Geoscience Reference
In-Depth Information
5
CHAPTER FIVE
Atmospheric instability,
cloud formation and
precipitation processes
LEARNING OBJECTIVES
When you have read this chapter you will:
n
know the effects of vertical displacements on the temperature of unsaturated and saturated air
parcels
n
know what determines atmospheric stability/instability
n
be familiar with the basic cloud types and how they form
n
understand the two main mechanisms leading to precipitation formation
n
know the basic features of thunderstorms and how lightning develops.
To understand how clouds form and precipitation
occurs, we first discuss the change of temperature
with height in a rising air parcel and tempera-
ture lapse rates. Then we consider atmospheric
stability/instability and what causes air to rise and
condensation to occur. Cloud mechanisms and
cloud classifications are described next, followed
by a discussion of the growth of raindrops and
precipitation processes, and finally thunderstorms.
temperature change, involving no subtraction
(or addition) of heat, is termed
adiabatic
. Vertical
displacements of air are the major cause of
adiabatic temperature changes.
Near the earth's surface, most temperature
changes are non-adiabatic (also termed
diabatic
)
because of energy transfer from the surface and
the tendency of air to mix and modify its
characteristics by lateral movement and turbu-
lence. When an air parcel moves vertically, the
changes that take place are generally adiabatic,
because air is fundamentally a poor thermal
conductor, and the air parcel tends to retain
its own thermal identity, which distinguishes
it from the surrounding air. However, in some
circumstances, mixing of air with its surroundings
must be taken into account.
Consider the changes that occur when an air
parcel rises: the decrease of pressure (and density)
A ADIABATIC TEMPERATURE
CHANGES
When an air parcel moves to an environment of
lower pressure (without heat exchange with
surrounding air) its volume increases. Volume
increase involves work and the consumption of
energy; this reduces the heat available per unit
volume and hence the temperature falls. Such a
