There are four basic types of lightning damage: physical damage, secondary effect damage, electromagnetic effect damage, and damage caused by changes in ground reference potential.
Physical damage is caused by current flow and heat. A typical lightning strike in the United States conveys between 25,000 and 45,000 amps, with the higher amperage strikes occurring in the south, where the storms build higher. Lightning is high current flowing over a short period of time.
The core temperature of a lightning channel is approximately 50,000 degrees Fahrenheit, or about five times the surface temperature of the sun. During a strike, the temperature rises from the ambient temperature to a temperature approaching 50,000 degrees over a very short rise time. It is this heat which causes the sap in a tree struck by lightning to turn to steam and expand, splitting the tree. Concrete never quite dries out; there is always latent moisture in concrete. When a concrete structure is struck, the latent moisture turns to steam, expanding and damaging the concrete structure. When the air surrounding the lightning channel is heated this rapidly, it expands in a shock wave. This shock wave can damage a structure. This is why lightning rods have a minimum length – to top lift this shock wave off the roof of the protected structure.
The secondary effect of a lightning strike can cause arcing and induced currents. During a lightning strike, the point at which the strike occurs is relatively vacated of ground charge. The area surrounding the point of the strike remains highly charged, causing an almost instantaneous potential gradient across the area. The surrounding area releases its charge to the point at which the strike occurred, causing a flow of current. This current flow can arc across any gaps in its path. If that arc takes place within a flammable material, it can cause a fire or explosion. If the arc takes place within a bearing, such as in a pump in a treatment plant, it can scar the bearing and cause premature wear. If it takes place on a circuit board, it can damage the circuit board.