The large Tesla coil shown above is part of a demonstration show called "The Wonders of Physics" by physics professor Clint Sprott of the University of Wisconsin. The primary consists of a small number of loops made from a large conductor. The secondary has a large number of turns of small wire. The high frequency input to the primary coil is transformed up to about a million volts, and therefore the current is very small. The high voltage is enough to produce spectacular electric discharges like those shown below.
One of the predictable things about such discharges is that they are unpredictable! The variety of discharge patterns above were obtained in less than a minute. This was in an auditorium where the conditions of the air around the Tesla coil were fairly stable. This is an example of a chaotic phenomenon. It illustrates the futility of trying to predict the path of lightning strikes, since there are many more variables in that case.