A theory of thunderstorm charge separation advanced by C. T. R. Wilson (1916). According to this theory, the lower negative charge of a thundercloud is generated by the accumulation there of raindrops that have captured predominantly negative ions in their descent through the cloud. The preferential capture of negative ions by such drops is said to be due to the polarization of the drops in the normal atmospheric electric field existing between the negatively charged earth and positively charged ionosphere. The lower halves of the falling drops therefore would attract and capture negative charges while their upper halves would be unable to draw in positive charges with comparable efficiency; thus a net negative charge would build up on the drops. This theory is generally regarded today as incapable of accounting for any important portion of thunderstorm charge separation, for it is quantitatively inadequate in view of typical ion densities. See precipitation current.