Why We Can't See Blue in the Sky on a Cloudy Day: Understanding Rayleigh Scattering

Have you ever noticed that the sky appears to lose its blue hue on a cloudy day? This intriguing phenomenon can be explained by the principles of light scattering, particularly Rayleigh scattering. In this article, we'll delve into how the sky appears blue on clear days and why clouds modify this appearance.

Rayleigh Scattering and Clear Skies

When the atmosphere is clear, the Earth's atmosphere performs a fascinating optical trick called Rayleigh scattering. Sunlight, which is a mixture of different wavelengths (or colors), interacts with the molecules in the air. Blue and violet light, with their shorter wavelengths, are scattered more than the longer wavelengths like red and yellow. This selective scattering is why the sky appears blue to us.

Clouds and Their Impact on Skies

When clouds enter the scene, something changes. Clouds are composed of water droplets or ice crystals, which are significantly larger than the atmospheric molecules responsible for Rayleigh scattering. These larger particles scatter all wavelengths of light more evenly.

This even scattering results in a diffuse, uniform gray appearance. Clouds essentially block the direct sunlight that contributes to the blue color of the sky. Instead, they scatter and reflect light in a way that makes the sky look gray or overcast. Consequently, we can't see the blue hues as clearly as on a clear day.

Visibility of Blue in Clouds

While it might seem that we can't see any blue at all on a cloudy day, the truth is more nuanced. Depending on the thickness of the cloud cover, you can still observe blue to some extent.

The underside of clouds often appears dark and reveals various shades of blue gray. Even in heavy storms, you might notice the clouds taking on a dark purple hue before they transform into pitch-black storm clouds. This is due to the extent to which light is scattered and absorbed by the cloud particles.

Cloud Composition and Scattering

Interestingly, clouds scatter light in a way that makes them appear white, not because they're made of air, oxygen, and nitrogen, but because of the size of their droplets or ice crystals.

Cloud particles are much larger than atmospheric gas molecules. As a result, they scatter light almost equally across the visible spectrum, giving clouds their characteristic white appearance. Moreover, cloud particles scatter light more strongly than gas molecules, contributing to the white coloration of clouds.

In the case of thunderclouds, known as stratocumulus, their great height can make them almost opaque. This is because the cloud droplets are so densely packed that light is scattered or absorbed, preventing it from passing through easily. This scattering effect is what makes thunderclouds appear dark and contributes to the phenomenon of the blue sky not shining through the clouds on a cloudy day.

During daytime, skylight is also considerably less intense compared to direct sunlight. This added factor further reduces the likelihood of seeing the blue coloration of the sky on a cloudy day.