WSAV NOW Weather: The colors of sunrise – sunset & Saharan dust


SAVANNAH, Ga. (WSAV) — We all enjoy the colors of the sunrise and sunset that we see every day. Why are they so colorful? And… how does the Saharan dust plume heading our way create a more colorful sky?

First, let’s talk about why the sky has color. During the day, it appears blue to us. This is because the atmosphere, the gasses that surround the earth, scatter visible light. The atmosphere is comprised of many gasses including nitrogen, carbon dioxide, and oxygen.

Each and every gas molecule has the ability to filter out visible light that comes from the sun. The process in which this occurs is called Rayleigh scattering.

Rayleigh scattering is the scattering (spreading) of light by particles that are smaller than the wavelength of the visible light spectrum.

Credit: National Weather Service

During the day, the sky appears blue because the atmosphere scatters the shorter wavelengths of visible light easily with a direct sun angle. The more direct sun angle means that the sunlight is passing though less of the atmosphere.

When the sun is low on the horizon in the morning and evening, the sunshine is passing through more of the atmosphere. Since the gasses scatter the blue light first, the only thing left are the longer wavelengths on light. Long wavelength visible light come in the form of yellow, orange, and red. This is what creates the colors of the morning sunrise and evening sunset.  

Credit: National Weather Service

Recently, a Saharan dust cloud moved off the coast of Africa. It traveled across the Atlantic Ocean, into the Caribbean Sea and Gulf of Mexico. Now the dust cloud is working its way into the Coastal Empire and Lowcountry and will stay with us for several days.

A thin layer of dust particles has the ability to scatter more of the longer wavelengths of visible light. This means that at sunrise and sunset, more if the yellow, orange, and red light is visible to us. A thick layer can make a hazy and milky looking sky.

One benefit to a thick Saharan dust layer over the Atlantic Ocean is that it suppress tropical cyclone develop. Locally the dust will lead to a hazy sky, reduced visibility, and air quality issues. The reduced air quality could aggravate allergies, asthma, and other respiratory problems.

Current Saharan dust plume

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