Three atmospheric elements affect the velocity of sound traveling through the atmosphere: temperature, wind velocity (direction and speed), and relative humidity. Of these elements, temperature and wind have the greatest affect on sound speed. Relative humidity changes produce only minimal sound speed changes.

TEMPERATURE. Temperature normally decreases with height through the atmosphere. This decrease in temperature causes sound waves to bend upward, away from the ground. This action is good with regard to blast waves, because blast noise is directed upward. On the other hand, when the temperature increases with height (an inversion) blast waves are bent downward, toward the ground, and property damage may result.

WIND VELOCITY. Wind direction and speed can have a positive or a negative effect on blast waves. The wind may aid in pushing a blast wave in a direction where the noise level can produce damage, or it may retard the blast wave. The positive or negative contribution of the wind is determined mathematically by comparing the true-wind direction to the vector (compass direction) between the blast point and the point of interest. The vector is the azimuth of interest. The blast wave travels along the azimuth of interest to reach the point of interest. The angle at which the true wind intersects an azimuth of interest determines whether the wind is having a positive, negative, or neutral affect on a blast wave.

Any component of the wind that assists in pushing the blast toward a point of interest is positive, while any component that deters a blast from reaching a point of interest is negative. A wind that blows perpendicular to the azimuth of interest neither aids nor retards a blast wave and is classified as neutral.

Figure 6-5-1, view A shows a vector of 290 degrees and the compass azimuth perpendicular to the azimuth of interest. A blast wave would travel along the 110-290 azimuth, or from the east-southeast to the west-northwest, toward the point of interest. View B shows the sectors and compass directions from which the wind must blow to aid or deter the blast wave from reaching the point of interest.

Wind and temperature information is usually obtained from the most recent radiosonde sound-ing taken in the proximity of the detonation or blast. For example, if bombing exercises are conducted at Bloodsworth Island, in the Ches-apeake Bay, the closest radiosonde station is at Wallops Island, Virginia. Under most circumstances, the latest sounding from Wallops Island is used to compute sound focus for Bloodsworth Island bombing runs.

NOTE: Care must be exercised in choosing a sounding. The upper-air station closest to the bombing site may not always pro-vide temperatures and winds representative of the bombing site. Topographical and syn-optic weather pattern differences may preclude the use of the sounding from the closest station.

Learning Objective: Identify the informa-tion and equipment required to manually component worksheet.

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