Hobie Science: Why is the Sky Red?

Red sky night, sailors delight… There may be some truth to this, but why does the sky appear red at dawn and dusk?

From last week’s blog it is understood that sunlight is the visible part of the electromagnetic spectrum and the sky is blue because the high energy blue wavelengths are being scattered and reflected while passing through our atmosphere.

What is happening during sunrise and sunset is that sunlight is passing through a much longer portion of the Earth’s atmosphere.

As can be seen in the image above, lightwaves from the sun are interacting with the atmosphere in a much more tangential manner. This means that lightwaves are not directly hitting the Earth’s surface, rather, they are passing through the atmosphere overhead more parallel to the Earth’s surface. It is only then we can see the red lightwaves being scattered in the sky.

The Earth’s lower atmosphere is much more dense than the air in the upper atmosphere. It is in the lower, dense portion of the atmosphere where most of the aerosols are found. Our atmosphere is mostly made-up of nitrogen and oxygen molecules. There are many other gases, e.g. argon, carbon dioxide, helium, neon, and methane, that vary in quantity depending on location. Many industrial cities have large quantities of manmade aerosols that hover in the lower atmosphere, often gaseous pollutants emitted from all sorts of factories. These areas often have brilliant, deep-red sunrises and sunsets as the lightwaves interact with particulate matter in the air. For us living in Southern California we are familiar with the amazing sunsets during fire season as forest fires pollute the sky with huge volumes of carbon (soot). Coastal cities are also well known for having beautiful sunsets, not due to pollution but because of the amount of salt that is in the lower atmosphere.

So the next time you catch an awe-inspiring sunset, enjoy! And revel in the fact that you now know why you’re seeing red.

Gary Larson


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