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Blue Haze and Red Sunsets
Nature Bulletin No. 727   October 12, 1963
Forest Preserve District of Cook County
Seymour Simon, President
David H. Thompson, Senior Naturalist

BLUE HAZE AND RED SUNSETS
October is the month with the year's calmest and most pleasant weather. It is the only time when temperatures are almost the same throughout the United States. In this region the nights may be chilly but the days are warm. About the middle of the month we get our first heavy frost followed, perhaps, by a taste of winter. Then, in late October or early November, comes a period of a few days or a week of mellow sunshiny weather called Indian summer. Supposedly it was given this name by the early settlers who knew that the blue haze at this season was smoke from prairie fires set by the Indians on their autumn hunts.

This bluish haze that dims our view of distant trees and hills is a light effect caused by smoke and dust in the atmosphere. So are the glowing colors of a sunset or sunrise.

Sunlight, or "white" light, is a mixture of light of all colors of the rainbow. When it passes through a smoky or dusty atmosphere the different colors of light are broken up or scattered -- some more, some less -- depending on the size and number of smoke and dust particles in the air. When the particles are extremely small the blue rays are scattered most and produce haze because the wave length of blue light is shorter than, for instance, yellow or red rays.

Tobacco smoke furnishes a good example. It scatters light and appears blue even though the smoke itself consists of small droplets of a yellow liquid. This can be proven by blowing tobacco smoke through a white handkerchief.

Frequently, when there is little air movement, dust particles concentrate in a layer near the earth, thus reducing visibility and giving the air a slightly veiled appearance. Against a dark background, this haze appears bluish, like smoke. Against a bright background, it gives a coppery tint to the atmosphere because the blue rays as well as some of the green and yellow rays are filtered out, allowing only the red and orange rays to get through. This also explains the predominantly red color effects on the clouds at sunrise and sunset. When the sun is near the horizon, its light must pass a longer distance through the atmosphere and, hence through more smoke and dust.

Blue hazes over the landscape can also be seen far from the smoke of cities and after rains have washed any dust from the atmosphere. These occur on sunny windless days in summer over croplands, forests and tropical jungles. Supposedly, in bright sunlight, vapors given off by plants such as the aroma of pine needles and the fragrance of a meadow are condensed into particles that scatter blue rays.

While tobacco smoke consists of droplets small enough to scatter light and give a blue color, the water droplets in fog or cloud range in diameter from forty to four hundred times as great -- too large to break up light and produce color effects. Thus, while the sun seen through a mixture of smoke and fog over a city looks red, it looks white through a cloud free from smoke.

The source of dust in the atmosphere is usually exposed soil dry enough and light enough to be raised and easily carried by the wind. An extreme example of a dust storm occurred in May 1934 when, following a drought, a huge cloud moved eastward from the "dust bowl" of the Western Plains. For days it hung as a yellow haze across the country and out over the Atlantic.

The greatest explosion in the world within historic times was the eruption in 1883 of Krakatoa, a volcano near Java in the East Indies. A vast quantity of fine dust was shot into the upper atmosphere where it spread around the world. For the next two or three years this dust caused a reddish brown corona around the sun and produced sunsets with spectacularly gorgeous colors.


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