On Dust Storms

Abstract

The author, meteorologist at the Military Aeronautical College at Tokorozawa near Tokyo, very often observed dust storms with interest, occurring there during recent ten years. He already contributed the reports in them twice to this Journal. In the present paper he classides the dust storm into the following three classes:- The first class, Funnel, shaped or smork column type. Most frequent, average height is 50cm; diameter near the ground is about 2 to 10 metres, duration of existence, 5 minute to quater an hour; sense of revolution is sometimes right and sometimes left. The s_??_cond class, Cylind_??_cal type; rather frequent, so-called dust devil in India; several columns with diameters of a few metres appear at the same time reaching the height about 300-600m. Their colour is dark; they have sharp, well defined edges at their middle parts, while rather diffused rims at their tops and feet. Their existing duration is about 10 minutes, after dissolving of the columns dust smork is still seen above the sky for some thirty minutes. Plate 13. shows “Dust devil” and Funnel shaped one. The third class, Group type: many whirls revolve round along one and the same circular orbit; the sense of revolution is the same as that of the rotation of each whirl.
The author noticed the sudden change of the sense of rotation of a whirl as Dr. E. H. Hankin⁽¹⁾ has formerly remarked. He observed it at the distance of about 4 metres. At first the rotation was of cyclonic sense, then suddenly it changed to anticyclonic and after about one minute the original rotation is resumed. Then the whirl seemed hesitating to rotate and all the sudden the anticyclonic rotation started again when the whirl displaced about 4 or 5 metres. After a while it vanished. The author hardly believe that he saw an ilusion. An aviator in the college once got into such a dust devil at the height of 400 meters. (March 13, 1924). All the radders of his machine fell off and he lost control of it. For a while came back again the resistance on radders and he was safe. The author once ventured to walk into the dust storm. He felt some lift with his body. He estimated the wind velocity at 30m/s. People in this vicinity sometimes were injured in the dust storm, which perhaps due to the sudden fall of pressure of some 5 or 6mm of mercury. Once a dust storm passed on across a wcoden house at a corner of the Aerodrome. Windor panes were broken and fell outside. A part of the roof of the house was torn off.
The progressive motion of the dust storm seems to be controlled by thegeneral flow of air at that place.
It was known that dust storm sometimes accompani_??_s electric discharge. On March 19, 1924 an air-ship filled with hydrogen, somewhat old and impure, exploded. The day was especially marked with dust storm and the author believes that the airship, being painted with metalic powder, must have the same electric potential as_??_the surrounding air. On encountering a dust storm which ascended from the ground, the inevitable fate must be the electric discharge dué to the great potential differerence between the airship and the dust columns and consequent explosion of hydrogen.
Meteorological conditions when the dust storm oceurs. The occurrence of the storm is confined in February, March and April. Two or three hours after the sunrise the storm begins to be formed and it is most vigorous at the noontide, and gradually subsides at the sunset. The dust-storm never occurs when it is raining. The humidity of air during the occurrence is very low, never exceeds 40%. On that occasion the wind velocity is always small, and temperature is rather high. The dust storm preferably occurs on a day of anticyclonic weather in which morning temperature is rather low which followed by a sudden rise caused by the solar radiation.