The Moving Velocity of Cumulus Humilis Clouds

Publisher Website
Google Scholar
Abstract
The horizontal movement of cumulus humilis clouds near Sapporo was measured by the stereophotogrammetric method in the summer seasons of the years 1969 to 1971 in Sapporo. The direction and speed of individual cloud movements were presented on charts. The environmental meteorological conditions were obtained from the Sapporo rawinsonde soundings. By comparing the analytical results of clouds with the environmental meteorological conditions, the following results were obtained. The direction of horiz ontal movement of the higher cumulus humilis clouds was nearly the same as that of prevailing wind direction at the cloud level (exactly speaking, near cloud base level), however, the direction of the lower clouds was not in agreement with the wind direction. The critical ground height for the agreement of cloud motion direction with wind direction was found to be about 700 meters. The speed of horizontal movement of cumulus humilis clouds was generally less than prevailing wind speed at the level of cloud base. It was also found that the speed of larger clouds was slower than that of the smaller clouds when their heights were the same. The general tendency was that the lower the clouds, the lower the motion speeds, and the greater the vertical wind shear, the lower the cloud speed. The slowness of cloud speed was qualitatively explained by considering the upward transportation of low horizontal momentum in thermal convections under a condition of the positive vertical wind shear.