Energy and Earth - Fundamentals of the Physical Environment - page 26

Potential energy is related to gravity. Because of the
apparent pull that Earth exerts upon objects within its
gravitational field, material is drawn towards Earth's
centre. Thus objects lying at greater distances from its
centre (for example, rocks on a hillside, water at the top
of a waterfall or the air near a mountain summit) possess
more potential energy. This energy is converted to kinetic
energy when the rock, the water or the air descends to
lower levels; some energy is converted to heat through
friction.

F

125

D

100

E

75

50

25

B C

0

A

-25

Time

Sensible and latent heat

Figure 2.3 The pattern of temperature and phase changes
for water. The temperature remains constant during each
phase change as long as pressure remains constant. Differ-
ences in specific heat of ice and water give different gradients
for the lines A-B and C-D.

In addition to the forms of energy outlined above, we have
two other forms of thermal energy which are very impor-
tant in the earth system. Sensible heat is the exchange of
warm air down a temperature gradient. By day, this will
normally be upwards, but at night there may be a weak
transfer of sensible heat down to the cooler ground
surface. It takes place because the air in contact with the
surface becomes warmer through conduction. Being
warmer, the air will be less dense than its surroundings
and, like a cork in water, will tend to rise until it has the
same density (temperature) as its surroundings. Occasion-
ally this process can be seen operating. If the ground is
being warmed intensely, the rate of sensible heat transfer
is high. The rising air can be seen as a 'shimmering' of the
air layer near the ground due to the variable refractive
indices of light through the air of different temperatures.
Replacing the rising warm air are pockets of cooler air
descending towards the ground.
The concept of latent heat can best be understood by
conducting a small experiment. Start with a large block
of ice out of a freezer and measure its temperature;
perhaps it may be -15 C. Then place it in a Pyrex glass
beaker and heat the beaker at a constant rate, monitoring
the temperature of the ice continuously. Keep heating the
beaker until all the ice has melted into water; eventually
it will reach boiling point and vaporize as steam. If the
temperature values are plotted against time, we find a
steady increase in temperature (representing heat input
from the heater and some heat flow from the air, which
will be warmer than the ice) until melting starts. Despite
the steady addition of heat, there is no increase in
temperature until the ice melts completely ( Figure 2.3 ).
A similar effect is found on vaporization. Where has the
heat that was being added continuously gone? It was being
used not to raise the temperature during melting or
vaporization but to change the physical state of the water,
either from solid to liquid or from liquid to vapour. As
the heat appears to be hidden, it is known as latent heat.

A change of state, from solid to liquid, or from liquid
to vapour, involves a considerable use of energy. In the first
case we need 3·33 10 5 J kg -1 ; this quantity of heat is
called the latent heat of fusion. In the second, much more
energy is needed. At 10 C the latent heat of vaporization
is 2·48 10 6 J kg -1 but it falls slightly with increasing

temperature. To get a better idea of this large quantity of
energy needed for evaporation, the amount consumed in
evaporating only 10 g of water is about the same as that
needed to raise the temperature of 60 g of water from 0 C

to boiling point (100 C). We tend to be most aware of

evaporational cooling after swimming. The effect of
evaporation leads to the extraction of heat from the skin
surface; sweating works in a similar way.
Overall, then, thermal, kinetic, chemical and potential
energy are important to Earth's system but operate
internally and so cannot be observed directly from space.
To understand the results of these different flows of
energy, we must look more closely at them, concentrating
on the forms of energy that have significance for the
physical geography of Earth.

Methods of energy transfer
The types of energy we have considered so far do not have
a uniform distribution over the globe. Both earth and air
experience major inequalities in energy receipts and
emissions. As a result of these differences, spatial transfers
of energy take place, for energy is redistributed to
minimize the inequalities, or to maintain (or to achieve)
an equilibrium.
To understand how energy is transferred we need
to consider a little further the principles of energy