A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Precipitation forms through two main processes: the Bergeron-Findeisen process in cold clouds where ice crystals grow at the expense of supercooled water droplets, and collision-coalescence in warm clouds where droplets collide and merge until heavy enough to fall. Precipitation type at the surface depends on the temperature profile of the air column below the cloud: rain falls through entirely above-freezing air; snow when the column is below freezing throughout; sleet forms when snow melts then refreezes in a cold layer near the surface; freezing rain occurs when snow melts but the surface layer is too thin to refreeze the drops in the air. Hail forms in strong updrafts in thunderstorms where ice embryos cycle repeatedly through the cumulonimbus.
Draw temperature profiles for each precipitation type and trace the water phase through each layer. Compare warm-process versus cold-process clouds using real tropical versus mid-latitude examples.
You know from cloud formation that water vapor condenses onto nuclei to form cloud droplets, but those tiny droplets — typically 10–20 micrometers across — are far too small to fall as rain. Something must make them grow by a factor of a million in volume before they become precipitation. The atmosphere uses two fundamentally different growth mechanisms depending on cloud temperature, and understanding both is the key to predicting what falls from the sky.
In warm clouds (entirely above 0°C, common in the tropics), the collision-coalescence process does the work. Larger droplets fall faster than smaller ones, sweeping them up in a chain reaction. A droplet that starts slightly larger — perhaps because it formed on a giant sea-salt nucleus — collects smaller droplets as it descends through the cloud, growing rapidly until it is heavy enough to fall as rain. This process is efficient in deep, warm, maritime clouds where droplets are large and varied in size, but it struggles in thin or continental clouds where droplets are numerous but uniformly small.
In cold clouds (with temperatures below 0°C, which includes most mid-latitude precipitation-producing clouds), the Bergeron-Findeisen process dominates. This mechanism exploits a remarkable physical asymmetry: the saturation vapor pressure over ice is lower than over liquid water at the same subfreezing temperature. When ice crystals and supercooled water droplets coexist in a cloud, the air can be supersaturated with respect to ice while undersaturated with respect to liquid water. The ice crystals grow by vapor deposition while the liquid droplets evaporate, effectively transferring mass from droplets to ice crystals. The crystals grow into snowflakes, which may aggregate into larger flakes or acquire a coating of supercooled water (riming) as they fall.
What reaches the ground depends entirely on the temperature profile of the air below the cloud. If temperatures stay below freezing from cloud to surface, snow reaches you. If the entire column below the cloud is above freezing, snow melts into rain. The interesting cases involve layered temperature profiles. If snow falls through a warm layer aloft (above 0°C) that melts it into rain, and then enters a deep cold layer near the surface (below 0°C), the rain refreezes into ice pellets — sleet. But if that near-surface cold layer is too shallow for the drops to refreeze in mid-air, they arrive at the surface as liquid and freeze on contact with cold objects — freezing rain, one of the most hazardous precipitation types. Hail follows a different path entirely: strong thunderstorm updrafts loft ice embryos repeatedly through the cloud, where each pass adds a new layer of ice until the stone is too heavy for the updraft to support and it falls to the ground.