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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
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This note was uploaded on 03/01/2012 for the course WILD 4520 taught by Professor Michealjenkins during the Spring '04 term at Utah State University.

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