
The standard international unit for cloud cover is the okta, which divides the sky into eighths. A trained observer (or, increasingly, an automated sensor) looks at the sky and estimates how many eighths are filled with cloud of any type or height. The scale runs from 0 to 8:
- 0 oktas: completely clear sky
- 1 okta: one eighth or less of the sky has cloud, but it’s not totally clear
- 3 to 4 oktas: roughly half the sky is cloudy
- 7 oktas: nearly full cloud cover with small breaks
- 8 oktas: fully overcast, no gaps at all
There’s also a special code, 9 oktas, used when the sky is completely obscured by fog, heavy snow, or another weather phenomenon that makes it impossible to judge actual cloud amount. Satellites measure cloud cover from above using reflected light, which is how NASA arrived at that 67 percent global average from nearly a decade of observations.
Cloud Cover in Aviation
Pilots and air traffic controllers use a slightly different system. Weather reports at airports (called METARs) describe sky conditions with four abbreviations, each tied to a range of coverage:
- FEW: 1/8 to 2/8 of the sky covered
- SCT (scattered): 3/8 to 4/8 covered
- BKN (broken): 5/8 to 7/8 covered
- OVC (overcast): 8/8, full coverage
Each abbreviation is followed by the height of the cloud base in hundreds of feet, so a pilot can quickly tell how much of the sky is blocked and at what altitude. A report reading “BKN025” means five eighths or more of the sky is cloudy at 2,500 feet. This matters for landing visibility, instrument flight rules, and turbulence planning.
How Clouds Control Temperature
Cloud cover acts like a thermostat with two competing settings. During the day, clouds reflect incoming sunlight back into space, keeping the ground cooler than it would be under a clear sky. At night, the effect flips: clouds trap heat radiating up from the Earth’s surface, keeping temperatures warmer than they would be on a clear night.
This is why clear days tend to be hot and clear nights tend to be cold, producing a wide swing between daytime highs and nighttime lows. Meteorologists call that swing the diurnal temperature range. When cloud cover increases, the gap shrinks because daytime heating drops while nighttime cooling slows. When cloud cover decreases, the gap widens in both directions. If you’ve ever noticed that overcast winter nights feel milder than clear ones, you’ve experienced the nighttime insulating effect firsthand.
Cloud Types and Altitude
Not all cloud cover is the same. The World Meteorological Organization divides clouds into three altitude layers, each with distinct cloud types that affect weather differently.
High-level clouds, including the thin, wispy cirrus and cirrostratus varieties, form in the upper troposphere. They’re mostly made of ice crystals and often let a good deal of sunlight through. Middle-level clouds, like altostratus and nimbostratus, sit lower and tend to be thicker. Nimbostratus is the classic gray blanket that produces steady rain or snow. Low-level clouds, such as stratus and stratocumulus, hug the ground and are responsible for the dull, flat overcast skies that can last for days. Cumulus and cumulonimbus clouds also have low bases but can tower upward through all three layers, producing anything from fair-weather puffiness to severe thunderstorms.
When a forecast mentions cloud cover, it’s describing total sky coverage regardless of which layer the clouds sit in. But the type and altitude of those clouds determine whether you’ll just see a hazy sun or no sun at all.
Effects on Solar Energy
Cloud cover is one of the biggest variables in solar panel performance. Atmospheric conditions including clouds, aerosols, and airborne dust can cut electricity output by up to 60 percent. Thin, high clouds might reduce output only slightly, while a thick overcast layer can slash it dramatically. Solar energy systems are typically sized and sited based on local cloud cover data for this reason, and regions with persistent cloud cover need more panel capacity to hit the same energy targets as sunnier areas.
Effects on Vitamin D Production
Your skin produces vitamin D when ultraviolet B radiation from sunlight triggers a chemical reaction in the outer layers of the epidermis. Cloud cover reduces the amount of UVB that reaches the ground, and heavy overcast conditions can suppress vitamin D synthesis completely, even at the equator. This is one reason people in consistently cloudy climates are more prone to vitamin D deficiency, particularly during winter months when the sun is already low in the sky. If you live in an area with frequent overcast skies, dietary sources or supplements become more important for maintaining adequate levels.