More On Cloud Microphysics

The formation of precipitation produces some other interesting phenomena. When very large precipitation particles are produced, their shapes can be very different from the spherical shape of small liquid droplets. The force of the moving air on a falling water droplet produces significant flattening of the shape when it becomes larger than about 1000 microns (1 mm). On the other hand, snowflakes often grow in a planar shape; colliding snow flakes can produce even more distorted shapes. When such large asymmetric particles fall through the air, the forces of the moving air try to align the particle in an orientation of maximum drag, with their largest dimension horizontal, but this position is unstable. In this situation, the particle falls by "side-slipping" as a compromise between gravity and the air resistance; thus, snow flakes flutter or "swirl" as they fall. In violent storms with collisions between ice and liquid particles, the background electrical charges (ions maintained by cosmic rays) are separated inducing very large voltages. When the voltages between different parts of the cloud or between the cloud and the ground reach sufficiently large values, the charge separation is suddenly eliminated by a current flow, which we call lightning. We hear the resulting shock wave as thunder. The frequency of lightning is greatly increased by stronger vertical motions, producing rapid particle collisions and heavy precipitation; lightning occurs much more frequently over land than over oceans.