Dust—An Active Meteorological Factor in the Atmosphere of Northern Africa

Abstract

In subtropical Africa dust is shown to be an active meteorological element, particularly below the T-inversions common at 2000–3000 feet under Etesian conditions. Actual instability of the lower atmosphere is then not necessary for dust to rise, and mere mechanical stirring by day by winds of about 20 miles per hour or more is sufficient to produce severe sand storms over desert areas under such conditions. A dust particle suspended in the atmosphere has been assumed to possess an ``air-pocket'' or ``surrounding eddy'' of its own. The radius of the ``air-pocket'' of a particle of radius ``a'' cm (less than 5 micron) is shown to be about 10a cm. It is doubtful whether one can assume the persistence of one and the same ``eddy,'' but with such an assumption made the average radius of the ``eddy'' is found to be 6a by observation, but on rejecting the assumption there appears to be a close agreement between theory and observation. The particles also share their excess of heat with their environment in a very small time, of the order of 10−2–10−3 second. The amount of heat that can be directly supplied to the lower atmosphere by baked dust blown in a dust storm may, by itself, be sufficient to produce convection up to a height of 3000 feet or more. The same range of height of convection is also attainable in a cloud of dust by the absorption of solar radiation scattered or diffusely reflected within the cloud. In this respect it is suggested, though not fully verified, that the effect of suspended dust should be taken into account when discussing problems of upper-layer instability in subtropical Africa. It has also been observed that in the special cases of ``squalls'' the intensity of the storm, notably the poorness of visibility, is proportional to the time during which the squall reaches its maximum. The feature has been given a mathematical explanation. The conclusion is that owing to increased absorption of solar radiation within dusty atmospheres, or to direct heating of the air by baked dust that may be blown by day in the warm season of the region, dust is found to play a part that cannot be ignored in modifying the processes of water vapor and heat transfer, by altering turbulent and radiative processes, and in modifying the properties of air masses invading the deserts.