S P E C I A L   F E A T U R E

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Of Blue Skies and Brilliant Sunsets

by Sue Hakala,
Master Gardener

When I first moved here from the Chicago area, I was amazed at the breathtaking blue of the Arizona sky. Even now, twenty-six years later, I still often sit in my yard with my chair tilted back, agog at the astonishing color display overhead.

Perhaps, like me, you've wondered why our sky is sooo blue. It's because somewhere in the earth's atmosphere there is a layer containing tiny floating water droplets called aerosols. These aerosols come in various sizes, and they scatter light coming to us from the sun to produce the color differences we see in our skies. They seem to scatter all the colors of the light spectrum about equally.

In climates more humid than the Sonoran Desert, such as in the Midwest and the East, the sky may look white, hazy, foggy, bright--but not blue. This is because there are more aerosols in the air in those parts of the country, and the light is more scattered. Here in the Valley of the Sun we have fewer aerosols in the atmosphere, so more of the color blue is allowed through to create, at least to our eyes, our fabulous blue sky.

Now, if we were looking at the sky through the eyes of, say, a honeybee or a hummingbird (both of whom see ultraviolet light which our eyes don't), the sky would appear to be a different color. And if our atmosphere were the same as Mars, I would be explaining why our daytime sky is such a beautiful shade of orange-red instead of blue.

Our state's spectacular sunsets--with the beautiful afterglow that we are fortunate enough to experience--are also related to aerosols in the atmosphere, as well as to the distance sunlight must travel to reach us at different times of the day and the amount of airborne dust in the sky.

As light zooms at us from the sun, it bounces off air and water molecules as it is entering our atmosphere and scatters in all directions. At noon when the sun is near to us, sunlight scatters out and diffuses the warm end of the color spectrum (red, orange, yellow), so that blue dominates. At sunset the light has to travel further, bouncing off a greater number of air molecules and scattering out the cool end of the spectrum (blue, green, purple), so that the warm colors dominate.

In the desert airborne dust attaches to the water droplets--those old aerosols again. This dust intensifies the dominant reds bouncing off the aerosols at dusk, creating our famous sunsets. So ironically, days of high pollution and airborne dust contribute greatly to our bright red sunsets.

During the calm of night dust settles out of the air, which is why the color of our sunrises aren't as spectacular as our sunsets. The best stargazing takes place after midnight, when skies contain the least amount of dust.

The next time you're enjoying a sunset, you may notice a blue band making its way across the sky. Relax. It's just the earth's shadow reflecting on our atmosphere, a comforting reminder that evening is on its way.


Maricopa County Master Gardener Volunteer Information
Last Updated May 28, 2003
Author: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
© 1997 The University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension in Maricopa County
Comments to Maricopa-hort@ag.arizona.edu 4341 E. Broadway Road, Phoenix, AZ 85040,
Voice: (602) 470-8086 ext. 301, Fax (602) 470-8092