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Chapter 26 In the early morning a temperature inversion may be present fol- lowingclear nigh [s with low wind speeds that inhibit mixing of Che atmosphere. The lack of cloud cover allows the temperature of the ground and air above it to drop rapidly creating the inversion. Blase ing done in the early morning will result in the loud noise levels illustrated in Figure 26-M. The incidence of the sun's rays on the earth's surface causes the ground temperature to rise, and the in- creased ground temperature warms the air in contact with it, This process continues throughout the day until late afternoon with the early morning inversion normally being eliminated by midmorning. At this time the favorable conditions illustrated in Figure 26-N pre- vail. Near sunset the temperature oC the eazth's surface begins to cool so that an inversion at ]ow altitude may exist on calm days when there is little atmospheric mixing caused by vends. The presence of cloud layers signifies the presence of a temperature inversion as the moisture condenses in the atmosphere at the top of an inversion where the temperature begins to decrease with altitude. The commonly accepted idea that noise is reflected from the bottom of cloud lavers is mistaken. The layers signify the presence of an inver- sion which normally does not extend to the earth's surface. However, low altitude clouds may signify temperature inversions which may extend to the earth's surface creating the positive gradient illustrated in Figure 26-M. Thick cloud layers prevent the sun's rays from reach- ing the earth's surface to provide the heat needed to eliminate the inversions. Changes in sound velocity with altitude may be caused by wind as well as temperature gradients. Wind gradients are highly directional while temperature gradients aze generally independent of azimuth. As shovm in Figure 26-0, a normal positive wind gradient causes an increase of sound speed with altitude. This results in downward re- fraction in the downwind direction and upward refraction in the upwind direction with the accompanying increase and decrease in noise levels. The effect of wind on noise levels is generally greatest in l .15.E Figure Z6-0. Propagation from positive wintl gradient (speed increesfng wtth altitude). 442 Vibration and Air Bfast _J 0000 LL I soon • 5 _- tooa 6 0 a soon ~ooo eooo Hodrontal DUtance - Ft. • m-•~ ..,-~• ~ nc gneve wmposne m ray path tliagrams was calculated at intervals of 2 Degrees elevation from ratliosontle tlata ohtainetl at Kennedy Airport, New York City. 443 •