With a mean annual rainfall of 6,813 mm (268 in) on 182 days a year, a high level even for the West Coast, Milford Sound is known as the wettest inhabited place in New Zealand and one of the wettest in the world. Rainfall can reach 250 mm (10 in) during a span of 24 hours. The rainfall creates dozens of temporary waterfalls (as well as a number of major, more permanent ones) cascading down the cliff faces, some reaching a thousand metres in length. Smaller falls from such heights may never reach the bottom of the sound, drifting away in the wind.
Accumulated rainwater can, at times, cause portions of the rain forest to lose their grip on the sheer cliff faces, resulting in tree avalanches into the sound. The regrowth of the rain forest after these avalanches can be seen in several locations along the sound
