Air rises and cools at the dry adiabatic lapse rate

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latent heat of condensation. Air rises and cools at the dry adiabatic lapse rate until it reaches its saturation point and then continues to rise and cool at the wet adiabatic lapse rate. What causes these differences in temperature lapse rates in the atmosphere? Well, there are several factors that contribute to varying temperature distributions, but the most important is heating and cooling at the earth's surface. See figure 3. As the earth loses its heat at night through radiation, the air in contact with the ground also cools. Conduction becomes an important process here, as air at lower levels cools faster than the air above. This creates a condition of cool, heavier air below warmer air. The layer of cool air above the surface deepens as the night progresses. In mountainous terrain, this condition is even more pronounced in the valleys, as cool air drainage from the slopes above helps to deepen the layer of cold air. The deepening of the cool air layer is also affected by the amount of cloud cover at night. Clear nights cool faster than cloudy nights. Warmer air just above a cool air layer creates a very stable air condition. Smoke or any other parcel of air that is forced to rise will stop when the warmer air is reached. This inversion layer discourages vertical air movement. Conditions usually begin to reverse after sunrise. As the earth's surface is heated by solar radiation, it warms the air in contact with it and above it by conduction and convection. The stable air at lower levels warms until it is no longer colder than the air above, and the temperature lapse rate approaches the dry adiabatic rate. Inversions usually disappear sometime before noon as unstable conditions continue to develop. Air near the surface is not much warmer than the air above, thus making it buoyant and able to rise. Any lifted parcel of air will continue to rise until it reaches a level of equal temperature. A smoke column could rise many thousands of feet. The lower atmosphere at night is usually always stable; whereas, during the daytime it is usually unstable. This is especially true if the weather is fair with mostly clear skies. Stable or unstable air conditions can develop under cloudy skies, but their degree of development is usually less. See page 6. From the two illustrations in figure 3, we can conclude that diurnal changes of temperature in the lower atmosphere occur due to heating and cooling at the earth's surface. We can also conclude that cooling from below promotes stability, while heating from below promotes instability. These diurnal changes in the lower atmosphere have a pronounced effect on fire behavior. Now do question 2; mark your choice or choices. In question 2, statement 2 is true. In the next portion of this unit, we will discuss how atmospheric stability affects fire behavior.
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  • Spring '04
  • MIchealJenkins
  • Cumulus cloud, lapse rate, 3°, cumulus, 1,000, solarradiation

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