If the air at about 10,000 feet also is moist, and temperatures decrease rapidly in the air above, the tiny cumulus cloud we see forming as a flat-bottomed head of cauliflower can grow to heights of 40,000 feet. At that level, where the stratosphere begins, stable air causes the ice crystals within the cloud to flatten out on top like an anvil. How lightning is generated is still one of the wonderful mysteries of science. Separate regions within the columnar, anvil-topped cloud, now called a cumulonimbus, contain opposite electrical charges. Scientists theorize these charges may be formed when hailstones collide with colder ice crystals, leaving the hailstones negatively charged and the ice crystals positively charged.
Hailstones fall to the bottom of the cloud, which becomes negatively charged.
The cloud causes the ground surface below it to be positively charged. Protruding objects such as trees, buildings and humans in open fields often build up a more dense positive charge than flat ground, setting up an electric potential for current to flow through. The lightning strike we see is actually a series of steps too quick for our eyes to distinguish. Charged electrons jump from the bottom of a cloud in an invisible line about 150 to 300 feet long. When the path is complete, large amounts of electrons flow to the ground.