Rainfall rate is generally described as light, moderate or heavy.
Light rainfall is considered less than 0.10 inches of rain per hour.
Moderate rainfall measures 0.10 to 0.30 inches of rain per hour.
Heavy rainfall is more than 0.30 inches of rain per hour.
Rainfall amount is described as the depth of water reaching the ground, typically in inches or millimeters (25 mm equals one inch).
An inch of rain is exactly that, water that is one inch deep.
One inch of rainfall equals 4.7 gallons of water per square yard or 22,650 gallons of water per acre.