Natural and artificial clouds

The motions which occur when an unstable layer of fluid breaks down have been found to be in the form of polygonal prismatic cells, in which the motion at the centre of the cell is upward when the fluid is a liquid. In air it is shown that while the motions in deep layers resemble those in liquids, except that the direction of motion is reversed, the motion in shallow layers of air made unstable by heating from below is different in appearance, and consists of a large number of ascending currents, surrounded by much slower descending currents. The motion of the air is made visible by cigarette smoke. When the air is bounded at its upper limit by a movable glass plate, it is found that the shearing produced by moving this plate will give long rolls extending through the whole length of the chamber, if the upper plate is moved sufficiently rapidly. With slower movements of the upper plate, the chamber is filled with distorted prismatic cells, and with very slow movement of the plate, the chamber is filled with rolls transverse to the direction of motion of the plate. These experimental results are applied to explain a variety of cloud forms, which are thus presumably to be explained as in part due to the effects of instability. Among these clouds are those which consist of small cloudlets on a background of blue sky, cloud sheets which show a series of clear holes, and clouds in rolls, which may be analogous to either the longitudinal rolls found in the laboratory with rapid shearing of the top plate, or to the transverse rolls found with slow shearing of the upper plate. Some comparison is given of the evidence of ascending currents in the atmosphere found by gliding pilots. A film exhibited at the meeting shows the forms taken by unstable layers during the process of breakdown, as well as of some clouds which closely resemble the structures obtained in the laboratory.