During the 20th century, research scientists, electric-utility companies, and weather bureaus have joined forces to study the causes and occurrences of lightning. Many useful facts have been learned. However, it is a complicated subject, and much more work must be done before lightning is fully understood. The Buildup and Discharge of Lightning Great electrical changes occur in the atmosphere when a large cloud forms above the Earth. Inside the cloud, rapid heating and cooling of air masses produce violent activity.
Rain falls, air currents rise, and water droplets freeze.
Some drops of water in the cloud become charged with positive electricity. These are carried to the upper part of the cloud by air currents and form a positive-charge center. Ice crystals that are formed have a negative charge. Due to their weight, they move down and produce a negative-charge center at the base of the cloud. To balance this negative charge, positive charges on the ground are concentrated just below the cloud.