Here is a good read "Lightning between Earth and Space". Basically it's lightning that goes up not down from the clouds. Since our sky is Blue it's hard to see it. <A HREF="http://www-star.stanford.edu/~vlf/optical/press/mende97sciam/#author" target="_new">http://www-star.stanford.edu/~vlf/optical/press/mende97sciam/#author</A>