The atmosphere is the superhighway in the sky that moves water everywhere over the Earth. About 3,100 cubic miles of water is in it at any one time. If it all rained down at once, it would only cover the globe to a depth of 2.5 centimeters. On average a drop of water may spend over 3,000 years in the ocean before evaporating. It spends only about nine days in the air before falling back to Earth. Warm tropical water supports high evaporation, and wind increases it.

As air rises it cools and the pressure drops. This makes some water vapor condense into small drops and form a cloud. The drops form around tiny aerosol particles, and without them clouds could not exist. A cloud floats because the moist air in it is less dense than dry air. Dew is water vapor that has condensed back onto Earth's surface. Condensation also happens when saturated air is cooled, such as on the outside of a glass of ice water.

Rain starts as cloud drops that are far too small to fall. It takes millions of them to make a single raindrop. Many people picture raindrops as tears falling, but that is a popular misconception. Small raindrops under 1 mm radius are round like a ball. As drops grow they flatten into the shape of the top of a hamburger bun. At about 4 to 5 mm the drop breaks apart. Rain can start as water drops or ice crystals in a cloud, but it always falls as liquid water.

Not all precipitation is rain. Sleet forms when a thin layer of warmer air comes between layers of cold air. Ice crystals melt as they fall through the warm layer, and then they refreeze. Freezing rain falls like rain but freezes as soon as it touches the ground. Hail forms inside thunderstorm updrafts as liquid drops freeze onto ice. Small hailstones fall at 9 to 25 mph. Very large ones can fall at over 100 mph.

Rain is not spread out evenly. In 2017 Colombia received about 3,240 mm of precipitation while Egypt received only 18 mm. On a global basis evaporation approximately equals precipitation. Over the oceans evaporation exceeds precipitation. Less water evaporates over land than falls onto it, and streams and rivers return the difference to the ocean. Human water use, land use and climate change all impact the water cycle. As of November 2026, a warmer atmosphere holds 7 percent more vapor for every 1 degree Celsius of warming, which can make storms rain more heavily.

Words to know

• atmosphere: air around Earth
• aerosol: tiny floating particle
• sleet: refrozen rain
• misconception: wrong idea