Cloud

• 1. Introduction

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  A cloud is an aerosol, a visible mass of tiny liquid droplets, ice crystals, or other particles suspended in the atmosphere. That clinical definition sits oddly beside what clouds have meant to people for thousands of years. The ancient Akkadians looked up and saw the breasts of the sky goddess Antu, and called rain her milk. The Book of Exodus describes Yahweh leading the Israelites through the desert as a pillar of cloud by day. In 423 BC, on the stage of the City Dionysia in Athens, a chorus of Clouds was hailed as the only true deities. So how did something so loaded with myth become a subject of precise scientific naming? Why do clouds carry Latin names like cirrus and cumulonimbus, and what does the difference between them tell us? And why, of all the variables in the atmosphere, are clouds called the single greatest uncertainty in how sensitive Earth's climate really is? The answers begin with water, cooling air, and a quiet observer in early 19th-century England.

• 2. Howard And Lamarck

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  Around 340 BC, the Greek philosopher Aristotle wrote Meteorologica, the first known work to treat a broad range of weather topics in a systematic way. He gave precipitation and the clouds it fell from the name meteors, from the Greek meteoros, meaning high in the sky. That single word seeded the modern term meteorology. But Aristotle worked by intuition and simple observation, not by what we would now call the scientific method, and centuries of speculation followed.

  Luke Howard changed that. A methodical observer in England with a strong grounding in Latin, he formally classified the various tropospheric cloud types during 1802. He believed that watching how cloud forms change in the sky could open the way to weather forecasting. His Latin names were published in 1803 and caught on quickly.

  Jean-Baptiste Lamarck, working independently in France the same year, lost the race. His scheme used unusually descriptive and informal French names, with twelve categories carrying labels that translate as hazy clouds, dappled clouds, and broom-like clouds. It failed to make an impression even at home. Howard's universally accepted Latin won out, and one measure of its popularity is striking. The German dramatist and poet Johann Wolfgang von Goethe composed four poems about clouds and dedicated them to Howard. An elaboration of Howard's system was formally adopted by the International Meteorological Conference in 1891, covering the tropospheric types he had mapped.

• 3. How Clouds Form

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  Saturation is where every cloud begins. Air becomes saturated when it is cooled to its dew point, the temperature at which condensation starts, or when enough moisture is added to raise the dew point up to the surrounding air temperature. From that single principle the whole field of cloud physics, the branch of meteorology also called nephology, unfolds.

  Adiabatic cooling drives most cloud formation, and it needs a lifting agent. Three exist. Convective lift comes from daytime solar heating at the surface, producing cumuliform clouds that can shower if the air is moist enough, and on rare occasions punch through the tropopause into the stratosphere. Frontal and cyclonic lift force stable air upward at weather fronts and around low-pressure centers by convergence. Orographic lift pushes air over a mountain, raising nothing more than lenticular cap clouds if the air is stable, but spawning showers or thunderstorms if it turns moist and unstable.

  Not every cloud needs lifting. Conductive, radiational, and evaporative cooling can chill air to its dew point right at the surface, and that is how fog forms. Water vapor can also be added directly, through evaporation from surface water or moist ground, through falling precipitation and virga, and through transpiration from plants. Condensation typically happens on cloud condensation nuclei, small particles of salt or dust light enough to stay aloft.

• 4. The Ten Genera

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  Five physical forms organize every cloud in the troposphere. There are stratiform sheets or veils, cumuliform heaps, stratocumuliform bands and ripples, cumulonimbiform towers often with fibrous tops, and cirriform wisps or patches. Cross these forms with altitude and you derive ten basic genera, the backbone of the World Meteorological Organization's Latin scheme.

  Altitude decides the prefix. Mid-level stratiform and stratocumuliform types take alto-, while their high-level cousins take cirro-. Low clouds carry no altitude prefix at all. There is a quirk in the stratocumuliform family. The prefix strato- attaches to the low-level genus but is dropped at mid and high levels to avoid double-prefixing with alto- and cirro-.

  High clouds form startlingly high, from 20000 to 60000 feet in the tropics. They include cirrus, fibrous white wisps of ice crystals that stand out against blue sky and never produce precipitation. Cirrostratus is a thin veil that gives rise to halos around the Sun and Moon. Cirrocumulus appears as small round masses rippled like sand on a beach. At mid level, altostratus is a gray veil that can drop light continuous rain, and altocumulus shows light-gray shading from its mix of droplets and ice. Down low, stratus resembles elevated fog and yields only drizzle or snow grains. Nimbostratus, the rain-maker ahead of a warm front, spans multiple levels. The towering types are the giants. Cumulus congestus, which the International Civil Aviation Organization calls towering cumulus, grows taller than it is wide with a cauliflower top. Cumulonimbus, the largest genus of all, rises like a mountain and can produce thunderstorms, flash-flooding downpours, hail, downbursts, and tornadoes. Lightning is the only event that requires a thunderstorm, because it is the lightning that creates the thunder. Below all of this, when a very low stratus cloud sinks to the ground, it sheds its Latin name. If surface visibility drops below 1 km it becomes fog, and if visibility holds at 1 km or more it is called mist.

• 5. Species And Strange Variants

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  Beneath the ten genera lies a finer hierarchy of species and varieties. Cirrus alone splits into fibratus filaments, uncinus with upturned hooks at the ends, and spissatus, opaque patches with light gray shading. A cumulus cloud climbs its own ladder of instability, starting as the small humilis cloudlet, growing into mediocris, then strongly convective congestus, then cumulonimbus calvus, and finally capillatus when supercooled droplets at the top freeze into a cirriform crown.

  Lenticularis clouds, lens-shaped and tapered at the ends, are most familiar as orographic mountain-wave clouds. Castellanus rises in turrets that resemble the battlements of a castle when viewed from the side. Volutus is a roll cloud, and one of its appearances is among the strangest in the sky. The Morning Glory rolls as a cylindrical cloud unpredictably over the Gulf of Carpentaria in Northern Australia, riding a powerful ripple in the atmosphere that glider pilots can literally surf.

  Varieties refine the picture further. Translucidus, perlucidus, and opacus describe opacity, while pattern names like radiatus, duplicatus, undulatus, and the honeycomb-holed lacunosus describe arrangement. Names can stack. A full technical label such as altocumulus stratiformis radiatus perlucidus identifies, in order, the genus, the species, and two combined varieties.

• 6. Supplementary Features

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  Incus is the most type-specific feature in the whole catalog, seen only with cumulonimbus of the species capillatus. It is the spreading anvil top, formed when rising air hits the stability layer at the tropopause and can climb no higher. Other features hang and bulge from clouds with their own Latin names. Mamma forms as downward bubble-like protuberances on a cloud base, once called mammatus before the World Meteorological Organization standardized the term in the 20th century. A tuba is a hanging column that can sharpen into a funnel cloud or tornado.

  Newer features keep entering the official vocabulary. Fluctus appears as regularly spaced crests under strong wind shear, known informally as a Kelvin-Helmholtz wave cloud and observed even in the Sun's atmosphere. Asperitas is a more chaotic wave-like form. Cavum is a circular fall-streak hole punched through thin supercooled altocumulus or cirrocumulus. Murus is a rotating wall cloud that can lead to tornadoes, and cauda is the tail cloud feeding into it.

  Some clouds are born from human and natural disruption. Flammagenitus clouds rise from large fires or volcanic eruptions, the latter reaching 57 km, with nuclear mushroom clouds extending up to 67 km. Cumulus homogenitus, Latin for man-made, covers industrial pyrocumulus. Cirrus homogenitus are aircraft contrails that spread into cirrus-like sheets. There is even stratus cataractagenitus, generated by the spray of waterfalls, and silvagenitus, a stratus cloud that forms over a forest canopy.

• 7. Above The Troposphere

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  Moisture is scarce above the troposphere, which is why the highest clouds are rare and confined to the coldest places. Polar stratospheric clouds sit in the lowest part of the stratosphere, restricted to polar regions in winter, in a single altitude band of roughly 15000 to 25000 m. Because they form as one undifferentiated type, they carry no Latin genera or species, only descriptive English names. The supercooled nitric acid and water variety, called type 1, has been identified as a cause of ozone depletion. The frozen nacreous types, type 2, are mother-of-pearl colored and undulating.

  Noctilucent clouds are the highest of all, near the top of the mesosphere at about 80 to 85 km, roughly ten times the altitude of tropospheric high clouds. Their Latin-derived name comes from the way they glow well after sunset and before sunrise, usually bluish or silvery white, occasionally red or orange. Convective lift in the polar summer mesosphere cools small amounts of water vapor to saturation, producing the coldest temperatures in the entire atmosphere just below the mesopause. The condensation nuclei may come from an unexpected source. Evidence points to smoke particles from burnt-up meteors. An increasing frequency of noctilucent clouds since the 19th century may itself be a sign of climate change.

  Clouds appear far beyond Earth too. Venus is wrapped in stratiform clouds of sulfur dioxide between 45 and 65 km. Jupiter and Saturn layer ammonia, ammonium hydrosulfide, and water clouds, with embedded cumulonimbus near Jupiter's Great Red Spot. Uranus and Neptune show the same structures built from methane, and Saturn's moon Titan carries cirrus believed to be largely methane. In 2013, astronomers announced high-altitude optically thick clouds on the exoplanet Kepler-7b.

• 8. Clouds And Climate

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  Reflectance of 70 to 95 percent across the visible spectrum is what makes deep clouds so bright. Water particles are packed so densely that sunlight bounces back out before it can penetrate, giving cloud tops their white color. As droplets grow heavier and fall as rain, the spaces between them widen, light sinks deeper, more of it is absorbed, and the cloud darkens toward gray and black. A simple proof is everyday experience. You can see farther in heavy rain than in heavy fog.

  Color tells stories of its own. Red, orange, and pink clouds appear almost entirely at sunrise and sunset, scattered by the atmosphere when the Sun sits below the horizon. A cumulonimbus with a greenish or bluish tint signals extremely high water, hail, or rain, a hint of a storm's severe potential though not proof of it. Yellowish clouds in late spring through early fall carry the color of nitrogen dioxide and forest-fire smoke.

  The deepest puzzle is climate. Clouds pull in two directions at once. White tops reflect shortwave radiation and raise Earth's albedo, cooling the surface. But cloud water also absorbs the infrared the warm ground emits and radiates some of it back down, warming the surface like a greenhouse. High cirrus tends to favor net warming, while mid-level and low clouds favor cooling, and NASA measurements indicate the cooling of low and mid clouds outweighs the warming of high ones. In a warmer future, more evaporation should mean more cloud, yet higher temperatures should also evaporate cloud away. Both effects are real, both are called cloud feedbacks, and the disagreement between climate models about which wins is the chief reason their climate sensitivities differ. That is why a thing children stare at for shapes remains the leading source of uncertainty in global warming projections.