Rain

• 1. Introduction

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  The biggest raindrops ever recorded on Earth fell over Brazil and the Marshall Islands in 2004, and some were as large as 10 mm. They did not look like the teardrop shape so many people picture. A large raindrop flattens on the bottom like a hamburger bun, and the very largest ones spread out like tiny parachutes. Rain is a form of atmospheric precipitation, water droplets condensed from vapor and pulled down by gravity. It is a major part of the water cycle and deposits most of the fresh water on Earth. From that single fact, a tangle of questions unfolds. How does invisible vapor in the air become a falling drop? Why does it rain more on Saturdays than on Mondays near a crowded coast? Why does the name of a nation's currency mean rain? And where on this planet does the sky open up so often that one summit sees rain on 360 days a year? The answers reach from ancient Sumer to a town in Colombia, and from the physics of a single drop to the salinity of distant oceans.

• 2. From Vapor To Drop

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  Warmer air can hold more water vapor than cooler air before it becomes saturated. That single rule sits at the heart of how a cloud is born. The amount of water in a given mass of dry air, called the mixing ratio, is measured in grams of water per kilogram of dry air. When a parcel reaches one hundred percent relative humidity, it saturates and forms a cloud, a group of visible tiny water or ice particles suspended above the surface. One reliable way to push air toward saturation is simply to cool it. The dew point is the temperature a parcel must reach to become saturated.

  Four main mechanisms cool air to its dew point. Adiabatic cooling happens when air rises and expands, lifted by convection, by large-scale motion, or by a barrier such as a mountain. Conductive cooling happens when air touches a colder surface, as when wind carries it from water onto colder land. Radiational cooling comes from the emission of infrared radiation by the air or the ground. Evaporative cooling occurs when moisture is added through evaporation, forcing the air down to its wet-bulb temperature.

  Condensation does not happen on nothing. Water vapor begins to gather on condensation nuclei such as dust, ice, and salt to form clouds. Stratus, a stable deck of cloud, tends to form when a cool, stable air mass is trapped beneath a warm one. It can also appear when breezy conditions lift an advection fog off the ground.

• 3. The Birth Of A Drop

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  Coalescence is the moment two water droplets fuse into one larger droplet. Inside a cloud, air resistance keeps the droplets nearly stationary, but turbulence makes them collide and merge. As these larger drops descend, the merging continues until they grow heavy enough to overcome air resistance and fall as rain. This warm rain process happens most often in cloud tops above freezing. In clouds below freezing, ice crystals must gather enough mass to fall, which usually requires more than coalescence alone, and supercooled droplets only survive where the cloud sits below freezing. Because the cloud is so much colder than the ground, those ice crystals can melt on the way down and arrive as rain.

  Raindrops range from 0.1 to 9 mm in mean diameter and tend to break apart at larger sizes. The smallest, called cloud droplets, are perfectly spherical. As a drop grows it flattens, turning oblate with its widest face meeting the oncoming air. Drops linked to melting hail tend to be larger than the rest.

  The final spread of drop sizes follows an exponential pattern known as the Marshall-Palmer law, named for the researchers who first described it. The parameters depend somewhat on temperature, and the slope scales with the rate of rainfall. Real, instantaneous spectra often deviate from this and have been modeled instead as gamma distributions.

  When rain finally lands, it hits at its terminal velocity, which grows with drop size because larger drops carry a higher mass-to-drag ratio. At sea level with no wind, a 0.5 mm drizzle drop strikes at 2 m/s, while a large 5 mm drop hits at around 9 m/s. The familiar sound of rain falling on water comes from bubbles of air oscillating beneath the surface.

• 4. What Makes It Fall

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  Weather fronts, three-dimensional zones of temperature and moisture contrast, are the major cause of rain production. Where enough moisture and upward motion meet, precipitation falls from convective clouds such as cumulonimbus, the thunder clouds, which can organize into narrow rainbands. What separates rain from snow or ice pellets is a thick layer of air aloft warmer than the melting point, which thaws frozen precipitation long before it reaches the ground. If only a shallow near-surface layer sits below freezing, the result is freezing rain that hardens on contact with cold surfaces.

  Convective rain falls from clouds like cumulonimbus and cumulus congestus as showers with rapidly changing intensity. Because convective clouds have limited horizontal extent, the rain covers a given area for a relatively short time. Most precipitation in the tropics appears to be convective, and graupel and hail are signs of convection at work. In the mid-latitudes it comes and goes, often tied to cold fronts, squall lines, and warm fronts.

  Orographic precipitation forms on the windward side of mountains, where moist air rising over a ridge cools adiabatically and condenses. On the leeward side, descending air heats and dries, leaving a rain shadow. In Hawaii, Mount Waiʻaleʻale on the island of Kauai is among the rainiest places in the world, with 373 in, and Kona storms bring heavy rains between October and April. In South America, the Andes block Pacific moisture and create a desert-like climate downwind across western Argentina, while the Sierra Nevada does the same in North America, forming the Great Basin and Mojave Deserts.

• 5. Cities Make Rain

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  There is a 22 percent higher chance of rain on Saturdays than on Mondays in heavily populated coastal areas such as the United States' Eastern Seaboard. The cause is human. Fine particulate matter from car exhaust and other pollution forms cloud condensation nuclei, raising the odds of rain. As weekday commuter and commercial traffic builds pollution over five days, that likelihood climbs and peaks by Saturday.

  The urban heat island effect warms cities 0.6 to 5.6 C above their surrounding suburbs and countryside. That extra heat drives greater upward motion, which can spark additional showers and thunderstorms. Rainfall rates downwind of cities rise between 48 and 116 percent. Monthly rainfall runs about 28 percent greater between 20 and 40 mi downwind of cities compared with upwind, and some cities push total precipitation up by 51 percent.

  The water that falls is rarely pure. The phrase acid rain was first used by Scottish chemist Robert Augus Smith in 1852. On America's East Coast, rain off the Atlantic typically carries a pH of 5.0-5.6, rain crossing from the west runs 3.8-4.8, and local thunderstorms can drop as low as 2.0. The acidity comes mainly from sulfuric and nitric acid. Over the past 20 years their concentrations in rainwater have fallen, a shift likely tied to rising ammonium, most likely ammonia from livestock production, which buffers the acid and raises the pH.

• 6. A Changing Climate

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  Over the contiguous United States, total annual precipitation has risen at an average rate of 6.1 percent since 1900. The largest gains came in the East North Central region at 11.6 percent per century and the South at 11.1 percent, while Hawaii was the only region to fall, by 9.25 percent. Globally there has been no statistically significant overall trend in precipitation over the past century, though the regional picture varies sharply. Eastern parts of the Americas, northern Europe, and northern and central Asia have grown wetter, while the Sahel, the Mediterranean, southern Africa, and parts of southern Asia have grown drier.

  Heavy downpours are climbing too. An analysis of 65 years of United States rainfall records shows the lower 48 states have seen more heavy downpours since 1950, with the largest increases in the Northeast and Midwest. Rhode Island leads the states with a 104 percent increase, and McAllen, Texas leads the cities with 700 percent. In this analysis, a heavy downpour is a day whose total precipitation exceeded the top one percent of all rain and snow days between 1950 and 2014.

  The oceans hold their own evidence. Decreased salinity in mid- and high-latitude waters implies more precipitation, while increased salinity in lower latitudes implies less rain or more evaporation. The most successful efforts to influence weather involve cloud seeding, used to boost winter precipitation over mountains and to suppress hail.

• 7. Faith And The Sky

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  In Botswana, the Setswana word for rain, pula, is the name of the national currency, chosen because rain carries such economic weight in a country with a desert climate. Across temperate climates, unstable or cloudy weather tends to leave people more stressed, an effect greater on men than on women. In dry places like India, or during a drought, rain lifts the mood. Many people find the scent during and just after rain distinctive, a smell called petrichor, an oil that plants produce, rocks and soil absorb, then release into the air when rain returns.

  The ancient Sumerians believed rain was the semen of the sky god An, falling to inseminate his consort, the earth goddess Ki, so she could give birth to all the plants of the earth. The Akkadians pictured the clouds as the breasts of Anu's consort Antu, and rain as the milk from them. In Jewish tradition, in the first century BC, the miracle-worker Honi ha-M'agel ended a three-year drought in Judaea by drawing a circle in the sand and refusing to leave it until his prayer for rain was answered.

  In his Meditations, the Roman emperor Marcus Aurelius preserved a prayer for rain that the Athenians made to the sky god Zeus. Various Native American tribes historically held rain dances to encourage rainfall, and rainmaking rituals remain important in many African cultures. Even in the present-day United States, state governors have called Days of Prayer for rain, including the Days of Prayer for Rain in the State of Texas in 2011.

• 8. The Wettest Places

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  Cherrapunji, on the southern slopes of the Eastern Himalaya in Shillong, India, is the confirmed wettest place on Earth, with an average annual rainfall of 11430 mm. Its highest recorded total in a single year was 22987 mm in 1861. Nearby Mawsynram, Meghalaya, holds a 38-year average of 11873 mm. Unlike these spots, which take most of their rain between April and September, some places are soaked almost evenly all year.

  Lloró, in Chocó, Colombia, is probably the rainiest place in the world, averaging 523.6 in per year. The Department of Chocó is extraordinarily humid. The small town of Tutunendaó in the same department averages 11394 mm, and in 1974 it received 26303 mm, the largest annual rainfall ever measured in Colombia. Its rain falls almost uniformly through the year. Quibdó, the capital of Chocó, takes more rain than any city on Earth with over 100,000 inhabitants, at 354 in per year.

  Mount Waiʻaleʻale on Kauaʻi receives 373 in of rain per year over its last 32 years of record, with a peak of 17340 mm in 1982, and its summit reports rain on 360 days a year. Across the globe roughly 505000 km3 of water falls as precipitation each year, with 398000 km3 of it over the oceans. Spread across the Earth's surface, that works out to a globally averaged annual precipitation of 990 mm, the quiet sum of every drop that ever flattened like a hamburger bun on its way down.