zx-5 - UNIT VI-LOCAL AND GENERAL WINDS This unit addresses...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
UNIT VI--LOCAL AND GENERAL WINDS This unit addresses winds, both local and general, and what firefighters should know about them. Before starting the unit, read the instructions to students on page 2; then the unit objectives on page 2 of your workbook. When you have finished, return to this text. See page 3. What is wind? By simple definition, wind is air in motion, especially horizontal, relative to the earth's surface. We are concerned with winds of two major scales in the atmosphere--the larger scale general wind and the smaller scale local wind. Collectively, these winds are measured at three levels in the atmosphere--the winds aloft, the 20-foot surface winds, and the winds at midflame height. See figure 1. The general winds or winds aloft are caused by broad scale circulation patterns high above the earth. This circulation of air throughout the atmosphere is the result of large-scale convective circulation between the equator and the polar regions, and of the earth's rotation on its axis. These are sometimes called the gradient winds. In the contiguous United States and Canada, high- and low-pressure patterns mostly move from west to east due to the prevailing westerlies. Winds aloft are measured at 1,000-foot intervals, since they can vary considerably at various altitudes. As the general air flow nears the earth's surface, it gradually becomes affected by the shape of the topography and by local heating and cooling over large areas. Frictional drag produced by the terrain usually slows the larger scale winds and can modify their direction. Next, consider the smaller scale, local winds. These are produced locally due to heating and cooling or temperature differences at the earth's surface. The general winds and the local winds may combine to produce the winds that we experience at the surface. The measurement of surface winds has been standardized at 20 feet above the ground in a clearing, or 20 feet above any vegetation. As surface winds drop closer to the ground, their speeds are reduced primarily due to friction. In doing calculations of fire behavior, we are concerned with the wind speeds at the level of the flames. This is referred to as the "midflame windspeed". This unit will discuss how each of these wind levels are measured and predicted. All of these wind levels can affect, either directly or indirectly, the behavior of wildfires, although we are generally not concerned with higher level winds unless fire intensities and convection columns are very high and long-range spotting becomes a problem. Most weather forecasts available to firefighters address the general and surface winds because these are more appropriate to fire danger predictions. Some special fire weather forecasts may predict midflame (usually eye level) winds, in which case the forecaster has reduced the surface windspeeds for you. First, we'll discuss surface or 20-foot winds. These affect the intensity, direction, and rate of
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 13

zx-5 - UNIT VI-LOCAL AND GENERAL WINDS This unit addresses...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online