Colored Rains
It’s time for the weird question: what color is rain? Would you say rain has no color because rainwater is always clear and transparent? But we’ll answer that rain can be colored. Moreover, it can come in many colors, like the rainbow. And we will also explain why.
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To Color the Sky
You’ve probably already guessed how rain can get different colors. For that to happen, the atmosphere must contain some colored particles, and impurities, which will color the water. These are «aerosols» — tiny particles that are light enough to float in the air for a while (i.e., a ball you throw to a friend will not be an aerosol). Aerosols enter the atmosphere in many different ways: dust and fine grains of sand lifted by the wind off the ground, soot from a campfire, industrial emissions, and plant pollen, among others.
Sometimes there are so many colored aerosols in the air that the entire sky takes on a hue. For example, sand from the Sahara, which the wind can carry to Europe or the Atlantic Ocean, can turn even a cloudless sky orange. But usually, the aerosols cannot be seen by the naked eye after all.
Armands Brants / Unsplash
When clouds form in the air and rain forms, the droplets capture the aerosols and fall with them to the ground. The color of rainwater as it is raining is hard to distinguish, but in puddles and traces of dried droplets, it becomes more evident. And now for the most frequent types of colored rain.
Bloody Rain
It sounds like the plot of a scary legend... Bloody rains have been known since antiquity and are mentioned even in Homer’s Iliad; they have always been considered a bad omen. But in reality, everything is quite friendly. The main purveyors of red rain are considered to be deserts. Sand in deserts can contain a lot of iron oxides, i.e., rust. Therefore, it is often red or orange. During sandstorms, grains of sand can rise to several kilometers and fly even to other continents. For example, most of the bloody rains in Europe come from the Sahara.
But it´s not just sand. Scientists recently determined that the bloody rains which periodically fell in the Indian state of Kerala and Sri Lanka get their red color not from sand but from algae of genus Trentepolia living on the rocks and bark of trees in the lichen (algae + fungi).
The famous bloody rain of 2001 in Kerala may have been caused by the wet weather in the region in the weeks before the rain. Due to the abundance of moisture, the lichens grew strongly and released many spores into the atmosphere, which colored the rain.
David Kristianto / Unsplash
In 2011, the same spores were found in the rain, but it was determined that this time they had been brought to India from Europe.
Milky Rain
Milky rain sounds much nicer than bloody rain, doesn´t it? You can scoop a glass from the milk river and drink it with your favorite chocolate chip cookie...
But milky rains are most common in areas where chalk deposits come to the surface. This is the same white chalk children use to draw on the asphalt and teachers write on blackboards at school. It was most actively formed at the end of the dinosaur era, 100 to 60 million years ago, in warm, shallow seas. Chalk consists of the skeletons of tiny organisms that lived and died in that water for millions of years. The same material, calcium carbonate, makes up snail shells, seashells, and egg shells.
Chalk deposits are often buried under other soil layers, but there are areas where chalk comes out, and the wind can pick it up. For example, the Negev Desert in Israel, the bottom of the ancient Tethys Ocean, is chalk.
Negev Desert, Israel. John Adeoye / Unsplash
Yellow rain
Yellow rain is not what you think it is. It is made yellow (or green) by pollen from plants: trees and grasses. They bloom yearly, so yellow rain is a common sight in many parts of the world, especially in temperate and subtropical latitudes.
Look out for puddles in the spring, and you’re bound to see yellow or green streaks on the water someday. Sometimes they have a very intense, unnatural hue.
There’s also black rain, and it comes from contamination with volcanic dust in areas of volcanic eruptions or areas with dark-colored soil during dust storms.
A volcanic eruption in Indonesia. Yosh Ginsu / Unsplash
Unfortunately, not all colored rains are natural. There have been cases where chemical emissions have resulted in acid green and other colored rains. People who have spent a long time under such rain can feel sick. So it is advisable to follow the news and be aware if there is a man-made accident in your area. But now, if you see green puddles on the street, you will be prepared.
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Jody A. Khomaro / Unsplash
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