UNIT IV--TEMPERATURE-MOISTURE RELATIONSHIP
Weather is the most variable and often the most critical determinant of fire behavior. This is the
first of several units that will deal with weather and its relationship to fire behavior. This unit
will discuss atmospheric temperatures, moisture, and the relationship between these two
Before starting the unit, read the instructions to students on page 1 of your workbook. On page 2,
you will find the unit objectives on which you will be tested at the end of this unit. Please study
these objectives. When you have finished, return to this text.
You should be familiar with the atmosphere that surrounds the earth and its life-supporting
elements of oxygen, moisture, and other gases that affect our activities and well-being. This
atmosphere is very dynamic, with conditions changing from moment to moment, that can impact
on our activities on very short notice.
These short term atmospheric variations are what we call weather. To most people, weather is
thought of in terms of temperature, humidity, precipitation, cloudiness, sunshine, visibility, and
wind. The fire manager is concerned with all of these factors, since his successes and failures are
often dependent on his keeping current with the weather. An understanding of weather processes
and the ability to observe and interpret atmospheric conditions are of great advantage to the fire
This unit will concentrate on several weather factors, especially those related to temperature and
moisture in the atmosphere. On page 3, we begin our discussion with temperature and heat. The
two are not synonymous, although often used in the same sense. Temperature is defined as the
degree of hotness or coldness of a substance; a measurement of its molecular activity. It is
measured by a thermometer on a designated scale such as Fahrenheit or Celsius. In this course,
all temperatures will be given in degrees Fahrenheit.
Where does heat come from? For the most part, all of the heating of the earth's surface and its
atmosphere comes from the sun through solar radiation. On a very small scale, heat may be
generated by a large, active forest fire or some other energy-releasing activity, but the sphere of
influence by these heat sources, worldwide, are relatively small. This discussion will concentrate
on solar radiation.
Fortunately, our planet basks in the radiant heat of "O1' Sol" each day and provides a climate
favorable to all living things. It's also fortunate that our day-night cycle is only 24 hours in
length; if longer, we might have much greater temperature differences between day and night.
Much of the heat incoming during the day is lost at night. The atmosphere and the earth retain a
certain amount of heat from day to day which reduces the wide temperature variations that might
The distribution of incoming solar radiation varies from day to day, depending on atmospheric