98 Fun Facts About Water Cycle

Where does more water evaporate than falls back as rain or snow?

Oceans: they give up about 86% of the world's evaporation but catch only about 78% of its rain and snow. Winds carry the surplus over land, and rivers bring it home.

What powers the water cycle?

Solar radiation: the sun's heat lifts water into the air as vapour, then gravity takes over, pulling it back down as rain and snow and draining it downhill to the sea.

After evaporating, about how long does water typically stay in the atmosphere?

9 days: all the vapour in the sky would cover the globe only about an inch deep, yet roughly a metre of rain falls each year, so the air is emptied and refilled nearly 40 times over.

Deep groundwater that rain no longer refills, like the store beneath the Sahara, goes by what name?

Fossil water: Libya's supply soaked in before the last ice age ended, when the Sahara was far wetter. Crews drilling for oil struck it in the 1950s, and it is piped north knowing it will not refill.

Each year, how does the world's precipitation compare with its evaporation?

About the same: some half a million cubic kilometres rise and fall each year, and the sky can never bank the difference, because at any moment it holds only about 0.001% of Earth's water.

About what share of all the water on Earth is fresh?

2.5%: rain falls fresh because evaporation lifts out only the water and leaves the salt behind, while rivers keep washing minerals out of rock and into the sea, which is how it turned salty.

What is the name for streaks of rain that evaporate before they reach the ground?

Virga: the vanishing rain chills the air around it, and that cold, heavy air can plunge to the ground as a dry microburst, a sudden downdraft that is a serious hazard to aircraft.

Where is most of Earth's fresh water found?

Glaciers and ice: they lock up about 69% of all fresh water, enough to raise the seas by more than 60 metres and flood every coastal city if it all melted.

Millions of people in the American West get their summer water from the slow melting of what?

Mountain snowpack: it works as a frozen reservoir, banking winter storms in the high country and releasing them as meltwater through the warm months, just when farms and cities are driest.

Sweat cools your skin through which process?

Evaporative cooling: the fastest molecules escape into the air and carry their heat away, which is why a muggy day feels hotter than a dry one, since sweat can barely evaporate into damp air.

Condensation is the reverse of which process?

Vaporization: water takes in heat to turn to vapour and gives it back on condensing. That released heat warms the air and keeps it rising, which is the engine that powers a hurricane.

Sublimation is the change from a solid directly into what?

A gas: snow and ice can vanish this way without melting first. The Chinook, a warm wind off the Rockies nicknamed the snow eater, is so dry that it can turn snow in its path straight to vapour.

Which instrument measures the humidity of the air?

A hygrometer: the classic design, built by a Swiss scientist in 1783, uses a stretched human hair, which changes length with the dampness of the air and moves a needle across a dial.

Plants release water vapour through small leaf pores called what?

Stomata: a pair of guard cells opens and closes each pore, and when the roots sense dry soil the plant shuts them to save water, at the cost of slowing its own growth.

Plants lose nearly all the water they take up because their leaf pores must stay open to absorb which gas?

Carbon dioxide: a leaf cannot let this gas in without letting water out. It is why cacti and many other desert plants keep their pores shut through the hot day and open them only at night.

Cooling air reaches 100% relative humidity at what temperature?

The dew point: rising air cools as it climbs, and cloud begins at the height where it reaches this temperature, which is why puffy fair-weather clouds all share the same flat base.

Luke Howard named the clouds cumulus, stratus and cirrus in the early 1800s, borrowing words from which language?

Latin: he named each cloud for its look, so cumulus means a heap, stratus a spread-out layer and cirrus a curl of hair. Howard was a London chemist who studied the sky as a hobby.

Thunderstorms and hail are produced by which type of cloud?

Cumulonimbus: its updraft climbs until the surrounding air is just as warm, near the top of the weather layer. There the cloud stops rising and spreads out flat, forming the storm's anvil top.

By definition, what separates fog from mist?

Visibility: under one kilometre it counts as fog, and beyond that it is only mist. Fog is simply a cloud whose base rests on the ground.

Dew forms most easily on grass, car roofs and bridges because those surfaces do what?

Radiate heat and cool quickly: cut off from the warmth of the deep ground, they chill faster than the air. It is why dew is heaviest after a clear, calm night, when no cloud cover sends that heat back down.

A snowflake's six arms grow separately, so why do they end up matching?

They meet the same conditions: each arm sprouts from a corner of one tiny hexagon of ice and rides through the same cold and damp on the way down. A different path shapes the next flake differently.

In winter, a deep lake freezes in which direction?

Top down: water is densest at about 4°C, so colder water and ice float above it. The ice lid then insulates the lake, which is why fish can overwinter in liquid water beneath it.

In US forecasts, hail counts as 'severe' once it reaches the size of what?

A quarter: that is one inch across, up from a penny-sized three-quarters of an inch before 2010. A stone falls only when it outgrows the storm's updraft, so bigger hail means a more violent storm.

A falling raindrop stops speeding up once it reaches what?

Terminal velocity: the air pushing back stops the drop from speeding up, and it also flattens the underside, so a falling raindrop is shaped like a hamburger bun rather than a teardrop.

Infiltration is the process by which surface water does what?

Enters the soil: from there it feeds plant roots or keeps sinking to join the groundwater, which seeps slowly into stream beds and keeps rivers flowing long after the last rain.

Rain that lands on a paved city street mostly becomes what?

Surface runoff: roofs and pavement act as a fast lane, rushing rain into storm drains and straight to streams instead of letting it soak in, which is why paved-over valleys flood more often and more severely.

The water table is the upper surface of which underground zone?

The saturated layer: below it every pore and crack in the ground is full of water. Dig a hole at the beach until it starts to fill and you have reached it, which is all that a well really is.

A rock unit that yields a usable quantity of water is called what?

An aquifer: despite the popular picture of underground rivers and lakes, it is rock or gravel with water filling the pores. That water helps hold the ground up, so heavy pumping can make the land above sink.

The Ogallala, a vast store of groundwater, lies beneath which US region?

The Great Plains: it lies under parts of eight states, from South Dakota to Texas, and waters one of America's main farm belts. Decades of pumping for irrigation have dropped its level by more than 100 feet in places.

Far from any volcano, how does a hot spring's water get hot?

Sinking deep underground: rock grows steadily hotter with depth, so rain that seeps down a mile or so comes back up warm. At Hot Springs, Arkansas, that round trip takes more than 4,000 years.

The deepest permafrost ever found, frozen ground reaching nearly a mile down, lies beneath which region?

Siberia: the frozen ground there runs about 1,500 metres deep, twice as far as Alaska's. It also works as a waterproof floor, which is why tundra that gets as little rain as a desert turns to ponds and bog each summer.

In North America, a drainage basin is commonly called what?

Elsewhere 'watershed' keeps its original meaning: the ridge line dividing one basin from the next.

A 'Pineapple Express' is an atmospheric river carrying moisture from where?

The Hawaiian tropics: warm, wet air streams from near the islands to the West Coast in a narrow band. Once ashore it can bring heavy rain and snow, and the strongest have caused severe floods.

Which chemical is most commonly used in cloud seeding to trigger ice crystals?

Silver iodide: its crystals are shaped so much like ice that supercooled droplets freeze onto them. Bernard Vonnegut, brother of the novelist Kurt, found the trick at General Electric in 1946.

The first standardized rain gauge, the Cheugugi, was invented in 1441 in which country?

Korea: King Sejong, who also gave the country its alphabet, had identical gauges set up in every province, so that rainfall could be recorded the same way across a kingdom that depended on its harvests.

Mawsynram and Cherrapunji, neighbouring towns that rank among the wettest places on Earth, are in which country?

India: they sit in Meghalaya, a state whose name means the abode of clouds. Summer winds heavy with sea moisture are forced up its steep hills, which wring out about 11 metres of rain a year.

Which desert, where some weather stations have never recorded rain, is the driest outside the polar regions?

The Atacama: the Andes block rain from the east, and a cold ocean current chills the coastal air into fog that never turns to rain. NASA found its soil so like that of Mars that it tests instruments there.

A rain shadow forms on which side of a mountain range?

The leeward slope: air sheds its rain climbing the side that faces the wind, then sinks, warms and dries on the way down. Death Valley is so dry because four mountain ranges stand between it and the Pacific.

What is the name for the earthy smell that rises when rain falls on dry ground?

Petrichor: two Australian scientists coined the word in 1964 from the Greek for rock and ichor, the blood of the gods in myth. The scent comes partly from plant oils that build up in dry soil and are released by the first drops.

In 1674 Pierre Perrault proved rain alone could feed a river by measuring which one?

The Seine: he worked out how much rain and snow fell on its upper basin in a year and found it was far more than the river carried away, so no hidden underground supply was needed.

At equal temperature and pressure, how does the density of water vapour compare with that of dry air?

Lower: a water molecule weighs less than the nitrogen and oxygen molecules it pushes aside, so damp air is lighter than dry air. That extra lift helps moist air rise and build into clouds.

A monsoon is traditionally defined as what?

A seasonally reversing wind: land heats and cools faster than the sea, so the wind blows ashore in summer, bringing the rains, and back out to sea in winter. The name is traced to the Arabic mawsim, meaning season.

Which category of dry spell is defined simply by a lack of precipitation?

Meteorological drought: it comes first, as a long run of dry weather. If the rain stays away, farm soils dry out and crops suffer next, and only later do rivers, reservoirs and wells run low.

Which of the Great Lakes holds more water than the other four put together?

Lake Superior: it averages nearly 500 feet deep, almost eight times Lake Erie's 62. That shallowness is why Erie warms so quickly in summer and often freezes over in winter.

Which lake is the deepest in the world?

Lake Baikal: it is more than 1,600 metres deep and, at over 20 million years old, also the oldest lake on Earth. That age and isolation produced wildlife found nowhere else, including a seal that lives only in fresh water.

Which river has the largest discharge volume of any in the world?

The Amazon: it carries about a fifth of all the river water that reaches the sea, enough to dilute the salt of the Atlantic for more than 100 miles from shore.

The Jordan River ends in which lake, where water escapes only as vapour and leaves its salt behind?

The Dead Sea: with no river flowing out, the salt has built up until bathers float like corks and no fish can survive. Its shore, more than 400 metres below sea level, is the lowest land on Earth.

Which island holds the only ice sheet on Earth outside Antarctica?

Greenland: ice more than three kilometres thick at its deepest covers about 80% of the island. The sagas say Erik the Red chose the inviting name a thousand years ago to tempt settlers to follow him there.

If all the water in the atmosphere fell as rain at once, how deep a layer would cover the globe?

About 1 inch is all it would amount to, because the atmosphere holds only around 0.001 percent of Earth's water. It works as a fast-moving conveyor, not a storehouse.

Air pressure flattens the underside of a falling raindrop into a shape that looks most like which of these?

A hamburger bun is the closest match, because air pushing up on a falling drop flattens its base while the top stays domed. The smallest drops stay spherical, held together by surface tension.

Which of these, far too small to see singly, crowd together to form the visible mass of a low cloud?

Liquid droplets form the visible mass of a low cloud, each one far too small to pick out alone. Water vapour is transparent, so the moisture shows up only once it condenses.

Which river carved the Grand Canyon, cutting down through nearly two billion years of rock layers in Arizona?

The Colorado is thought to have settled into its course there about five to six million years ago, and it has been deepening and widening the canyon ever since.

Which Central Asian lake, once among the world's largest, shrank to a tenth of its size after its rivers were diverted?

The Aral Sea began shrinking in the 1960s, when Soviet planners diverted its two feeder rivers to grow cotton in the desert. By 2007 it had split into four smaller lakes.

Worldwide, which human activity takes about 70% of the fresh water people withdraw from rivers, lakes and wells?

Agriculture is easily the thirstiest thing people do, mostly because crops need irrigating. Industry and households share the rest.

Which moon has a cycle much like Earth's water cycle, with methane rain feeding rivers, lakes and seas?

Titan runs its version at about −179 °C, cold enough for methane to rain down and pool in polar lakes. It is the only world besides Earth known to keep stable lakes and seas on its surface.

Rainwater seeping through cracks slowly dissolves which rock, hollowing out the largest water-carved caves on Earth?

Limestone is mostly calcium carbonate, a mineral that slightly acidic rainwater slowly dissolves. Dripping water later lays it back down as stalactites and stalagmites.

Pumping groundwater has helped make which Asian capital sink, one reason its government decided to build a replacement?

Jakarta's northern districts are subsiding as wells drain the ground beneath them, which worsens coastal flooding. In 2022 Indonesia laid the legal groundwork to move its capital to Nusantara.

Ice sheets locked up so much water in the last ice age that which sea passage off Alaska became dry land?

The Bering Strait averages only 40 to 50 metres deep, so it drained once growing ice sheets pulled the seas down. The exposed bridge to Asia was up to 1,000 km wide.

Through which process does a large oak tree give off about 40,000 gallons of water vapour from its leaves each year?

Transpiration supplies roughly a tenth of the moisture in the atmosphere, with the rest rising from seas, lakes and rivers. An acre of corn gives off thousands of gallons a day.

NASA's twin GRACE satellites tracked shrinking groundwater stores from orbit by measuring tiny changes in what?

Earth's gravity varies slightly with the mass lying below, so the twin craft could map where water was being gained or lost over time. Their data exposed groundwater depletion in California.

Frost forms when water vapour turns straight into ice on a cold surface, skipping the liquid stage, by which process?

Deposition happens when the air touching a surface is already below freezing, so the vapour has no chance to settle as dew first. It is how hoar frost coats leaves and the ground.

Which word for water trickling down through soil and rock is also used for brewing coffee through a filter?

Percolation is the slow downward trickle that refills stores of groundwater. Plato and Aristotle were already speculating about it more than 2,300 years ago.

Which name do scientists give to all of Earth's frozen water, from sea ice to permafrost?

The cryosphere also takes in snow cover, frozen lakes and rivers, and the great ice sheets. It overlaps with the hydrosphere, since frozen water is water all the same.

Which science deals with the movement, distribution and management of water across Earth and even other planets?

Hydrology lends the water cycle its other name, the hydrologic cycle. The Roman engineer Vitruvius sketched an early version in the first century BC, with mountain rain feeding lowland springs.

Which process, driven by uneven heating of the ground, sends warm air rising in columns known as thermals?

Convection happens because warm air is less dense than the air around it, so it floats upward while cooler, denser air sinks. Moisture carried aloft this way builds puffy cumulus clouds.

Which lowest layer of Earth's atmosphere holds about 99% of its water vapour and nearly all of its weather?

The troposphere takes its name from the Greek for 'turning', after the constant churning that mixes its air. On average it is only about 13 km thick.

Storm clouds keep towering upward because condensing water vapour releases which form of energy, warming the rising air?

Latent heat was named in the 18th century by the Scottish chemist Joseph Black, from a word meaning lying hidden. It changes water's state without changing its temperature.

Scientists use which term for the average stay of a water molecule in one store, such as a lake?

Residence time in the sea averages about 3,200 years, against only a month or two for moisture held in soil. Dividing a store's volume by its flow gives the figure.

At Harpers Ferry the Shenandoah River flows into the larger Potomac, which makes the Shenandoah which kind of stream?

A tributary is labelled left-bank or right-bank by looking downstream, so the Shenandoah is a right one. The opposite, a branch that leaves the main channel, is a distributary.

River deltas take their name from the triangular mouth of which river, shaped like the Greek capital letter delta?

The Nile spreads into a triangle of silt so distinctive that the Greek historian Herodotus refers to 'the Delta' fourteen times in his Histories.

Which word describes the water of an estuary, saltier than a river but less salty than the open sea?

Brackish comes from the Middle Dutch word brak. Mangrove swamps thrive in such water, fringing estuaries where the saltiness shifts with each tide.

Marshes, swamps and bogs belong to which group of ecosystems, prized for storing floodwater and refilling groundwater?

Wetlands may be flooded permanently or only for a season. Under the Ramsar treaty that protects them, even shallow coastal sea counts, down to six metres deep at low tide.

Which fertile spot in a desert forms where underground water reaches the surface, often beneath a canopy of date palms?

An oasis can be fed by natural springs or by wells dug down to the water. Its date palms form an upper layer that shades smaller trees such as figs, olives and apricots.

Which river's basin, the largest in the United States, drains the land between the Rockies and the Appalachians?

The Mississippi gathers water from about thirty states and two Canadian provinces. Engineers work to stop its lower course shifting into the Atchafalaya and bypassing New Orleans.

Which line along the Rockies decides whether rainwater ends up in the Pacific or in the Atlantic and Arctic?

The Continental Divide runs the whole length of the Americas. It meets the continent's northern divide at Triple Divide Peak in Montana.

The Everglades are really a slow-moving river about 60 miles wide, flowing south from Lake Okeechobee in which US state?

Florida's broad, creeping sheet of water earned the nickname 'River of Grass', a phrase the writer Marjory Stoneman Douglas used for it in 1947.

The Great Artesian Basin, one of the world's largest stores of underground fresh water, lies beneath which country?

Australia depends on it as the only source of fresh water across much of the inland. The water can come up at anything from 30 to 100 °C.

The Three Gorges Dam, a hydroelectric giant near Yichang in central China, spans which river?

The Yangtze dam helps protect millions of people downstream from severe floods. Building it displaced more than 1.1 million residents.

Sulfur and nitrogen oxides from smokestacks react with moisture in the air to cause which problem?

Acid rain got its name from Robert Angus Smith, who had linked it to Manchester's polluted air back in 1852. It peels paint, corrodes steel and eats into stone buildings and statues.

Which Pacific warming, named by Peruvian fishermen for arriving around Christmas, reshuffles rainfall around the world?

El Niño means 'The Boy', after the Christ Child. The opposite, cold phase of the same seesaw is called La Niña.

Which light precipitation consists of drops smaller than half a millimetre across, finer than those of ordinary rain?

Drizzle is normally produced by low, layered clouds. It is distinct enough from rain to get its own code in aviation weather reports, DZ.

Which line-shaped clouds form when water vapour in aircraft engine exhaust freezes at cruising altitude?

Contrails are short for 'condensation trails' and are made mostly of ice crystals. In cold, humid air they can last for hours and spread until they resemble natural cirrus.

Which process turns seawater into fresh water for cities in Israel and Saudi Arabia, and for ships and submarines?

Desalination works either by boiling and distilling the water or by forcing it through a membrane that holds back dissolved salts. Both take so much energy that it costs more than river or groundwater.

Raindrops that stay liquid below freezing, then turn to ice the instant they hit a road, are described as what?

Supercooled raindrops coat everything they touch in clear glaze ice, and a heavy fall of them is called an ice storm. Pure water can stay liquid down to about −48 °C.

Which term describes the lift that forces moist air up and over rising terrain, turning it into cloud and rain?

Orographic lift chills the climbing air until its moisture condenses. It explains why high ranges wring so much rain and snow out of passing weather.

Which precipitation, also called soft hail, forms when a falling snowflake gets coated in frozen cloud water?

Graupel is a German word for crisp, opaque balls of rime only 2 to 5 mm across. The balls are so fragile that they typically fall apart when pressed.

Which term covers rain that never reaches the soil because leaves and branches catch it on the way down?

Interception can be surprisingly large: trees such as the Norway maple hold back roughly 38% of the precipitation that falls on them in a temperate climate.

Which plant tissue carries water upward from the roots to the leaves, where most of it escapes into the air?

Xylem takes its name from the Ancient Greek word for wood, and wood is the best-known example of it.

What draws water up a paper towel, a paintbrush or a thin tube without any pump or outside push?

Capillary action was first recorded by Leonardo da Vinci, and it was the subject of Albert Einstein's first scientific paper, submitted in 1900.

Which term describes huge blocks of ice breaking away from the edge of an ice shelf to become icebergs?

Calving is often announced by a loud crack or boom, before blocks of ice up to 60 metres high break loose and crash into the sea.

Which English astronomer showed that evaporation from the Mediterranean could account for the rivers flowing into it?

Edmond Halley, better remembered for the comet that bears his name, also studied the saltiness of the sea as a clue to the age of the Earth.

Qanats, gently sloping tunnels that bring groundwater to the surface without pumps, were developed in which country?

Iran's invention is about 3,000 years old and spread both east and west, becoming the kārēz of Afghanistan and the foggāra of Algeria.

Salar de Uyuni, the world's largest salt flat, was left when a series of prehistoric lakes dried up in which country?

Bolivia's vast salt crust covers a pool of brine exceptionally rich in lithium. It is so flat that satellites use it to calibrate their altimeters.

The Okavango River never reaches the sea, spreading into a vast inland delta in the Kalahari Desert of which country?

Botswana's inland delta returns nearly all its water to the air, evaporated by the sun or given off by plants. It is one of the few deltas that never meet a sea.

Which term describes a basin, like that of the Caspian Sea, whose rivers drain inland and never reach the open sea?

Endorheic joins Greek roots meaning 'within' and 'flow'. With no outlet, such basins lose water only by evaporation and seepage, and they are often simply called sinks.

Which word describes old granular snow that has outlasted a summer but has yet to be compressed into solid ice?

Firn comes from a Swiss German word for 'last year's'. It looks like wet sugar, yet it is hard enough to resist a shovel.

Which Old Testament book observes that 'all the rivers run into the sea, yet the sea is not full'?

Ecclesiastes also has the wind circling back on its own circuits, and some scholars read the passage as a complete description of the water cycle.

Which flood barrier running alongside a riverbank takes its name from a French verb meaning 'to raise'?

A levee was first recorded under that name in New Orleans, a few years after the city's founding in 1718. The failure of the city's flood banks during Hurricane Katrina in 2005 was catastrophic.