Precipitation

• 1. Introduction

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  Precipitation is the name meteorologists give to any form of condensed atmospheric water vapor that falls from clouds under the pull of gravity. Each year, approximately 505,000 cubic kilometers of water descend onto the Earth in this way: roughly 398,000 cubic kilometers landing on oceans, and 107,000 cubic kilometers on land. That works out to a globally averaged annual total of 990 millimeters, though over land the figure drops to 715 millimeters. What falls is not always rain. Drizzle, snow, ice pellets, graupel, hail, and the mixed form called sleet in Commonwealth usage all qualify. Fog and mist, by contrast, do not count as precipitation at all, because their water vapor never condenses enough to actually fall. The questions worth asking about precipitation go deeper than a weather report. How does a water droplet become heavy enough to escape a cloud? What turns a snowflake into a hailstone the size of a golf ball? And how are cities, mountains, and a distant moon of Saturn all reshaping the story of falling water?

• 2. From Cloud to Ground

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  A cloud particle begins its journey downward only when atmospheric motions can no longer keep it aloft. That tipping point depends on coalescence, the process by which smaller droplets collide and fuse into larger ones, or freeze onto an ice crystal through what is called the Bergeron process. Droplets of different sizes fall at different terminal velocities, which drives those collisions. Turbulence within the cloud accelerates the process further. As the growing drops descend, they keep collecting smaller droplets, until they are heavy enough to push through air resistance and reach the surface as rain.

  Raindrops have mean diameters ranging from 5.1 to 20 mm. Above that size they tend to break apart. Despite the cartoon image, a falling raindrop does not look like a teardrop. Smaller drops remain spherical; as a drop grows, it becomes more oblate, with its widest face turned into the oncoming air. High-intensity rain tends to be short-lived, while lower-intensity rain can persist for a long time. Rain drops associated with melting hail are typically larger than ordinary raindrops. The METAR weather code for rain is RA, and for rain showers, SHSN.

• 3. The Architecture of Ice

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  Snow crystals begin as supercooled cloud droplets roughly 10 micrometers in diameter that freeze solid. Once frozen, a crystal grows in the supersaturated environment around it, drawing water vapor away from the remaining liquid droplets, which then evaporate. This is the Wegener-Bergeron-Findeisen process. The crystals can grow to hundreds of micrometers, eventually becoming heavy enough to fall. They may collide and stick together into aggregates called snowflakes.

  The shape a snowflake takes is set by the temperature and humidity at the moment of its formation. Rarely, near -2 C, snowflakes form with threefold symmetry rather than the familiar sixfold pattern, producing triangular crystals. Because each flake travels through a unique and constantly shifting column of temperature and humidity on its way down, no two are identical. Guinness World Records lists the world's largest snowflakes as those measured in January 1887 at Fort Keogh, Montana, where one allegedly reached 38 cm wide.

  Hail forms by a different mechanism entirely. Supercooled water droplets inside a storm cloud freeze on contact with condensation nuclei such as dust or dirt. The storm's updraft then carries the hailstone upward to the cloud's top. When the updraft weakens, the stone falls back down, only to be lofted again. With each ascent it gains another ice layer. If latent heat from further freezing partially melts the outer shell, the stone enters what is called wet growth, accumulating smaller hailstones around itself. Hailstones must reach at least 5 mm in diameter to qualify; the METAR code GR designates larger stones of at least 6.4 mm and derives from the French word grele. Hailstones can grow to 15 cm and weigh more than 500 grams.

• 4. Mountains, Fronts, and Convection

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  Three broad mechanisms generate most of the precipitation that reaches the ground: convective processes, stratiform processes, and orographic effects. Convective precipitation arises from strong vertical motions in clouds such as cumulonimbus, overturning the local atmosphere within an hour and delivering heavy, short-lived showers. Stratiform precipitation comes from weaker upward motion, producing steadier, less intense rainfall over wider areas.

  Orographic precipitation falls on the windward side of mountain ranges, where a large-scale flow of moist air is forced to rise, cool adiabatically, and condense. The leeward side receives far less moisture. Compressional heating of descending air on that side creates dry conditions, and, taken to an extreme, desert climates. The Sierra Nevada range in North America produces this effect, helping to form the Great Basin and Mojave Deserts. In South America, the Andes block Pacific moisture, leaving a desertlike climate across parts of western Argentina.

  In Hawaii, Mount Waialeale on the island of Kauai holds the distinction of receiving the second-highest average annual rainfall on Earth, at 460 inches, driven by the consistent trade winds that push moisture against its slopes. Storm systems batter Hawaii with heavy rains between October and March, while the leeward Kona sides of each island remain notably drier and sunnier.

• 5. Measuring the Fall

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  Rainfall is typically measured with a rain gauge and expressed in millimeters of depth, which is equivalent to liters of water per square meter of surface. The standard gauge comes in 10 cm plastic and 20 cm metal varieties. The inner cylinder holds up to 2.5 cm of rain; overflow passes into the outer cylinder. Plastic gauges carry printed markings; metal gauges require a calibrated measuring stick. In Australia before metrication, rainfall was measured in points, each defined as one-hundredth of an inch.

  Solid precipitation uses a snow gauge, measuring depth in centimeters. The snow can then be melted to produce a water-equivalent figure in millimeters. The relationship between snow depth and water content varies with the snow's density, so the water-equivalent figure is only an approximation of actual depth. Hail, snow pellets, and mixed precipitation can all be melted and expressed the same way.

  Where gauges cannot reach, satellite sensors fill the gap. Thermal infrared sensors record emissions at around 11 micron wavelength and read cloud-top temperatures; colder tops typically indicate higher-altitude clouds and more vigorous precipitation. Mathematical algorithms convert those readings into rainfall estimates. The modern global precipitation record depends substantially on this satellite data, because vast stretches of ocean and remote land make surface gauges impractical. Volunteer networks such as CoCoRAHS also collect ground-level measurements across the United States, submitting data through the internet to supplement official station records.

• 6. Climate, Cities, and Change

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  Precipitation has generally increased over land north of 30 degrees North from 1900 to 2005 but has declined over the tropics since the 1970s. A 2018 study using a high-resolution global dataset spanning more than 33 years found no evidence of a statistically significant increase in precipitation at the global scale despite observed global warming, though regional trends varied widely. Eastern portions of North and South America, northern Europe, and northern and central Asia have become wetter. The Sahel, the Mediterranean, southern Africa, and parts of southern Asia have grown drier.

  Over the contiguous United States, total annual precipitation increased at an average rate of 6.1% per century since 1900. The greatest increase appeared in the East North Central climate region at 11.6% per century, and the South at 11.1%. Hawaii was the only region to show a decrease, at -9.25%.

  Cities generate their own precipitation signal. The urban heat island effect warms cities between 0.6 and 5.6 degrees Celsius above surrounding suburban and rural areas, driving greater upward motion in the air above them. Rainfall rates downwind of cities increase between 48% and 116% compared with upwind areas. Monthly rainfall runs about 28% higher in the band 20 to 40 miles downwind of a city. Some cities induce a total precipitation increase of 51%.

  The reach of precipitation extends beyond Earth. Saturn's largest satellite, Titan, hosts methane falling as a slow drizzle, with rain puddles observed at both its equator and polar regions.