How airborne algae can color rain
Rain does not turn red because algae grow inside the clouds. In some cases, tiny algal cells or fragments are already suspended in the air. As raindrops fall, they collect these particles and carry them to the ground. When enough pigment-rich material lands together, the water can look pink, rust-colored, or crimson.
Some algae make red or orange pigments that help shield them from intense sunlight. Those pigments can tint a sample even when the organisms themselves are too small to see clearly without magnification. Red-colored rain can also have other causes, including airborne dust, so color alone does not identify what was in the water.
From strange weather to an omen
Before people could examine rainwater under a microscope, an unusual red downpour offered no obvious explanation. Its resemblance to blood made it easy to read as a warning of disaster or divine displeasure. The spectacle was real; the apocalyptic meaning came from how people interpreted it.
The memorable twist is that a sky that appears to be raining blood may be carrying ordinary microscopic life—or other airborne particles—down to earth. A drop’s color can be dramatic while its cause is small enough to miss.
