Last update: August 29, 2025

Why Is Earth So Often Cloudy?

Global Cloud Cover — This map of **global cloud cover** represents an **"average cloudiness"** of all observations from NASA satellites between July 2002 and April 2015. The colors range from dark blue (no clouds) to light blue (few clouds) to white (frequent clouds). There are three major "bands" where Earth's skies are often cloudy. Deserts (Tibetan Plateau desert, Death Valley, Atacama Desert in South America) are clearly visible.
Image Credit: NASA Earth Observatory by Jesse Allen and Kevin Ward, using data provided by the MODIS Science Team, NASA Goddard Space Flight Center.

What is the Importance of Earth's Cloud Cover?

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

Satellites from NASA's EOS program (such as Terra, Aqua, and Aura) collect global data every 1-2 days, measuring cloud cover and radiative balance. However, these averages do not reflect daily variations, seasonal changes, 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 clouds.

What Exactly Do We See in This Image of Earth's Average Cloud Cover?

There are three major bands where Earth's skies are often cloudy:

The first equatorial band (ITCZ) results from the collision between warm, humid air from the equator and cold air from the tropics, causing storms and clouds.

The other two bands, at 60° North and South, are caused by the clash between polar air and mid-latitude air. Conversely, between 15° and 30°, descending air inhibits cloud formation, favoring deserts.

Clouds are also observed off the west coasts (South America, Africa, North America), due to upwelling: cold water from the depths cools the humid air, forming stratocumulus clouds (the most common, covering 20% of Earth).

Mountain ranges block air currents and rain, creating deserts on the leeward side (Tibetan Plateau desert, Death Valley in California east of the Sierra Nevada, Atacama Desert in South America).

Why Is 67% of Earth's Sky Cloudy?

Earth is covered by 67% clouds on average, mainly due to two key phenomena:

First, massive evaporation from the oceans (which cover 71% of Earth's surface) continuously supplies the atmosphere with water vapor. This vapor condenses into clouds as it rises and cools.

Second, atmospheric movements (such as the Intertropical Convergence Zone or polar fronts) promote cloud formation by pushing warm, humid air upward, where it cools and condenses.

Satellites, like those in NASA's EOS program, confirm this dominant cloud cover, with variations depending on the region (oceans are cloudier than continents) and seasons.