ON THE DESICCATION OF A CLOUD BANK BY A PROPAGATED PRESSURE WAVE

Abstract

On December 6, 1949, an accelerated microbarograph in Washington, D. C., produced a unique pressure trace at the time that a peculiar cloud bank passed overhead. The rear (western) edge of this cloud bank ended very abruptly and had the appearance of an Antarctic ice barrier. A study of the synoptic conditions of the day revealed that this pressure variation was produced by an expansion-type pressure wave which travelled eastward from the Midwest and was propagated between two inversion surfaces with a speed far in excess of the wind speed in that layer. The cloud bank also travelled in this same atmospheric layer but was advected by the prevailing winds. The conclusion is reached that the pressure wave which progressively overtook the cloud bank was associated with a rapid drop of the isentropic surfaces which in turn produced the adiabatic heating necessary to desiccate the cloud partially in the Midwest and completely over Washington. Confirmation of this conclusion is found in solar radiation records.