Storm clouds can form when humid, sun-warmed air near the ground rises to meet cooler air above. As these distinct air masses churn together, atoms near the top of the cloud become positively charged, while those near the bottom become negatively charged. At the same time, the negative charge at the cloud's bottom repels negative charges on the ground, causing the area directly below the cloud to take on a positive charge. When the bottom of the cloud has accumulated enough of a negative charge, electrons in the cloud move rapidly toward the ground, completing the cloud-ground connection. As the electrons move downward, they collide with atoms and molecules in the air. It's these high-energy collisions that produce the characteristic crackling light we associate with lightning strikes.
In this way, lightning strikes aren't any different from other static electricity shocks. Negative charges build up in one area; when they reach high enough levels, electrons jump to a more positively charged area. Such is the case when your feet and body acquire a negative charge from a carpeted floor. When your hand, for example, comes close to an object with a positive charge, like a metal doorknob, electrons jump from your skin to the object -- sometimes, as we all know, painfully so.
Excess electrons, whether on your skin or accumulated near the base of a thundercloud, jump to some substances far more easily than others. Many metals, like the copper wire attached to the rocket in this video segment, conduct electrical charges quite well. This is because the electrons surrounding their atoms move easily from one atom of the material to another. As a result, electrons at the base of the clouds into which the lightning researchers launch their rockets readily follow the path the copper wire provides.
The ability of some metals to attract lightning strikes allows scientists and engineers to analyze in great detail this remarkable, yet poorly understood phenomenon. By doing so, they may discover ways to better protect people and buildings from electrical storms, and to perhaps one day harness lightning's tremendous electrical energy as a renewable energy source for humans.