Shallow clouds and those in the tropics do not reach freezing level, so ice crystals don't form (A). Instead, a larger-than-average cloud droplet may coalesce with several million other cloud droplets to reach raindrop size. Electrical charges may encourage coalescence if droplets have opposite charges. Some raindrops then break apart to produce other droplets in a chain reaction, which produces an avalanche of raindrops. Most rainfall in the mid-latitudes is the result of snowflakes melting as they fall (B). It takes many millions of droplets and ice crystals to make a single raindrop or snowflake heavy enough to fall from a cloud. Yet a snowflake can be grown from ice crystals in only 20 minutes. Large hailstones need strong upcurrents of air in order to form (C). A 30 mm (1.2 in) diameter hailstone probably needs an updraft of 100 km/h (60 mph). The turbulent air currents in a thunderstorm turn a frozen water droplet into an embryonic hailstone. The abundant supercooled moisture droplets in a storm will readily freeze on to its surface. It is swept up and down by the currents and accumulates numerous layers of thick ice, which are alternately clear and milky. The opaque layer is made when air bubbles and sometimes ice crystals are trapped during rapid freezing in the cloud's cold upper levels. The clear layers form in the cloud's warmer, lower levels, where water freezes slowly. There can be as many as 25 layers in a hailstone (D) and the last –a clear layer of ice, which is often the thickest – develops as the hailstone falls through the wet, warm cloud base.
Precipitation is distinguished from cloud, fog, dew, and frost, in that it must fall and reach the ground. Measured by rain and snow gauges, the amount of precipitation is expressed in millimeters or inches of water depth.
Precipitation occurs with the condensation of water vapor in clouds into water droplets that coalesce into drops as large as 7 millimeters (0.25 inch) in diameter. It also forms from melting ice crystals in the clouds. Drizzle consists of fine droplets, and snow of masses of many-sided ice crystals. Sleet is formed when raindrops freeze into small ice pellets, and hail when concentric layers of ice in cumulonimbus clouds freeze, forming lumps measuring from 0.5–10 centimeters (0.2–4 inches) in diameter.