Rain formation: From Cloud Droplets to Downpours
What falls from clouds as liquid drops? Rain is the answer, and rain formation begins with water vapor that condenses into droplets before gravity draws them toward the ground.
Water vapor enters the air from oceans, other bodies of water, wet land, and plants. The amount present can be expressed as relative humidity, a percentage tied to the air's temperature. Warmer air can contain more water vapor before becoming saturated than cooler air can. Cooling therefore provides a route from invisible vapor to a visible cloud. The dew point is the temperature at which a parcel of air becomes saturated. Within clouds, tiny particles of dust, ice, and salt give condensing water places to gather, forming suspended droplets or ice particles above Earth's surface. Cool or dry air moving over warmer water also gains moisture, as does air receiving precipitation that evaporates on its way down from clouds above.
Rising air helps set rain formation in motion because it expands and cools. Convection can lift it, broad atmospheric movements can raise it, and a mountain can force it upward. Other routes to cooling begin closer to the surface. Air loses heat when it touches colder ground, while radiation carries energy away from the air or the surface beneath it. Evaporation can also cool air as moisture enters it. Along weather fronts, contrasting masses of air create broad zones of upward movement, producing cloud layers from which precipitation can fall over an extensive area.
Inside a cloud, droplets collide and merge in a process called coalescence. Turbulence brings them together, and larger droplets continue gathering water as they descend. Eventually their weight overcomes the resistance that has kept smaller droplets suspended. This warm-cloud route is one part of rain formation; another begins with ice. In clouds below freezing, ice crystals gain mass and begin falling. A substantial temperature difference between cloud level and the ground can carry those crystals into warmer air, where they melt. Water that began its descent as ice can therefore arrive at the surface as rain.
Air shapes each falling drop. Very small drops are almost spherical, while larger ones flatten underneath as they meet the oncoming airflow. Broad drops resemble hamburger buns rather than the pointed teardrops familiar from drawings. At still larger sizes they become unstable and break apart. A 2004 study reported drops measured over Brazil and the Marshall Islands with maximum dimensions of at least 8.8 millimeters, possibly reaching 10 millimeters. Their size was linked to condensation on large smoke particles or collisions within small regions containing unusually large amounts of liquid water. Drops associated with melting hail also tend to be larger than other raindrops.
The terrain beneath a cloud can give rain formation a strong local pattern. Moist air approaching a mountain rises along its windward slope, cools, and sheds water. Beyond the ridge, descending air becomes warmer and drier, producing a rain shadow. In North America, the Sierra Nevada helps create the dry conditions of the Great Basin and Mojave Deserts. Across western Argentina, the Andes block moisture arriving from the Pacific. On Hawaii's islands, slopes facing the east to northeast trade winds receive much more rain than leeward areas, where sunnier and drier conditions prevail beneath less cloud cover.
In the tropics, rising air produces showers whose intensity can change rapidly over short distances. These convective clouds have limited horizontal extent, so their rain often passes over a particular area for a relatively short time. Seasonal shifts bring a different rhythm. Savanna climates and monsoon regions commonly have wet summers and dry winters, while tropical rainforests receive rain more evenly through the year. During a warm-season wet period, rain often falls in the late afternoon and early evening. Tropical cyclones can bring enormous quantities of water, sometimes delivering a year's worth of rainfall to places along their paths.
The journey does not always reach the ground. Falling precipitation can evaporate beneath a cloud, leaving visible streaks called virga, a phenomenon common in hot, dry climates. Drops that do reach water create sound through bubbles of air oscillating beneath the surface. On loose material, including newly fallen ash, impacts can leave dimples that later become fossilized impressions. Rain gauges collect and measure rainfall at the surface, while weather radar estimates its amounts across a wider area. The speed of impact varies with drop size: larger drops fall faster because their mass is greater in relation to aerodynamic drag. In still air at sea level, drizzle falls much more slowly than the broad drops of a heavy shower. Once deposited, the fresh water supports crop irrigation, hydroelectric generation, and ecosystems, continuing its movement through Earth's water cycle.