Firewhirls are much less common but present serious

This preview shows page 5 - 7 out of 8 pages.

Firewhirls are much less common but present serious safety and security problems on the fireline. Firewhirl development will be discussed in detail in Unit 9 of this course. Any convective lifting at the surface causes indrafts from adjacent areas to replace the rising air. These indrafts can be gusty and erratic.
Image of page 5

Subscribe to view the full document.

Smoke convection columns rise much higher in unstable air. Chimneys, of a sort, develop with indrafts feeding the fire at the base of the column and strong convective currents rising through the column. The greater the instability and fire intensity, the stronger the indrafts and convection column updrafts. Of primary concern is the spotting potential of high, well developed convection columns due to the rise of firebrands in the column. Convection column development and spotting by aerial firebrands will be further discussed in Unit 9. We'll give you some more examples of instability which occur on a daily basis. Figure 9 illustrates rising bubbles of air from the surface which are called updrafts or thermals. Sailplane pilots seek out rising air currents or thermals which can give their aircraft lift and prolong flight. Large soaring birds also take advantage of thermals to sustain flight. Uneven heating at the surface produces thermals at some locations more readily than others. Cumulus clouds are often good indicators of thermals produced from below. These clouds may develop at specific locations and then drift with the prevailing winds. We have described one of the common lifting processes which produce clouds in an unstable atmosphere. There are other lifting processes that also aid cumulus cloud buildups. See page 13. The cloud most likely to cause problems for the firefighter is the cumulonimbus or thunderhead. There are four lifting processes which can cause thunderstorm development. Under item C, list the following: Convection or thermal, orographic, frontal, and convergence. You should understand each of these processes. Figure 10 illustrates three of the lifting processes. We have already discussed thermal lifting. Orographic lifting occurs in mountainous terrain when a mass of moving air is forced to rise because of the presence of slope. Air that is forced upward cools adiabatically. If this air reaches its saturation point, clouds develop. Orographic lifting and thermal lifting often work together to produce cumulus clouds in mountainous areas. The third process is frontal lifting. Here a moving, cooler air mass pushes its way under and lifts a warmer air mass. Again, this lifting action can produce cumulus clouds if saturation occurs. Cumulus cloud development is usually associated with cold front passages, while stratus clouds generally accompany a warm front. The fourth lifting process, convergence, is illustrated in figure 11 on page 14. Here you see the relationship of convergence to subsidence. In a high-pressure cell, air is piled very high, thus exerting more weight and pressure on air within the cell. The result often is "subsidence," and "divergence," or air flowing out of the a cell at the surface. Diverging air from the high-pressure
Image of page 6
Image of page 7
You've reached the end of this preview.
  • Spring '04
  • MIchealJenkins
  • Cumulus cloud, lapse rate, 3°, cumulus, 1,000, solarradiation

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern