plume, the smoke from a forest fire, or the debris falling
from a nuclear mushroom cloud.
Sand particles may be picked up from dry surfaces
by the wind at wind speeds as low as 21 knots and
carried to moderate heights. Stronger winds may carry
sand to extreme heights. The term sand storm refers to
blowing sand that reduces visibility from 5/16 to 5/8
mile, while the term heavy or severe sand storm means
that visibility is less than 5/16 mile.
The only hazard to aviation caused by haze and
smoke is reduced visibility. But dust, ash, and sand can
also clog engine intakes and be very abrasive to moving
components. Aircraft flying through these conditions
may experience fatal engine failure.
Hydrometeors are liquid or solid water particles
falling through, suspended in, or condensing/subliming
from the atmosphere, as well as solid or liquid water
blown from the surface by wind. The term refers to all
forms of condensation, such as clouds, fog, dew, and
frost; all forms of precipitation, such as rain, drizzle,
snow and hail; and all forms of moisture blown about by
Many of the weather elements identified as
hydrometeors are formed by the condensation or
sublimation of water vapor in the air or on surfaces.
Clouds and fog are hydrometeors of suspended liquid or
solid moisture suspended in the air. Dew and frost are
hydrometeors of moisture that condense or sublime
directly on surfaces or on the ground.
CLOUDS.Clouds are the visible form of water
vapor, and consist of minute suspended droplets of
liquid water or ice particles. Fog is a cloud on the earths
surface. Liquid water droplets develop from gaseous
water vapor by the process of condensation. Solid water
particles or ice crystals develop by the process of
sublimation. During sublimation, gaseous water vapor
bypasses the liquid state and goes directly from a gas to
a solid, thereby releasing heat into the atmosphere.
Three factors are necessary for cloud formation:
sufficient moisture, hygroscopic nuclei, and a cooling
Moisture is supplied by evaporation and is
distributed vertically by convection currents and
horizontally by winds.
Hygroscopic nuclei are small particles on which
water vapor can condense or sublime. Hygroscopic
nuclei actually attract water vapor. The most effective
hygroscopic nuclei are the by-products of combustion,
sulfuric acid and nitric acid particles, and salts (such as
sodium chloride raised from the sea surface). Dust
particles may contain sufficient salts or acids to become
hygroscopic nuclei, but dust particles in general are not
effective hygroscopic nuclei. The presence of
hygroscopic nuclei is a must for water vapor to
condense. Air has been super-saturated in laboratories
to over 400% before condensation began in the absence
of hygroscopic nuclei.
In actual conditions, in the
presence of abundant hygroscopic nuclei, condensation
may begin at relative humidities near 70%. Saturation
of the air is reached when the relative humidity reaches
100%. At this point, the evaporation rate from liquid
water droplets to water vapor equals the condensation
rate from water vapor to liquid water, or theoretically,
the sublimation rates from gas-to-solid and from solid-
to-gas are exactly equal.
Hygroscopic nuclei are also called condensation
nuclei and sublimation nuclei when referring to the
specific process of condensation or sublimation.
A cooling process aids in condensation, since it
increases the humidity of the air without increasing the
amount of water vapor present.
The higher the
humidity, the easier condensation proceeds.
cooling process most frequently associated with
condensation is adiabatic expansion. When a parcel of
air is lifted higher in the atmosphere (where the pressure
is lower), it expands and its temperature decreases.
Another important cooling process is radiational
cooling. Simply put, as the sun goes down, the air cools
because the heat source, the sun, is no longer available
to maintain the heating.
FOG.Fog is a suspension of small visible water
droplets (or ice crystals) in the air that reduces
horizontal and/or vertical visibility at the earths
surface. Fog is a stratus cloud on the surface of the
earth. It is distinguished from smoke, haze, or dust by
its dampness and gray appearance. Fog usually does not
form or exist when the difference between the air
temperature and the dew-point temperature is greater
than 4 Fahrenheit degrees (2 Celsius degrees).
However, at temperatures below -2O°F (-29°C),
freezing fog, or ice fog, may form when the dew-point
temperature is as much as 8°F (4 Celsius degrees) lower
than the air temperature.
Freezing fog is composed
entirely of ice crystals that sparkle brilliantly in light.
When the air temperature is between 32°F and -20°F,