slower rate than land does. The slower rate of heating
and cooling of water is the reason temperature extremes
occur over land areas while temperatures over water
areas are more consistent.
The specific heat of various land surfaces is also
different, though the difference between one land
surface and another is not as great as between land and
water. Dry sand or bare rock has the lowest specific
heat. Forest areas have the highest specific heat. This
difference in specific heat is another cause for
differences in temperature for areas with different types
of surfaces even when they are only a few miles apart;
this difference is important in understanding the
horizontal transport of heat (advection) on a smaller
Advection is a form of convection, but in
meteorology it means the transfer of heat or other
properties HORIZONTALLY. Convection is the term
reserved for the VERTICAL transport of heat. In this
manual the words convection and advection are used to
atmospheric properties, respectively.
Horizontal transfer of heat is achieved by motion of
the air from one latitude and/or longitude to another. It
is of major importance in the exchange of air between
polar and equatorial regions. Since large masses of air
are constantly on the move somewhere on Earths
transporting more heat from place to place than any
other physical motion. Transfer of heat by advection is
achieved not only by the transport of warm air, but also
by the transport of water vapor that releases heat when
What is the definition of Temperature?
What are 20 C converted to Fahrenheit?
Name the zones of the earth's atmosphere in
What are the four methods of heat transfer?
What is the horizontal transport of heat
LEARNING OBJECTIVE: Describe how
moisture affects the atmosphere.
More than two-thirds of Earths surface is covered
with water. Water from this extensive source is
continually evaporating into the atmosphere, cooling
by various processes, condensing, and then falling to
the ground again as various forms of precipitation. The
remainder of Earths surface is composed of solid land
of various and vastly different terrain features.
Knowledge of terrain differences is very important in
analyzing and forecasting weather. The worlds terrain
varies from large-scale mountain ranges and deserts to
minor rolling hills and valleys. Each type of terrain
significantly influences local wind flow, moisture
availability, and the resulting weather.
Moisture in the atmosphere is found in three
statessolid, liquid, and gaseous. As a solid, it takes
the form of snow, hail, and ice pellets, frost, ice-crystal
clouds, and ice-crystal fog. As a liquid, it is found as
rain, drizzle, dew, and as the minute water droplets
composing clouds of the middle and low stages as well
as fog. In the gaseous state, water forms as invisible
vapor. Vapor is the most important single element in the
production of clouds and other visible weather
phenomena. The availability of water vapor for the
production of precipitation largely determines the
ability of a region to support life.
The oceans are the primary source of moisture for
the atmosphere, but lakes, rivers, swamps, moist soil,
snow, ice fields, and vegetation also furnish it. Moisture
is introduced into the atmosphere in its gaseous state,
and may then be carried great distances by the wind
before it is discharged as liquid or solid precipitation.
WATER VAPOR CHARACTERISTICS
There is a limit to the amount of water vapor that
air, at a given temperature, can hold. When this limit is
reached, the air is said to be saturated. The higher the air
temperature, the more water vapor the air can hold
before saturation is reached and condensation occurs.
(See fig. 1-10.) For approximately every 20°F (11°C)
increase in temperature between 0°F and 100°F (-18°C
and 38°C), the capacity of a volume of air to hold water
vapor is about doubled. Unsaturated air, containing a
given amount of water vapor, becomes saturated if its
temperature decreases sufficiently; further cooling
forces some of the water vapor to condense as fog,
clouds, or precipitation.