When rain and snow fall from the sky in colors of the rainbow rather than clear or white, is it anything to be worried about? Centuries ago, our ancestors believed “blood rain” and other strange weather events were bad omens. Today, meteorology explains they have less to do with catastrophe and more to do with a common nuisance visitor: dust.
The Dirt on Dust
No, dust doesn’t just exist within the lonely corners of your home and under your bed. Dust lives outdoors, too. Sand, soil, salt from sea spray, pollen, algae, soot, and ash—are all sources of tiny outdoor specks of dust called “aerosols.” In fact, this is where most household dust comes from.
But unlike household dust, you likely won’t take much notice of aerosols when outdoors. Not only are they less than a millimeter in size, but they often travel high above our heads in the upper atmosphere.
So How Does This Dust Color Rain or Snow?
Winds from low-pressure systems and weather fronts carry these aerosols over great distances—from one city, state, or country to the next—unbeknownst to us. Along the way, these dust clouds meet and mingle with actual clouds, and as raindrops or snow crystals fall through aerosol plumes, they attract the dust-like particles on their way to the ground. It’s the reason why the air feels cleaner after a rainstorm. If the aerosols that dissolve in rainwater and snow moisture are highly pigmented, they’ll actually stain raindrops and snowflakes in midair.
What colors you’ll get in the rain or snow all depends on which minerals present within the aerosols. If the dust is rich in iron oxides, you’ll see a Mars-like landscape of reds and oranges and get “red rain” or “blood rain.” Sulfate-heavy aerosols will yield a sunny yellow hue. And if the air happens to be soot-laden, ink-colored rains are a possibility.
The thicker the dust plume or shorter the rain or snow shower, the more vibrantly colored the precipitation will be. And if dust concentrations are light, you might see pinks, butter yellows, and other pastel-colored precipitation coating your house, car, and outdoor furniture.
Is Red Rain Dangerous To Plants?
Seeing outdoor plants and trees covered with Crayola-colored rain and snow can be a distressing sight, especially if you’re a gardener. After all, the rainwater or snowmelt will eventually soak into your plants’ leaves and roots.
The good news is, colored rain and snow isn’t necessarily toxic to vegetation. Many of the minerals which color precipitation, including iron oxide and sulfates, are considered micronutrients—elements essential to the growth of healthy flowers and crops.
And if you’re worried about your plants getting too much of a good thing, don’t be. Blood rains occur so rarely (typically only a few times a year in Europe, and less frequently than that in the U.S. and Canada) that the micronutrient boost they’ll get isn’t typically cause for concern.