Why is heavy rain always associated with tropical cyclones?

Question

Why is heavy rain always associated with tropical cyclones?

Answer 1

Tropical cyclones derive their energy from condensation of water vapor.  The water vapor comes from evaporation from very warm ocean waters and the warmer the ocean is the more water vapor that can be evaporated into the air, so the stronger the tropical cyclone will be.  This condensation of water vapor primarily occurs in the cumulonimbus rainclouds of the storms eyewall, so these storms can be very big rain producers, since the winds that are feeding them are so moisture-laden.

All tropical cyclones are not created equal, though, and some produce a lot more rain than others.  The ones that produce the most rain are typically large and slow moving.