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<br />5/29/2019 35 <br /> <br />• Winds are most likely to change following calm or low wind speeds, and when winds are <br />from east or northeast directions. <br /> <br />Fire behavior is most unpredictable when temps are rising during the morning hours. <br /> <br />Temp rise of 20 F from sunrise to midafternoon reduces the Relative Humidity (RH) by one-half <br />and cold-fronts drop humidity. <br /> <br />Preferred RH is from 25 to 50%. Under certain conditions a RH 20 to 80% can produce <br />satisfactory results. <br /> <br />When 50% RH, fires may not burn area completely. <br /> <br />Recommendation for grass lands 20-80% RH <br /> <br />Grasslands have been burned 9 hrs. after .5 inch rain. <br /> <br />Stability is important. A small decrease in temperature occurs with increase in altitude. With <br />inversions, an increase in temperature occurs. <br /> <br />With unstable air, a large decrease in temps occur with high temperatures. Will get strong <br />convective fire behavior. Helps to confine fire to prescribed area. If too unstable, the fire can <br />experience strong in-drafts and spotting. It no longer meets the Rx and the fire should be <br />extinguished. <br /> <br />Clouds in layers, steady winds, smoke, and haze hanging near the ground are signs of stability. <br />Unstable air = dust devils, gusty winds, good visibility, and clouds with vertical growth. <br /> <br />