Defines an area of rain.

num_drops: The number of drops of rain in the
	   area (maximum)

ramp_up: The time (in seconds) over which
	 the area should ramp up to the
	 maximum number of drops.
	 -1 in this field will make the rain start
	 and stay at max (num_drops).

ramp_down: The time (in seconds) over which
	   the area will ramp down from the
	   maximum number of drops.
	   -1 in this field will make the rain stay
	   at max after ramping up from 0.