Whether a wind condition can disperse a radioactive plume depends on whether conditions will allow the plume to be swept away, to spread out, and to eventually descend. These conditions are tied to the atmospheric stability and the temperature gradient in the atmosphere (11). A number of different thermal conditions contribute to this atmospheric gradient:

(a) Potential temperature decrease (lapse rate) with height is due to a temperature decrease as the pressure decreases. Assuming a dry adiabatic lapse rate without heat exchange to other air masses, there would be an upward cooling rate of 5.4°F/1000 ft. Such an adiabatic temperature con­dition ensures that the air will be unstable, because daytime hotter air nearer the earth is constantly changing position with upper colder air.

(b) The normal environmental lapse rate is not adiabatic, and it has some heat exchange, resulting in a lapse rate of 3.5°F/1000 ft. Superadiabatic rates give greater cooling with altitude than the normal environmental lapse rate. Also possible are subadiabatic rates giving a lesser cooling gra­dient. Both conditions are unstable.

(c) In some conditions the gradient can reverse itself, giving rise to inver­sion conditions, where the temperature increases with height. In inversion conditions the air layers are stable, cooler air remains below, and warmer air remains above. Such morning conditions can remain until the sun warms the lower regions to reverse the gradient and make the air mass unstable.

(d) In practice there may be several layers, each with a different thermal gradient.

Such wind speed and thermal stability considerations determine the wind — induced dispersal of any radioactive cloud following an inadvertent release of effluent. The wind conditions have to be characterized in terms of stability, persistence, shear, and direction. This characterization has been performed by Pasquill (12) and is used in dispersion calculations.