In moist air the breakdown field is much smaller. We try to concentrate on the main features only. Since lightning is a breakdown of the air by electric fields we need to know the electric field at each point in space. We model the whole thing by breaking space up into a grid. When the random number, r, in some region is large, then the square is brighter and it is easier for this region to break down.
Dark squares correspond to regions where it is harder for breakdown to occur.
It is because we don't know which is more important, the field or the fluctuations in the properties of the air. We can vary to power alpha to change the relative importance of electric field or of the random number. Run the simulation and see which value of alpha gives results that look most like real lightning.