Rain — and by extension the Earth’s water cycle — is an essential planetary process that makes all life possible. Rain supports crops, fills lakes, and tops off reservoirs. And because salt remains behind during evaporation, rain is also a major source of fresh water. Here are six fascinating facts about rain to provide some new perspective when the next rainy day comes your way.
Water itself is odorless, of course, but rain, particularly after a dry spell, produces a pleasant, earthy scent, known as “petrichor.” The word is a combination of two Greek words — petros, meaning “stone,” and ichor, referring to the mythological fluid that fills the veins of the Greek gods. This name is actually an apt description for where the smell originates, because when rain hits porous soil or rock, microorganisms called actinobacteria release an organic compound called geosmin into the air, which contributes to the odor we associate with petrichor. Humans are better at sniffing out this compound than sharks are at smelling blood in water, and some scientists theorize that this particular nasal sensitivity helped our hunter-gatherer ancestors find water sources.
What we perceive as rain is actually water vapor at the end of a long journey. Precipitation forms when water vapor condenses into water droplets along the surface of certain aerosols that serve as “condensation nuclei.” As these droplets begin their journey toward Earth, they often freeze to form ice crystals at high altitudes, falling as snow. It’s only when the snow meets warmer air at lower elevations that the precipitation becomes hail or rain. (Freezing rain occurs when snow meets a pocket of warm air, melts, and then encounters freezing temperatures near the surface. Because the precipitation doesn’t have time to reform as snow, the rain instead freezes on contact with the ground, creating one of the most dangerous types of wintry conditions.)
The largest desert in the world isn’t the Sahara, the Arabian, or the Gobi. In fact, those three deserts combined don't make up the entire surface area of the Antarctic polar desert. (Although many of us associate deserts with sand dunes and cacti, they’re actually categorized as such based on their arid climates.) Antarctica as a whole receives very little precipitation, but the driest place by far is an area called the McMurdo Dry Valleys. Thanks to a phenomenon known as katabatic wind, which occurs when gravity pulls cold, dense mountain air downhill, this extremely parched region likely hasn’t seen any rain for an estimated 2 million years. In part because of this, though, it serves as a good analog for the Martian surface, a planet-wide desert in its own right that hasn’t seen precipitation in billions of years.
Mars might be dry as a bone, but Earth’s other planetary neighbor is another story. Thanks to large amounts of sulfur dioxide in Venus’ atmosphere, the planet experiences precipitation in the form of extremely corrosive sulfuric acid. Acids and bases are measured by the pH scale, with “0” being a strong acid and “14” being a strong base. Earth’s rain, for example, typically has a slightly acidic pH of around 5.6, but during powerful volcanic eruptions, as more sulfur dioxide is injected into the atmosphere, the resulting acid rain can have a pH as low as 2.5 (similar to vinegar). The acid rain on Venus, meanwhile, is estimated to have a pH of 1 or even lower, which is extremely hostile to any sort of life. Of course, this rain never actually reaches the planet’s surface, which is a roiling 900 degrees Fahrenheit. (Sulfuric acid evaporates at around 572 degrees Fahrenheit.) In any case, you should probably scratch Venus off of your solar system bucket list.
When kids draw raindrops, they’re often big, blue, and tear-shaped. In reality, however, a raindrop doesn’t look anything like a tear. While hovering in clouds, water droplets take on a spherical shape. As a droplet increases in size, it eventually falls to Earth, colliding with other droplets along the way. The bottom of the water droplet faces wind resistance as air also rushes past its sides, forming a jelly bean shape (though NASA describes it as a hamburger bun). Then, when the raindrop grows to about 4 millimeters in diameter, the pressure from the wind resistance flattens the droplet even further into a thin, umbrella shape before it eventually splits it into smaller spherical droplets. This may seem like a nitpicky fact, but knowing the exact shape of raindrops helps radar instruments on orbiting satellites monitor precipitation levels more accurately.
One inch of rain may not sound like a lot — especially when you consider that some places get more than 460 inches of rain per year — but all that water adds up. According to the United States Geological Survey (USGS), 1 inch of rain over 1 acre of land is equal to 27,154 gallons and weighs around 113 tons. To put that into perspective, 1 square mile contains 640 acres, which means 1 inch of rain in 1 square mile is more than 17 million gallons of water. If you continue to scale up, 1 inch of rain over the entire United States is equal to more than 61 trillion gallons.
The stats are even more mind-boggling when you consider that the contiguous U.S., for example, experiences enough rain in one year to cover the entire Lower 48 in 30 inches of water — which works out to 1,430 cubic miles of water weighing approximately 6.6 billion tons. By some accounts, an average of 1 billion tons of rain falls on the Earth every minute.
Of course, this could shift with climate change. A warmer planet means more water will evaporate in the atmosphere, and that extra moisture could lead to more frequent “heavy precipitation,” which causes soil erosion and increases flood risk. Heavy precipitation doesn’t necessarily mean areas will see an increase of average rainfall; rather, it refers to the nature and intensity of dramatic, storm-filled events. Like so many of Earth’s natural processes, rain will not escape the reality of our warming world.