From the US Federal Meteorological Handbook-
Light
From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.
Moderate
Individual drops are not clearly identifiable; spray is observable just above pavements and other hard surfaces.
Heavy
Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of several inches is observed over hard surfaces.
Rather hard to apply while you are flying.