This theory assumes that many ions are present in the atmosphere which get themselves attached to tiny dust and water particles. It further assumes the presence of an electric field in the earth’s atmosphere during fair weather directed towards the earth. The field intensity is roughly 1 V/cm at the earth’s surface and progressively decreases with height so that at 10,000 m it is only about 0.02 V/cm. Because of the atmospheric electric field, raindrops become polarized, the upper side gets a negative charge and the lower side a positive charge as shown in figure 4.4(a)
Later, the lower part of the drops attracts negative ions from the atmosphere, acquiring an overall negative charge while leaving an abundance of positive charges in the air. The upward motion of air currents takes positively charged air and smaller drops to the top of the cloud. Heavier rain drops settle on the base of the cloud as shown in figure 4.4(b). Thus, according to Wilson’s theory, a thundercloud is bipolar with a positive charge at the top and negative at the bottom, normally separated by several kilometers. When the electric field strength becomes more than the breakdown value, lightning discharge is commenced.