Precipitation of liquid water drops with diameters greater than 0.02 in. (0.5 mm). When the drops are smaller, the precipitation is usually called drizzle. Raindrops may form by the coalescence of colliding small water droplets or from the melting of snowflakes and other ice particles as they fall into warm air near the ground. Hawaii's Mount Waialeale, with a 20-year annual average of 460 in. (11,700 mm), is the Earth's wettest known point; the driest areas are in parts of deserts where no appreciable rain has ever been observed. Less than 10 in. (250 mm) and more than 60 in. (1,500 mm) per year represent approximate extremes of rainfall for all the continents.