Q33. Where can shipboard observers obtain visibility
range information at sea?
Q34. When is sector visibility reported?
Q35. When is differing level visibility reported?
WEATHER AND OBSTRUCTIONS TO
VISION
LEARNING OBJECTIVES: Identify six types
of lithometeors. Compare the condensa-
tion/sublimation and precipitation forms of
hydrometeors. Explain wind-blown forms of
hydrometeors.
Describe two types of
electrometeors. Define and list four types of
photometeors.
The occurrence of weather and the presence of
obstructions to visibility are directly related to sky
condition and the visibility. Observing the type of
weather occurring and the presence of any obstructions
to visibility is usually the third task undertaken in an
observation. A pilot may use this information to
determine the impact of the conditions at a station on the
aircraft being flown. In this discussion, we use the term
weather to refer to any particles suspended in or
precipitating from the atmosphere, or to the process that
causes these particles to precipitate from the
atmosphere. Observable weather elements may be
broken down into four groups: lithometeors,
hydrometeors, electrometeors, and photometeors.
LITHOMETEORS
A lithometeor is any dry particle suspended in or
falling from the atmosphere. The particles are usually
formed on earth’s surface and then are carried aloft by
either wind or thermal currents. Haze, smoke, dust,
dust-devils, ash, and sand are all lithometeors.
Haze
Haze is composed of suspended dust, plant pollen,
or salt particles that are so small that they cannot be seen
by the unaided eye. It is opalescent, reducing visibility.
Haze typically produces a bluish tinge when viewed
against a dark background. It produces a dirty yellow or
orange tinge when viewed against a brighter
background because of the scattering of light. When
haze is present and the sun is well above the horizon, its
light may have a silvery tinge. Haze particles are
hygroscopic—they attract moisture. Because they
attract moisture, they are good condensation nuclei.
When conditions are favorable, haze may attract
sufficient moisture to thicken into fog as the sun sets and
the temperature drops.
Smoke
Smoke is composed of fine ash particles and other
by-products of combustion. When concentrated at its
source, smoke may appear white to bluish-black, or
yellow to brown, depending on its composition and the
amount of water vapor present. After it is dispersed in
the atmosphere, smoke is distinguished from haze by its
characteristic reddish tinge, especially near the horizon
at sunrise and sunset.
Dust
Dust is composed of fine solid matter uniformly
distributed in the air. It typically imparts a tan or gray
hue to distant objects. The sun’s disk may appear pale
and colorless, or may have a yellow tinge when viewed
through dust. Although dust and haze appear similar,
when the visibility is less than 7 miles, dust may be
differentiated from haze or fog by the low relative
humidity associated with dusty conditions. In certain
areas of the world, suspended dust may reduce visibility
to less than a mile. Normally, the lower visibility
associated with dust is limited to blowing dust—dust
picked up and carried by the wind. The term dust storm
usually refers to blowing dust reducing visibility to 5/16
to 5/8 of a mile, while the term heavy or severe dust
storm is reserved for use with blowing dust that restricts
visibility to less than 5/16 mile.
Dust/Sand Whirl
Dust/sand whirls or dust devils, are rotating
columns of dust or sand-laden air, caused by intense
solar radiation. They are best developed on calm, hot,
clear afternoons and in desert regions. Warm,
ascending air in a dust devil may carry leaves and other
small debris to a height of a few feet or a few hundred
feet.
Ash
The phenomenon called ash in a surface
meteorological observation usually refers to the heavier
volcanic ash particles falling from a volcanic cloud. It
may also be used to identify heavier solid particles
precipitating and falling from an industrial smoke
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