and a dirty yellow or orange tinge when viewed against
a bright background. Differences in air temperature
may cause a shimmering veil over the landscape called
optical haze.
SMOKE
Smoke is fine ash particles suspended in the
atmosphere. When smoke is present, the disk of the Sun
at sunrise and sunset appears red, and during the
daytime has a reddish tinge. Smoke at a distance, such
as from forest fires, usually has a light grayish or bluish
color and is evenly distributed in the upper air.
DUST
Dust is finely divided solid matter uniformly
distributed in the air. It imparts a tan or grayish hue to
distant objects. The Suns disk is pale and colorless or
has a yellow tinge during the day. Blowing dust consists
of dust raised by the wind to moderate heights above
the ground and restricting horizontal visibility to less
than 7 miles. When visibility is reduced to less than
five-eighths of a mile but not less than five-sixteenths of
a mile, it is classified as a dust storm and, if less than
five-sixteenths of a mile, as a severe dust storm.
SAND
Fine particles of sand picked up from the surface by
the wind and blown about in clouds or sheets constitute
a troublesome lithometeor in some regions. Blowing
sand consists of sand raised by the wind to moderate
heights above the ground, which reduces horizontal
visibility to less than 7 miles. When the visibility is
reduced to less than five-eighths of a mile but not less
than five-sixteenths of a mile, it is classed as a
sandstorm and, if less than five-sixteenths of a mile, as a
severe sandstorm.
DUST DEVILS
Dust devils, or whirling, dust-laden air, are caused
by intense solar radiation, which sets up a steep lapse
rate near the ground. They are best developed on calm,
hot, clear afternoons and in desert regions. As the
intense surface heating sets up a steep lapse rate, a small
circulation is formed when the surrounding air rushes
in to fill the area of the rising warm air. This warm
ascending air carries dust, sand, leaves, and other small
material to a height of a few hundred feet.
REVIEW QUESTIONS
Q5-6. Name one way that smoke is distinguished
from haze.
Q5-7.
When and where are dust devils usually
observed?
PHOTOMETEORS
LEARNING OBJECTIVE: Identify the
characteristics of photometeors and describe
the characteristics of light, reflection, and
refraction.
Photometeors are any of a number of atmospheric
phenomena that appear as luminous patterns in the sky.
While they constitute a variety of fascinating optical
phenomena, photometeors are not active elements; that
is, they generally do not cause adverse weather.
However, many are related to clouds that do cause
adverse weather. Therefore, they help in describing the
state of the atmosphere.
LIGHT
Light, acting in conjunction with some of the
elements of the atmosphere, produces a variety of
atmospheric phenomena, such as halos, coronas,
mirages, rainbows, and crepuscular rays. This lesson
discusses the theories of light and the resulting
photometeors.
Light is the portion of the electromagnetic
spectrum that can be detected by the human eye. It
travels at the same speed as all other electromagnetic
radiation (186,000 miles per second). However, the
characteristics of light are considerably different from
other regions of the electromagnetic spectrum because
of the differences in wavelength and frequency.
Sources of Light
There are two sources of lightnatural and
artificial. Nearly all natural light is received from the
Sun. Artificial light is light such as that produced by
electric lamps, fires, or fluorescent tubes. Luminous
bodies are those bodies that which produce their own
light, such as the Sun and the stars. Illuminated or non
luminous bodies are those bodies which merely reflect
the light they receive and are therefore visible because
of this reflection. The Moon is an example of an
illuminated body.
5-13