Precipitation is the process by which atmospheric water vapor, condensed into clouds, falls back to the Earth’s surface due to gravity. This return of water is a fundamental part of the global water cycle, continuously circulating water between the atmosphere and the land or oceans. The specific form the precipitation takes—whether liquid, solid, or a mixture—depends entirely on the temperature profile of the air column from the cloud to the ground.
Forms of Liquid Precipitation
Liquid precipitation reaches the ground as water droplets, requiring the entire air column from the cloud base to the surface to maintain a temperature above freezing. The two primary forms are rain and drizzle, distinguished mainly by the size of their droplets and intensity.
Rain consists of drops typically larger than 0.5 millimeters in diameter, falling at a relatively fast rate. These larger drops usually form in thicker clouds where droplets have had sufficient time to collide and coalesce.
Drizzle is composed of numerous fine droplets, each less than 0.5 millimeters across, which fall much more slowly, often appearing to float or drift. This lighter form often originates from shallow, low-lying stratus clouds that lack the strong vertical air currents needed to grow droplets into full-sized raindrops.
Forms of Solid Precipitation
Precipitation that remains completely frozen as it descends is classified as solid, a category dominated by snow and hail. Snow requires the atmospheric temperature to be at or below the freezing point throughout the entire descent, preventing any melting.
Snow begins in the cloud when water vapor deposits directly onto a tiny nucleus, such as a dust particle, forming a hexagonal ice crystal through deposition. As the crystal falls, it continues to grow as more water molecules freeze onto it, and it may aggregate with other crystals to form a snowflake. The intricate shape of the resulting snow crystal is highly dependent on the temperature and humidity conditions it encounters.
Hail forms under dramatically different conditions within strong thunderstorms known as cumulonimbus clouds. Hailstones start as ice particles lofted by powerful updrafts into the extremely cold upper regions of the storm.
As the ice particle is repeatedly carried up and falls back down, it collides with supercooled water droplets that freeze onto its surface, causing it to grow. This cyclical process builds up layers of ice until the hailstone becomes too heavy for the updraft to support, causing it to fall to the ground.
Mixed and Transitional Forms
Sleet and freezing rain are transitional forms involving phase changes on their way to the surface. Both require a temperature inversion where a layer of warm air sits above a layer of sub-freezing air near the ground.
Sleet, technically ice pellets, forms when snow falls into the elevated warm layer and melts completely into a raindrop. The raindrop then falls into a sufficiently deep layer of freezing air near the surface, allowing it time to refreeze into a solid ice pellet before impact.
Freezing rain occurs when the warm layer is deep, melting the snow into rain, but the sub-freezing layer near the ground is very shallow. The raindrops pass through this shallow cold layer without fully freezing, becoming supercooled liquid water.
These supercooled drops remain liquid until they strike an object that is at or below freezing, such as a road or tree branch. They instantly freeze on contact to form a glaze of clear ice. The thickness of the cold air layer near the surface determines whether the precipitation freezes in the air as sleet or freezes upon impact as freezing rain.