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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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Update: June 2012
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