I knew an artist once who painted nothing but clouds. With their endless richness in texture and color, they provided a challenge and a vehicle for all the shapes he might hide in them. They are among the most constant features of nature, not only on Earth but also on other planets.
Clouds on Earth are made of water—either liquid drops, ice crystals, or both, depending on what type of cloud. The droplets are suspended in the air, sometimes condensing around particles of dust, and remain so until they become large enough by merging to become heavier than the force of the updrafts holding them up; then they fall as rain, snow, sleet, or hail. Then water returns to the atmosphere through evaporation from rivers, lakes, and seas to become new clouds, and the “water cycle” essential for land life on Earth continues.
The scientific study of clouds began with Aristotle (if not before), who wrote Meteorologica, the first known book on meteorology. Meteoros, in ancient Greek, meant “high in the sky” and Aristotle’s usage of the word included all atmospheric phenomena, such as clouds. Aristotle’s student Theophrastus wrote a book on how to forecast the weather by atmospheric signs, such as a halo around the moon. Their books remained the most authoritative sources in meteorology for about 2000 years.
Cloud classification by altitude of occurrence
Our modern classification of clouds was created mainly by scientist Luke Howard in 1803, with a few types added later by others. Howard named the three main cloud shapes: cirrus (wispy), stratus (layered sheets), and cumulus (fluffy). He also named some combinations of these, such as cumulostratus and nimbus—the complex combinations of cloud types typical of thunderstorms. Later meteorologists added further combinations and names for the different subtypes that occur in various layers of the atmosphere.
High cirrus upper-left merging into cirrostratus and some cirrocumulus upper right
Clouds have more functions than decorating the sky and making rain. They play a significant role in regulating climate and have been one of the most problematic unknowns in the effort to predict climate change. On the one hand, clouds cool down the Earth by reflecting the sun’s short-wave light back into space. On the other hand, they also heat up the Earth by absorbing light reflected off the surface and radiating it back down. The cooling effect of clouds seems to be dominating so far, according to NASA.
Cumulonimbus cloudscape over Borneo
Storm clouds are special. The reason they’re dark is not because they’re large. We often see large, bright white clouds because water is highly reflective. Storm clouds are dark because the water droplets in them are larger than the drops in white clouds, so they can absorb more light. The colors of clouds result from several different effects. Clouds reflect the colors of the light reaching them through the atmosphere, making them red and orange near dawn and sunset. But greenish and bluish clouds are showing that they contain more water than others, while yellow clouds reveal pollutants in the smoke.
Isolated cumulonimbus cloud over the Mojave Desert, releasing a heavy shower
This is not to say we’d be missing out on clouds if we travel to other worlds. In our solar system, Venus, Jupiter, Saturn, and Uranus also have clouds; in fact, it’s all we can see of them. But their clouds are not made mainly of water. The clouds of Venus are made of sulfur dioxide. The outer layers of clouds around Jupiter and Saturn are made of ammonia. Further inside are hydrogen sulfide and water clouds. Uranus, Neptune, and Saturn’s moon Titan are wrapped in clouds of methane. And some of the planets outside our solar system, such as Kepler 7-b and several others, appear to have clouds as well.
Although ethereal, clouds are an essential part of the natural balance supporting life on Earth, and they are a sign that other worlds have atmospheres and chemical cycles, making life more likely. And hey, it’s also fun to lie on your back and try to see pictures in them.
“Clouds come floating into my life, no longer to carry rain or usher storm, but to add color to my sunset sky.” —Rabindranath Tagorev
“In the presence of eternity, the mountains are as transient as the clouds.” —Robert Green Ingersoll
“The clouds travel like white handkerchiefs of goodbye. The wind, travelling, waving them in its hands.” ―Pablo Neruda
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