THE NON-EQUILIBRIUM ELECTRIFICATION OF RAINDROPS BY THE ASSOCIATION OF CHARGED CLOUD DROPLETS

Abstract

Whenever the number of collisions experienced by raindrops growing by coalescence with cloud droplets is limited either by a finite distance of fall or by the drop size, then the accumulated electrical charge is similarly limited. It is shown that under these non-equilibrium conditions the accumulated drop charges, averaged without regard to sign, are proportional to the charge on the parent-cloud droplets and to the square root of the number of charged transferring collisions; whereas the accumulated net charges are directly proportional to the number of collisions. When the parent-cloud droplets are weakly electrified, there are usually too few collisions to accumulate random equilibrium charges on a typical raindrop. On the other hand, equilibrium may be established on raindrops formed from randomly charged cloud droplets that are, themselves, highly electrified by atmospheric electric fields. Whenever the population density of the positively and negatively charged parent cloud droplets are notably different, drop charges as large as those usually observed may be established even when the number of collisions is limited. The analysis shows that the non-equilibrium regime is important in the electrification of rain.