Image taken
from: http://www.mthurricane.com/
Rain clouds become charged
through a complicated and largely unknown process. There are two major categories of hypotheses that describe the
process: those that include ice as a key factor and those that do not. The ice hypothesis holds that the charges
are a result of the separation of ice and water droplets within the cloud, as
experiments have shown that freezing a dilute solution of water causes the ice
to gain a negative charge and the water to keep a positive charge. Inside a cloud, rising air pulls small
droplets of water up and away from the larger falling ice particles, causing
the upper half of the cloud to gain a positive charge and the lower half to
gain a negative charge. Other
experiments have shown that small, slowly falling drops of water become
positively charged while larger, quickly falling drops of water become
negatively charged, thus causing the upper half of the cloud to gain a positive
charge while the lower half gains a negative charge. It is most widely believed that ice is a contributing factor in
the charging process, as lightning is not generally seen until ice has formed
in the upper layers of thunderclouds.