The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- Water vapor, an odorless, colorless gas, changes from one state of
matter (solid, liquid, or gas) to another at the temperatures and pressures experienced near
Earth's surface. The processes involved are evaporation, condensation, melting,
freezing, sublimation, and deposition. During each change, latent
(hidden) heat is either absorbed or released.
- Humidity is the general term to describe the amount of water vapor in
the air. The methods used to express humidity quantitatively include (1) mixing
ratio, the mass of water vapor in a unit of air compared to the remaining mass of
dry air; (2) vapor pressure, that part of the total atmospheric pressure
attributable to its water-vapor content; (3) relative humidity, the ratio of
the air's actual water-vapor content compared with the amount of water vapor required for
saturation at that temperature; and (4) dew point, that temperature to which a
parcel of air would need to be cooled to reach saturation. When air is saturated, the
pressure exerted by the water vapor, called the saturation vapor pressure,
produces a balance between the number of water molecules leaving the surface of the water
and the number returning. Because the saturation vapor pressure is temperature-dependent, at
higher temperatures more water vapor is required for saturation to occur.
- Relative humidity can be changed in two ways. One is by adding or
subtracting water vapor. The second is by changing the air's
temperature. When air is cooled, its relative humidity increases.
- The cooling of air as it rises and expands owing to successively lower air pressure
is the basic cloud-forming process. Temperature changes in air brought about by compressing
or expanding the air are called adiabatic temperature changes. Unsaturated air
warms by compression and cools by expansion at the rather constant rate of 10°C per 1000
meters of altitude change, a figure called the dry adiabatic rate. If air
rises high enough, it will cool sufficiently to cause condensation and form a cloud. From
this point on, air that continues to rise will cool at the wet adiabatic rate, which varies
from 5°C to 9°C per 1000 meters of ascent. The difference in the wet and dry
adiabatic rates is caused by the condensing water vapor releasing latent heat,
thereby reducing the rate at which the air cools.
- Four mechanisms that can initiate the vertical movement of air are (1)
orographic lifting, which occurs when elevated terrains, such as mountains,
act as barriers to the flow of air; (2) frontal wedging, when cool air acts as
a barrier over which warmer, less dense air rises; (3) convergence, which
happens when air flows together and a general upward movement of air occurs; and (4)
localized convective lifting, which occurs when unequal surface heating causes
pockets of air to rise because of their buoyancy.
- The stability of air is determined by examining the temperature of the
atmosphere at various altitudes. Air is said to be unstable when the
environmental lapse rate (the rate of temperature decrease with increasing
altitude in the troposphere) is greater than the dry adiabatic rate. Stated
differently, a column of air is unstable when the air near the bottom is significantly
warmer (less dense) than the air aloft.
- For condensation to occur, air must be saturated. Saturation takes
place either when air is cooled to its dew point, which most commonly happens, or when water
vapor is added to the air. There must also be a surface on which the water vapor can
condense. In cloud and fog formation, tiny particles called condensation
nuclei serve this purpose.
- Clouds are classified on the basis of their appearance
and height. The three basic forms are cirrus (high, white, thin,
wispy fibers), cumulus (globular, individual cloud masses), and
stratus (sheets or layers that cover much or all of the sky). The four
categories based on height are high clouds (bases normally above 6000 meters),
middle clouds (from 2000 to 6000 meters), low clouds (below 2000
meters), and clouds of vertical development.
- Fog is defined as a cloud with its base at or very near the ground.
Fogs form when air is cooled below its dew point or when enough water vapor is added to the
air to bring about saturation. Various types of fog include advection fog,
radiation fog, upslope fog, steam fog, and
frontal (or precipitation) fog.
- For precipitation to form, millions of cloud droplets must somehow join
together into large drops. Two mechanisms for the formation of precipitation have been
proposed. (1) In clouds where the temperatures are below freezing, ice crystals form and
fall as snowflakes. At lower altitudes the snowflakes melt and become raindrops before they
reach the ground. (2) Large droplets form in warm clouds that contain large
hygroscopic (“water-seeking”) nuclei, such as salt
particles. As these big droplets descend, they collide and join with smaller water droplets.
After many collisions, the droplets are large enough to fall to the ground as rain.
- The forms of precipitation include rain, snow, sleet, freezing rain (glaze),
hail, and rime.
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