Answer · confidence: partial

Why is the sky gray on cloudy days, and do water drops matter?

Research question: Why does the sky appear white or gray during overcast conditions rather than blue, and what role do water droplet sizes play in this shift?

The records show that cloud droplet size and number concentration control how much solar radiation a cloud reflects ³. A model cloud had a wide spread of droplet radii, peaking near 4 microns ², and in the visible range its water droplets scatter essentially all extinction with no absorption, with a single-scattering albedo of 1.0 at 0.45 μm ⁵. Mie's 1908 work on how waves scatter off spheres is the basis for the term Mie scattering ⁴. The facts do not explain why this scattering looks white or gray rather than blue, so the question is only partly answered. The newest fact is from 2021, about five years before October 2026 ³⁶.

Checked 2026-10-14. Only documented and supported facts are used.

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Sources

[1] documented

The paper reports that the angular intensity and polarization patterns of water clouds and hazes depend strongly on the droplet size distribution and size range.

"These show a strong dependence of angular intensity and polarization patterns on the size distribution, the size range, and the dielectric and absorbing properties of water droplets at each wavelength."

Scattering and Polarization Properties of Water Clouds and Hazes in the Visible and Infrared (1964)

[2] supported

The model cloud used in the paper has a wide spread of droplet radii with the highest concentration at a radius of about 4 microns.

"a model cloud of spherical water droplets, with a wide distribution in droplet radius and a maximum concentration at a 4- μ radius."

Scattering and Polarization Properties of Water Clouds and Hazes in the Visible and Infrared (1964)

[3] supported

The size and number concentration of cloud droplets control the reflectance of the cloud, which in turn affects how much solar radiation is reflected.

"the reflectance of the cloud, the formation of precipitation"

cloud droplet size spectra (2021)

[4] supported

Gustav Mie's 1908 work computed how an electromagnetic wave scatters off a homogeneous dielectric sphere, the basis for the term Mie scattering.

"Mie worked on the computation of scattering of an electromagnetic wave by a homogeneous dielectric sphere"

Gustav Mie

[5] supported

In the visible range, the computed water cloud droplets have a single-scattering albedo of 1.0 at 0.45 μm, meaning essentially all extinction is scattering with no absorption.

"0.45 μ 1.34 0 0.1206 1.0 0.1056 1.0 16.33 1.0"

Scattering and Polarization Properties of Water Clouds and Hazes in the Visible and Infrared (1964)

[6] supported

Droplet size distribution is recognized as important for Earth's energy balance through the cloud albedo effect.

"through the so-called cloud albedo effect (Twomey, 1977)"

cloud droplet size spectra (2021)

[7] documented

The paper's model reproduces features of natural fog such as an extremely bright, narrow aureole, cloudbows, and glories, all arising from scattering by water droplets.

"Prominent observational features characteristic of natural fog, such as an extremely bright and narrow aureole, cloudbows, and glories, are reproduced in a model cloud of spherical water droplets"

Scattering and Polarization Properties of Water Clouds and Hazes in the Visible and Infrared (1964)