Measuring rain starts with a simple tool. A rain gauge must stand in an open area away from trees and buildings, and a standard National Weather Service 4-inch gauge holds exactly 1 inch of rain in its inner cylinder. The totals grow large once they are counted over land: one inch of rain falling on just a single acre results in 27,154 gallons of water on the landscape. A standard gauge holds exactly 1 inch.
Weather radar measures the power returned from the rain, which is expressed as a reflectivity factor Z. Sensors on a suite of NASA satellites observe and measure water on land, in the ocean and in the atmosphere. GRACE uses two satellites in tandem about 220 kilometers apart to measure dips and bumps in Earth's gravity, which are affected by the mass of water.
Precipitation is concentrated in some parts of the world and scarce in others, and it can vary substantially from season to season and year to year. In 2017 Colombia received about 3,240 mm of precipitation while Egypt received only 18 mm. Most precipitation falls as rain, and most precipitation lands in the oceans. On a global basis evaporation approximately equals precipitation, but over the oceans evaporation exceeds precipitation, while less water evaporates over land than falls onto it; streams and rivers return the difference to the ocean.
About 500,000 cubic kilometers of water move through the atmosphere and fall as precipitation every year. Only about a thousandth of 1 percent of Earth's water is water vapor in the atmosphere, yet about 495,000 cubic kilometers cycle through the air each year, as if the air's water were replaced nearly 40 times a year. About 3,100 cubic miles of water is in the atmosphere at any one time; if it all rained down at once, it would only cover the globe to a depth of 2.5 centimeters. The air is a fast road for water, not a large lake.
What happens to rain on land depends on the ground. Clays absorb less water at a slower rate than sandy soils, and soil already saturated like a wet sponge cannot absorb much more. Soils absorbing less water result in more runoff overland into streams, and water falling on steeply sloped land runs off quicker and infiltrates less than water falling on flat land. Vegetation can slow the movement of runoff, allowing more time for it to seep into the ground. Water can stay in the soil for a month or two, though this varies greatly.
Water can also wait in the cold. Water can be stored as snowpack or ice until it melts, and about 1.7% of Earth's water is stored in polar ice caps and glaciers, as of November 2026. Sublimation is the conversion between the solid and gaseous phases of matter with no liquid stage, and it is a common way for snow to disappear in the arid West. The opposite of sublimation is deposition, where water vapor changes directly into ice, such as snowflakes and frost. The melting of glacial ice is a major contributor to sea level rise.
People learned all this slowly. Perrault experimentally showed that only one-sixth of the annual rainfall was necessary to sustain the river flow, so rain alone could feed the river, and Edmé Mariotte, working independently, later confirmed it with more sophisticated measuring techniques. Halley found in 1687 that water exhales the thickness of 60 parts of an inch in two hours from its whole surface. Even after these experiments, Derham argued in 1720 that springs have their origin from the sea, and not from rains and vapors. Some water isotopes now act as natural tracers in the water cycle.
Groundwater is hard to measure. About 1.7 percent of all of Earth's water is groundwater, which is about 30 percent of Earth's freshwater, and aquifers are a huge storehouse that people around the world depend on. Estimates of groundwater are particularly difficult to make and vary widely, and estimates of remaining storage might vary from decades to millennia, as of November 2026. Scientists say they do not actually know how much is stored in each of these aquifers, and GRACE cannot measure exactly how much water is left in one.
A warmer atmosphere holds more water vapor: on average, for every 1 degree Celsius of warming it can hold 7 percent more, as of November 2026, and that extra vapor can make storms rain more heavily and lead to more flooding. In the Northeast and Midwest United States, the precipitation falling on the heaviest 1% of days increased 60% and 45% from 1958 to 2021, according to the Fifth National Climate Assessment. As of November 2026, scientists say that air pollution particles called aerosols have generally offset some of the effect of warming on the water cycle in recent decades. Human water use, land use and climate change all impact the water cycle.
Words to know
- infiltrates: soaks in
- runoff: surface flow
- saturated: soaked full
- sublimation: solid-to-gas change
- deposition: gas-to-solid change
