UNIT VII--ATMOSPHERIC STABILITY AND INSTABILITY
The stability or instability of the atmosphere is a concern to firefighters. This unit discusses how
changes in the atmosphere affect fire behavior, and how firefighters can recognize existing and
changing atmospheric conditions. Before starting the unit, read the instructions to students on
page 1 and then the unit objectives on page 2 of your workbook. When you have finished, return
to this script.
In earlier weather units of this course, we discussed the facts that the earth's gaseous mantle,
called "the atmosphere," is very fluid, with air constantly moving and mixing, and that air
changes in temperature, moisture, pressure, and other properties. By now you know that air
moves horizontally or vertically in response to the earth's rotation, to large and small scale
pressure gradients, to various lifting mechanisms, and to gravity.
Unit 6 concentrated on the horizontal movement of air or wind. In this unit, we will discuss
vertical air movement, what causes it, and what it means to firefighters. This vertical movement,
either upward or downward, is generally influenced by the degree of stability or instability of the
atmosphere at any particular time.
What is atmospheric stability? On page 3, we give a simple definition of stability as the
resistance of the atmosphere to vertical motion. Air may be stable, neutral, or unstable,
depending on its temperature distribution at various levels in the atmosphere.
Temperature distribution and lapse rates were discussed in Unit 4, where you learned that
temperatures normally increase as we get closer to the earth's surface. This is due in part to the
greater molecular activity of denser, more compressed air at lower altitudes. These conditions
change throughout a 24-hour period, as the daytime solar heating and nighttime heat loss to and
through the atmosphere tend to modify the temperature distributions.
The term "adiabatic process" was used in Unit 4, which simply means warming by compression,
or cooling by expansion, without a transfer of heat or mass into a system. As air moves up or
down within the atmosphere, it is affected by this process. See figure 1. This temperature
difference will be 5-1120 decrease per 1,000 feet increase in altitude. This is also termed the dry
adiabatic lapse rate. The atmosphere may or may not have a temperature distribution that fits the
dry adiabatic lapse rate. Usually it does not.
The actual lapse rate may be greater or less than the dry adiabatic lapse rate and may change by
levels in the atmosphere. This variation from the dry adiabatic lapse rate is what determines
whether the air is stable or unstable. If the air is unstable, the vertical movement of air is
encouraged, and this tends to increase fire activity. If the air is stable, vertical movement of air is
discouraged, and this usually decreases or holds down fire activity. The importance of this
atmospheric property will become evident by the time you have completed this unit.
The actual temperature lapse rate in a given portion of the atmosphere could range from a plus