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Behind the Forecast: Why the sky isn’t always the same shade of blue

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Published: Oct. 8, 2021 at 11:22 AM EDT
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LOUISVILLE, Ky. (WAVE) - We see our skies as blue unless, of course, if clouds cover them. During the Fall and Winter, skies appear more vibrantly blue than during the summer months.

In the summer, skies often look hazy with a white, milky sheen. Haze can be caused by emissions from vegetation, but most often, it’s from air pollution. The tiny droplets that make up the pollution scatter light differently than air molecules. That’s why the sky appears more whitish than blue. The pollution also decreases the amount of direct sunlight that reaches the planet’s surface. The summer humidity also plays a role in the milkiness of summer skies. The water vapor droplets in the atmosphere work a lot like pollutant particles, scattering light and making the sky appear more white than blue. This works mainly when the particles are close to but no bigger than the wavelength of light.

Fall and Winter cold front are experts in clearing the skies allowing us to truly enjoy its vibrant blue color. Cold fronts usher in cooler air which is typically less humid and contains less water vapor. Behind cold fronts, the air is usually clear of clouds since there isn’t enough moisture to sustain them. Since the Northern Hemisphere is tilted away from the sun in the Fall months (this tilt is why we have seasons), the sun angle is lower. A lower sun angle decreases the green and red color wavelengths that reach our eyes, allowing us to see a more intense blue.

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