A dust storm passed through Sydney, Australia, Wednesday morning. While a thick cake of orange dust covered everything that didn't move, residents described a soft-pink, deep-red, or even yellow sky. Why would a cloud of orange dust turn the sky pink, red, and yellow?
Because tiny particles can scatter sunlight. Light interacts differently with minuscule particles than with objects in the macroscopic world. If the sky were filled with dust particles that were each significantly wider than the largest wavelength of visible light—i.e., if each one were much more than 750 nanometers wide—then the atmosphere would appear to be approximately the same color as the particles themselves. (In this case, orange.) But many of the dust particles hanging over Sydney were probably less than 750 nanometers. Sunlight scatters when it hits such small particles; its various color components are redirected in a complicated pattern, and only limited wavelengths of light pass through to the observer. In such situations, which physicists still don't understand perfectly, the atmosphere can take on any number of colors, from blue to deep red, and can even look different depending on where the observer is standing.
The same principle applies to a cloud of tobacco smoke, which appears bluish-gray even though the loosely bound smoke particles that compose the haze are actually yellow. (Try blowing smoke into a handkerchief if you don't believe it—or check out the teeth of a lifelong chain-smoker.) Another classic example is the sky. Air molecules in their gaseous states are colorless, but the sky appears blue. That's because when sunlight passes through a clean atmosphere, it is scattered by the many molecules that constitute air, which direct the shorter blue wavelengths toward our eyes.
Many observers described Sydney's rosy haze as lasting until about noon. This is partly because the greatest dust concentration had passed by that time, but the positioning of the sun played a role as well. During the morning and evening, light comes in at a more acute angle and passes through more of the troposphere and stratosphere—the two lowest levels of the atmosphere, which contain the most molecules and dust particles—on its way to the ground. The more distance the light has to travel at these lower elevations, the more particles it will interact with, and the more significant the scattering effect. As the sun approaches high noon, the light comes straight down and spends far less time fighting its way through the atmospheric dust. This is also why sunsets appear red or orange, even though the daytime sky is blue. The composition of the atmosphere does not change significantly at night, but sunlight passes through more particles and becomes more scattered.
Bonus Explainer: Why is the dust orange in the first place? Because there's so little vegetation. Southeastern Australian soil is composed of weathered ferric rocks. The iron makes the resulting clay minerals—like nontronite, saponite, and volkonsokite—orange-ish. This process is certainly not unique to the land Down Under. Many regions started out orange but eventually transitioned to brown or black as vegetation sprang up in the fertile clay and composted into dark organic matter. The climate around Sydney is too arid for trees and shrubs to proliferate, so the area retains its original hue. The lack of vegetation also explains the frequent dust storms. Clay is flaky, and there aren't many trees or roots to prevent it from sweeping across the plains.
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Explainer thanks Stephen F. Corfidi of the National Oceanic and Atmospheric Administration's National Weather Service, Mark Green of the Desert Research Institute, and Enrique Merino * of Indiana University.