Environmental Engineering Reference
In-Depth Information
do not change position. This is the process of conduction of heat.
Temperature is the measure of the average speed or kinetic energy of the
molecules, with higher temperatures being associated with greater speed. In the upper
atmosphere, gas molecules move at high speeds and so have high temperatures
(Figure 3.1). As the density of air is low at these heights and there are few molecules
the heat energy is small. We use a variety of scales to measure temperature. The most
basic one is the Kelvin scale. It uses the temperature at which atoms and molecules
possess a minimum amount of energy and theoretically no thermal motion. This is
termed absolute zero or 0 K. On this scale, the freezing point of water is 273 K. More
popularly we have the Celsius or Centigrade scale based on 0° C as the freezing point
of water and 100° C as its boiling point. The Fahrenheit scale uses 32° and 212° for
these boundaries.
Figure 2.4
Energy exchange at the soil surface. The size of
the arrows is only approximately to scale.
movement of substances containing heat, such as water or air, and is not possible in a
solid.
Conduction
is the transfer of heat through a medium from molecule to molecule
(see box on heat and temperature).
These three processes of transfer are often closely related. Thus energy may be
conducted through the soil to the surface and then radiated or convected into the
atmosphere. Similarly, in the air, convection currents may raise warm air masses to
higher levels and then conduction to the surrounding cooler atmosphere may occur (see
Chapter 4), while condensation of water vapour releases latent heat. Convection is very
important as an energy transfer mechanism because it transfers energy in two forms. The
first is the
sensible heat
content of the air, which is transferred directly by the rising and
mixing of warmed air. It can also be transferred by conduction. The other form of energy
transfer by convection is less obvious, as there is no temperature change involved, hence
its name,
latent heat
. The evaporation of water into vapour or the melting of ice into
water involves a supply of heat to allow the change to take place. When the reverse
process operates, from vapour to liquid, or water to ice, this heat is released. We shall
return to these mechanisms in more detail in Chapters 3 and 4.
Transfers also occur between other forms of energy. If two objects with different
kinetic energies are brought together, a transfer takes place between the two which tends
