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upon several factore, chief among which are convection currents. These currents are strongest when the ground is dry and warmer than the air and when the sun is shining brightly. On the other hand, at night and in the early morning, the ground surface is usually cooler than the surrounding air, and there are no convection currents, so that gas clouds have much less vertical rise under such conditions. This rise will vary

Average Concentration as a Percentage of Mean Concentration of Lowest b'Stratum

Seal« for Curve C

Height of Cloud (in Feet) 51 from about 10 per cent of the distance traveled under the roost favorable conditions to as much as 35 per cent under adverse conditions.

Drag Effect—Since the wind velocity along the ground is practically «to, but increases rapidly upward, gas clouds are carried over the ground with a sort of rolling motion, which causes the cloud to incline forward and stretch out in length more at the top than the bottom. This increase in the length of the cloud is called the drag effect and, for cloud* released on the ground, amounts to from 10 to 30 per cent of the distance traveled, depending upon the nature of the terrain. For gas released from airplanes, up to 100 ft. above ground, the drag effect is equal to 100 per cent of the distance traveled, owing principally to the high velocity of the airplane.

Variation of Concentration.—The concentration of vapor in a gas cloud varies inversely as the logarithm of the height above ground. Thus, if Ci is the concentration at a height Hi and Ct the concentration at a higher point H*f then . „

This causes the lower layers of the gas cloud to be more dense and more toxic than the upper layers, as is clearly shown in Chart III, in which curve A shows the general relation between heights above ground (ordinates) and the concentrations at these heights (abscissas) and indicates the form of the relation between height and concentration.

Average Concentration.—In a gas cloud, the average concentration is not in the geometrical center of the cloud, but varies therefrom as the height of the cloud increases. In Chart III, curve B shows the change in the position of the average concentration, as the height varies.

If the concentration at any height in a toxic cloud is known, the concentration at any other height may be computed from Eq. (1). It the concentration is not known for any given height, then the total height and the average concentration of the toxic cloud may l>e .computed from a given quantity of chemical as shown on pages 64-66. The position of the average concentration is then located from curve B in ('hart III. With the average concentration and its position thus determined, the concentration at any other height in the cloud can be computed from Eq. (1). Also the effective concentration in the lowest 6-ft.

Average Concentration as a Percentage of Mean Concentration of Lowest b'Stratum

Seal« for Curve C

layer of the cloud may be determined from curve C in Chart III.

Smokb Clouds

Density.—When chemicals create a smoke they either burn or hydrolyze in the air, thus generating heat which warms the air imme-

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