away from the gravitational pull of the center of Earth.
However, the mass remains the same even though the
gravitational pull is different. Gravity also varies with
latitude. It is slightly less at the equator than at the poles
due to the equators greater distance from the center of
The weight of an object is a measure of its
gravitational attraction. The weight depends upon the
mass or quantity that it contains and the amount of
gravitational attraction Earth has for it. Weight is a
force, and as such it should be expressed in units of
force. Since gravity varies with latitude and height
above sea level, so must weight vary with the same
factors. Therefore, a body weighs more at the poles
than at the equator and more at sea level than atop a
mountain. In a comparison of mass and weight, mass
remains constant no matter where it is, but weight
varies with latitude and height above sea level.
Volume is the measure of the amount of space that
matter occupies. The volume of rectangular objects is
found directly by obtaining the product of their length,
width, and depth. For determining the volume of liquids
and gases, special graduated containers are used.
The mass of a unit volume of a substance or mass
per unit volume is called density. Usually we speak of
substances being heavier or lighter than another when
comparing equal volumes of the two substances.
Since density is a derived quantity, the density of an
object can be computed by dividing its mass (or weight)
by its volume. The formula for determining the density
of a substance is
where D stands for density, M for mass, and V for
From this formula, it is obvious that with mass
remaining unchanged, an increase in volume causes a
decrease in density. A decrease in volume causes an
increase in density.
The density of gases is derived from the same basic
formula as the density of a solid. Pressure and
temperature also affect the density of gases. This effect
is discussed later in this unit under Gas Laws.
CHANGES OF STATE
A change of state (or change of phase) of a
substance describes the change of a substance from a
solid to a liquid, liquid to a vapor (or gas), vapor to a
liquid, liquid to a solid, solid to vapor, or vapor to a
solid. In meteorology you are concerned primarily with
the change of state of water in the air. Water is present in
the atmosphere in any or all of the three states (solid,
liquid, and vapor) and changes back and forth from one
state to another. The mere presence of water is
important, but the change of state of that water in the air
is significant because it directly affects the weather.
The solid state of water is in the form of ice or ice
crystals. The liquid state of water is in the form of
raindrops, clouds, and fogs. The vapor state of water is
in the form of unseen gases (water vapor) in the air.
Energy is involved in the various changes of state
that occur in the atmosphere. This energy is primarily in
the form of heat. Each of the changes of state processes
either uses heat from the atmosphere or releases heat
into the atmosphere. The heat used by a substance in
changing its state is referred to as the latent heat and is
usually stated in calories.
The calorie is a unit of heat energy. It is the amount
of heat required to raise the temperature of 1 gram of
water 1°C. A closer look at some of the major changes
of state of the atmosphere helps to clarify latent heat.
Refer to figure 2-5 during the following discussions.
Liquid to Solid and Vice Versa
Fusion is the change of state from a solid to a liquid
at the same temperature. The number of gram calories
of heat necessary to change 1 gram of a substance from
the solid to the liquid state is known as the latent heat of
fusion. To change 1 gram of ice to 1 gram of water at a
constant temperature and pressure requires roughly 80
calories of heat. This is called the latent heat of fusion.
Fusion uses heat. The source of this heat is the
The opposite of fusion is freezinga liquid
changes into a solid. Since it requires 80 calories to
change 1 gram of ice to 1 gram of water, this same
amount of heat is released into the air when 1 gram of
water is changed to ice.