Once in a Red Sun or Blue Moon

Weather Talk

POSTED: Tuesday, February 4, 2014 - 1:27pm

UPDATED: Wednesday, February 5, 2014 - 10:14am

What in our Atmosphere Creates Our Spectacular Borderland Sunrises and Sunsets?

Here in the Borderland we have some of the most amazingly beautiful sunrises and sunsets! This is because of all the light scatters we have floating around in our atmosphere. Why is it that the midday sun seems intensely white, while at sunrise or sunset it appears to be yellow, orange or red? In today’s “Weather Talk” I will take a look at what the atmosphere does to scatter visible light, what are the many tiny particles that scatter light in our atmosphere and what causes some of the unique colors.

At noon when the sun is high in the sky, the light from the sun is the most intense. All the wavelengths of visible light are able to reach the eye with about the same intensity and the sun appears white. Now remember looking directly at the sun, especially at midday, can cause irreparable eye damage to the eye. Normally we get only glimpses of the sun out of the corner of our eye.

Near sunrise or sunset, the sun’s rays strike the atmosphere at a low angle. They must pass through much more atmosphere than at any other time during the day. The math works out like this. When the sun is 4º above the horizon, the sunlight must pass through an atmosphere more than 12 times thicker than when the sun is directly overhead. By the time sunlight has penetrated this large amount of air, most of the shorter waves of visible light have been scattered away by air molecules. Just about the only waves from the setting sun that make it though the atmosphere on a fairly direct path are the yellow, orange and red. Upon reaching the eye these waves produce a bright yellow, orange sunset.

Bright, yellow-orange sunsets only occur when the atmosphere is relatively clean, such as after a recent rain. If the atmosphere contains many tiny particles whose diameters are a bit larger that air molecules, the slightly larger waves of yellow light are scattered away. Only the orange and red waves will penetrate through to the eye and the sun would appear red-orange. When the atmosphere is loaded with particles, only the largest red wavelengths are able to make it through the atmosphere and we see a red sun.

Natural events may produce red sunrises or sunsets. Over the ocean for example, the scattering characteristics of small, suspended sea salt particles and water vapor are responsible for the brilliant red suns seen from the beach.

Volcano eruptions blast massive amounts of tiny particles into the atmosphere. These tiny particles are moved along by the upper level winds circling the globe, producing amazing sunrises and sunsets for months or even years.

Sometimes our atmosphere becomes so thick with dust, smoke and pollution that even the red waves are unable to make it though the dirty air. A weird, effect occurs because no visible waves reach your eye; the sun literally disappears before it reaches the horizon. This happens when there are large forest fires or when the Borderland experiences a strong dust storm towards sunset. Hopefully, we shall never see this effect caused by pollution, but there are reports of this happening in some large cities in China. Check out my "Weather Talk entitled:  http://www.ktsm.com/weather/weather-talk/our-smoky-brown-haze-enhanced-w...

The scattering of light by large quantities of atmospheric particles can cause some rather unusual sights. If the volcanic ash, dust or smoke particles are roughly uniform in size, they can selectively scatter the sun’s rays. We have plenty of dust storms here in the Borderland. Read my “Weather Talk”; Are You Ready for Another One Bites the Dust? http://www.ktsm.com/weather/weather-talk/are-you-ready-%E2%80%9Canother-one-bites-dust%E2%80%9D
Even at midday, a variety of colored suns have been seen from orange, green suns to suns that are even blue in appearance! Although rare, the same phenomenon can happen to moonlight, making the moon appear blue! That is where the expression “once in a blue moon” came from. The moon can also look red or orange depending on what type of scatter particles are floating around.

Here in the Borderland we have plenty of dust, unfortunately a good amount of pollution and even sea salt carried into the upper level flow from the oceans. We are mainly dry all year, but yes sometimes we even have water vapor and droplets to help scatter the light of the setting sun too! Of course the most famous effect of water vapor and droplet light scatter is a rainbow! All these tiny particles, debris and droplets work to create beautiful desert sunrises and sunsets! We get the traditional brilliant yellows, oranges and reds, but we, on occasion, will see bight magentas and purples mixed in! Amazing! I always force myself to stop, appreciate and most of the times snap a photo of our spectacular Borderland sunsets! Please send me your pictures to cdebroder@ktsm.com and I will use them on the air or on my Face Book pages.

Chuck DeBroder, Chief Meteorologist
KTSM, NewsChannel 9, NBC, El Paso, TX
www.twitter.com/ Chuck DeBroder NC9 @wxchuckNC9

Comments News Comments

Good article Chuck! :D

Good article Chuck! Thanks for sharing! :D

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