Cloud cover is the fraction of the sky obscured by clouds when viewed from a specific location. At any given moment, about 67 percent of Earth’s surface is covered by clouds, making them one of the most influential features of our atmosphere. Cloud cover affects temperature, solar energy, weather forecasts, and even agriculture, which is why meteorologists track it carefully.
How Cloud Cover Is Measured
Meteorologists express cloud cover using a scale called oktas, which divides the sky into eighths. An observer (or automated instrument) looks up and estimates how many eighths of the visible sky are hidden by clouds. The World Meteorological Organization defines the scale like this:
- 0 oktas: Clear sky, categorized as “fine”
- 1–2 oktas: Up to a quarter of the sky covered, still considered fine
- 3–5 oktas: Three- to five-eighths covered, labeled “partly cloudy”
- 6–7 oktas: Six- to seven-eighths covered, labeled “cloudy”
- 8 oktas: The entire sky is covered with no breaks, labeled “overcast”
Weather stations historically relied on trained human observers to make these estimates, but most airports and reporting stations now use laser-based instruments called ceilometers. A ceilometer fires a pulse of light straight up and measures the signal that bounces back off cloud bases. The instrument detects a cloud when it registers a sudden spike in reflected light, followed by a rapid drop-off. By timing the return signal, it calculates the height of the cloud base. These devices run continuously, giving updated readings every few seconds.
On a larger scale, satellites provide a global picture. NASA and other agencies use satellite imagery to map cloud fraction across the planet, producing the kinds of data that let scientists track long-term patterns and calculate that two-thirds-of-the-Earth figure.
Why Cloud Cover Matters for Temperature
Clouds play a dual role in Earth’s energy balance. They reflect incoming sunlight back into space, which cools the surface. But they also trap heat radiating up from the ground, functioning much like greenhouse gases. Which effect wins depends largely on the cloud’s altitude and thickness.
Low, thick clouds are powerful reflectors. On a completely overcast day with low clouds, only about 30 percent of the sun’s energy reaches the ground, meaning those clouds block roughly 70 percent of incoming solar radiation. This is why an overcast winter day feels noticeably colder than a sunny one at the same air temperature. High, thin clouds like cirrus behave differently. They let most sunlight pass through (more than 80 percent still reaches the surface) but are effective at trapping heat radiating upward from the Earth. So high clouds tend to have a net warming effect, while low clouds tend to cool.
This balance matters enormously for climate. If cloud cover shifts even slightly toward more high clouds or fewer low clouds, the warming effect grows. This is one reason cloud behavior remains a critical factor in climate projections.
Cloud Height and Type
The atmosphere is divided into three cloud levels, and knowing which level a cloud occupies helps explain both its appearance and its effects. In temperate regions, these levels break down as follows:
- Low clouds (surface to about 2 km): Stratus, stratocumulus, cumulus, and cumulonimbus. These are the clouds closest to the ground, often responsible for drizzle, fog-like conditions, or dramatic thunderstorms in the case of cumulonimbus.
- Middle clouds (2 to 7 km): Altostratus, altocumulus, and nimbostratus. These often appear as gray sheets or layers and can produce steady, prolonged rain.
- High clouds (5 to 13 km): Cirrus, cirrocumulus, and cirrostratus. These wispy, ice-crystal clouds are thin enough that sunlight passes through easily.
Some clouds don’t stay neatly in their level. Cumulonimbus, the towering thunderstorm cloud, starts near the surface but can punch all the way up into the high level. Nimbostratus, which brings steady rain, typically forms in the middle level but often extends both upward and downward.
Where Clouds Are Most and Least Common
Cloud cover varies dramatically by geography. Oceans are significantly cloudier than land: slightly more than 70 percent of the sky over oceans is typically cloud-covered, compared to just under 60 percent over continents. Almost a fifth of land surfaces sit under large areas of persistently clear sky, while less than 10 percent of the ocean surface stays clear.
The cloudiest zones on Earth are the tropics and the temperate midlatitude storm belts, where warm, moist air rises frequently. The subtropics and polar regions see 10 to 20 percent less cloud cover. If you’ve ever noticed that desert regions tend to cluster around 20 to 30 degrees latitude (think the Sahara or the Arabian Peninsula), that’s partly because those subtropical zones have sinking air that suppresses cloud formation.
How Cloud Cover Is Changing
A 42-year analysis of global cloud data from 1979 to 2020, published in Atmospheric Chemistry and Physics, found a clear and diverging trend: cloud cover is decreasing over most continents while increasing over tropical and subtropical oceans. The pattern holds across South and North America, the Congo Basin, most of Asia, Europe, and the polar regions, all of which are losing cloud cover. Meanwhile, ocean cloud cover is growing, particularly in tropical and subtropical waters.
The driving factor appears to be declining humidity over land surfaces, which is dropping at a rate of 1 to 2 percent per decade. Less moisture in the air means fewer clouds form. This has practical consequences beyond temperature. Reduced cloud cover over land means more direct solar radiation hitting the surface, amplifying warming. It also signals increasing stress on freshwater supplies, since the same drying trend that reduces clouds also reduces the moisture available for precipitation. Desert areas and the Indian subcontinent are exceptions, showing a slight increase in cloud cover against the broader continental trend.
Practical Effects of Cloud Cover
Cloud cover directly shapes daily life in ways that go beyond whether you need sunglasses. Solar energy generation drops substantially under overcast skies. Thick, low clouds can reduce the solar radiation reaching panels by up to 80 percent, which is why solar output forecasts depend heavily on cloud cover predictions. High, thin clouds have a much smaller impact, letting most energy through.
For agriculture, cloud cover influences both temperature and the amount of light crops receive. Persistent overcast conditions can slow growth, while clear skies in arid regions can increase heat stress and evaporation. Aviation relies on cloud cover reports for flight planning, since low ceilings affect visibility and landing procedures. And for anyone planning outdoor activities, the okta reading in a weather report tells you at a glance how much of the sky will be open or blocked.