CLOUDS - HOW DO THEY FORM?
Wind is the horizontal movement of air, transporting energy
transferred from the earth's surface as sensible and latent heat.
Sensible heat is transferred by the processes of conduction and
convection. Conduction transfers energy within a substance, and
convection transfers energy through the vertical movement of the
heated substance. Latent heat is the transfer of energy by
transforming the substance itself. As you recall, water has the
ability to exist as liquid, gas or solid. The transformation from
liquid to gas is called evaporation; the reverse process, from gas to
liquid, is called condensation; from liquid to solid is known as
solidification (freezing); and from solid to liquid, fusion (melting).
Water can also be transformed directly from solid to gas
(sublimation), or the reverse, through a process called deposition.
We will see these various processes in the formation of clouds.
Clouds are formed when air contains as much water vapor
(gas) as it can hold. This is called the saturation point, and it
can be reached in two ways. First, moisture accumulates
until it reaches the maximum amount the volume of air can
hold. The other method reduces the temperature of the
moisture filled air, which in turn lowers the amount of
moisture it can contain. Saturation, therefore, is reached
through evaporation and condensation, respectively. When
saturation occurs, moisture becomes visible water droplets
in the form of fog and clouds.
It should be noted that condensation by itself does not cause
precipitation (rain, snow, sleet, hail). The moisture in clouds must
become heavy enough to succumb to gravity and return to earth's
surface. This occurs through two processes. In cold clouds ice
crystals and water droplets exist side by side. Due to an imbalance
of water vapor pressure, the water droplets transfer to the ice
crystals. The crystals eventually grow heavy enough to fall to earth.
In the second process, water droplets in warm clouds collide and
change their electric charge. Droplets of unlike charge attract one
another and merge, thereby growing until they have sufficient
weight to fall.
There is no difference between fog and clouds other than altitude.
Fog is defined as a visible moisture that begins at a height lower
than 50 feet. If the visible moisture begins at or above 50 feet, it is
called a cloud. Two common types of fog are called radiation fog
and advection fog. Radiation fog forms during the night as the
earth's surface cools and the air immediately above it cools in turn
by conduction. If the air is moist enough, the cooling causes it to
reach saturation and visible water droplets form. We often call this
type of fog ground fog because it lies so close to the surface.
Advection fog forms when warm moist air moves over a colder
surface (advection means to move horizontally). A perfect example
is on the west coast of continents. Prevailing westerly winds move
moist air from over a warm ocean area to over the colder waters off
the coast. Fog forms and is carried by the westerly over the land.
