Why Is the Sky a Deeper Blue Today? — The Science Behind Sky Color

Ask a Meteorologist: Why does the sky look deeper blue in some places?

This week a reader asked why parts of the sky sometimes look a much deeper blue than others, and why the shade can change from one day to the next. Meteorologist Jonathan Belles explains that the answer is optics: everything between you and the sun determines the color you see.

How sunlight becomes blue

Sunlight is made of many colors. When sunlight passes through Earths atmosphere, tiny gas molecules like nitrogen and oxygen scatter shorter wavelengths (blue and violet) more efficiently than longer wavelengths. This selective scattering is called Rayleigh scattering, and it is why the daytime sky appears blue.

Why not purple? Violet light is scattered even more than blue, but human eyes are much more sensitive to blue. Also, some violet is absorbed by ozone, so blue dominates our perception.

Why the blue looks deeper or paler

Several factors change how intense or washed-out the blue appears:

• Particle size and type: Larger particles such as dust, smoke, ash, droplets and pollution scatter light differently (Mie scattering). These particles tend to scatter all wavelengths more evenly, which mutes or whitens the deep blue.
• Humidity and haze: Moisture and tiny droplets increase scattering and make the sky look paler or more milky.
• Aerosols and pollution: Wildfire smoke, volcanic ash or urban pollution often reduce the deep blue and can produce striking red and orange sunsets.
• Sun angle and path length: At sunrise and sunset sunlight travels through a much longer column of air, which removes short wavelengths and leaves reds and oranges.

Temperature, seasonal changes and altitude

Warm conditions often increase evaporation, convection and mixing that lift moisture and aerosols into the air column; combined with daytime haze, this tends to wash out the blue and produce paler skies. Cold, dry air contains fewer scatterers, so the sky appears crisper and a deeper, darker blue — especially in winter or at high latitudes.

At high elevations, you are above much of the atmosphere, so there is less scattering between you and space; the sky looks noticeably darker and can take on a purplish-blue cast in very thin, cold air.

Night-sky viewing

The same principles apply to stargazing: clearer, colder, drier air usually provides the best visibility for stars because there are fewer particles to scatter or absorb light.

Bottom line

The sky's changing shades are driven by scattering physics (Rayleigh and Mie scattering) plus variations in particle size, humidity, temperature and elevation. Those factors determine how sunlight is filtered before it reaches your eyes, producing everything from pale, washed-out blues to deep, vivid or even slightly purplish skies.

Have a question for the meteorologists at Weather.com? Write to us at morning.brief@weather.com and we may answer it in a future column.

Credit: Meteorologist Jonathan Belles