Why Is Earth So Often Cloudy?

What is the importance of Earth's cloud cover?

A predominantly cloudy planet

Satellite observations and astronaut photographs reveal that 67% of Earth's sky is on average covered by clouds, with a predominance over the oceans (only 10% of clear sky) versus 30% over the continents.

The contribution of observation satellites

Satellites of NASA's EOS program (such as Terra, Aqua and Aura) collect global data every 1-2 days, measuring cloud cover and the radiation budget. However, these averages do not reflect daily and seasonal variations, cloud altitude or the presence of multiple layers.

N.B.:
MODIS detects clouds better over dark surfaces (oceans, forests) than over bright surfaces (ice), and cirrus clouds are harder to identify than thick cumulus.

What exactly can be seen in this image of Earth's average cloud cover?

There are three major zonal bands where the skies are often cloudy. To these three main bands are added regional phenomena, linked to coastlines and relief, which do not form a continuous band around the globe.

The equatorial band (ITCZ)

The first cloud band is the Intertropical Convergence Zone (ITCZ). It results from the collision between the warm, humid air of the equator and the cooler air of the tropics. This warm, humid air rises, cools and condenses, causing frequent storms and an almost permanent cloud cover. This band migrates through the year, following the position of the Sun.

The cloud band at 60° North

This band lies around 60° North latitude. It is caused by the northern polar front, the meeting zone between cold polar air and the milder air of mid-latitudes. This thermal contrast generates disturbances and frequent clouds. Conversely, between 15° and 30° latitude, descending air inhibits cloud formation and favours deserts.

The cloud band at 60° South

This band lies around 60° South latitude. It is produced by the southern polar front, where polar air meets mid-latitude air. As in the Northern Hemisphere, this thermal clash favours condensation and cloud formation. Between 15° and 30° South latitude, descending air instead maintains clear skies and desert regions.

Coastal and orographic clouds

In addition to these three zonal bands, there are regional clouds. Off the west coasts (South America, Africa, North America), upwelling cools the humid air and forms stratocumulus, the most common clouds on the planet. These formations remain localized, however, and do not constitute a continuous band.

Mountain ranges block air currents and rainfall, creating leeward deserts (Tibetan Plateau desert, Death Valley desert, Death Valley in California east of the Sierra Nevada, Atacama Desert in South America). These orographic effects are also regional and depend on the relief.

How can we explain that 67% of Earth's sky is cloudy?

An ocean, the slow boiler of the atmosphere

The oceans do not merely evaporate: they store and redistribute heat. Their thermal inertia allows vast marine regions to remain warmer than the air in winter and cooler in summer. This lag maintains a permanent latent heat flux, which transforms water into vapour without a sudden rise in temperature. The vapour thus produced is then transported by the winds, sometimes over thousands of kilometres, before condensing into clouds.

Circulation cells that sculpt the clouds

The distribution of clouds is not due only to disordered movements. The atmosphere is organized into Hadley, Ferrel and polar cells, which ensure a transfer of energy from the tropics to the poles. At equatorial latitudes, air rises in the Intertropical Convergence Zone, but this is only part of the system. Between 15° and 30°, subsidence dries the air, while at mid-latitudes, depressions and polar fronts concentrate humidity and form continuous cloud bands.

Cloud microphysics and aerosols

For a cloud to appear, water vapour must find microscopic surfaces on which to condense. These condensation nuclei are made of dust, sea salt, pollen or sulphates. Without them, the air could remain supersaturated without forming visible droplets. Once the droplets are created, their size and number determine the cloud's albedo, and therefore its ability to reflect solar radiation. Aerosols thus play a key role in global cloud cover.

What satellites really reveal

Satellites do not merely confirm an average of 67%. They characterize clouds in three dimensions. Passive radiometers, such as MODIS on Terra and Aqua, measure cloud height, their optical thickness and their phase, whether ice or liquid water. But these instruments remain blind to the thinnest layers, notably cirrus.

To break through this limit, active satellites such as CALIPSO and CloudSat carry a lidar and a millimetre-wave radar respectively. The lidar emits a laser beam that reveals the most tenuous clouds, while the radar detects thicker droplets and crystals. By combining their vertical profiles, they reconstruct a cross-section of the atmosphere and reveal stacks of cloud layers invisible to conventional radiometers.

These observations show that cloud cover fluctuates strongly on hourly, seasonal and regional scales. The 67% average is only a statistical summary: it masks skies that are sometimes perfectly clear, sometimes entirely overcast, and vertical structures of great complexity.