6 Precipitation Processes and Types

How precipitation develops

Precipitation develops when moist air rises and cools, forming cloud droplets or ice crystals. These particles are initially too small to fall, so they must grow first. In warmer clouds, droplets collide and merge; in colder clouds, ice crystals grow as water vapor deposits on them.

The temperature layers between the cloud and the surface largely determine which form reaches the ground. Depending on those layers and on how the particles grow, precipitation may arrive as rain, snow, sleet, freezing rain, or other forms.

Forms of precipitation

Rain consists of liquid drops. It can form when droplets merge in a warm cloud or when snow crystals melt as they fall through warmer air. Drizzle consists of very small, closely spaced drops and often falls from low clouds.

Snow consists of ice crystals, often joined into flakes. It reaches the ground as snow when it does not melt completely on the way down. Snowflake shapes reflect the conditions in which the crystals grew.

Sleet (ice pellets, in U.S. usage) and freezing rain both pass through a warm layer aloft, where falling snow melts. Their difference depends on the depth of the cold air near the surface: a sufficiently deep subfreezing layer lets the drops refreeze into hard pellets before reaching the ground, while a shallow layer allows them to reach the ground as liquid and freeze on contact. Freezing drizzle is made of tiny liquid drops that also freeze on contact.

Hail grows inside strong thunderstorms. Updrafts carry ice particles through regions containing supercooled water droplets, which freeze onto the particles. Repeated growth can produce large, layered stones; hail falls when it becomes too heavy for the updraft to support it.

Graupel (snow pellets) forms when supercooled droplets freeze onto snow crystals, creating soft, opaque pellets. It is generally smaller and more fragile than hail. Virga is precipitation that evaporates before reaching the ground, so it is not surface precipitation.

Measuring precipitation and snow

Precipitation amount is usually reported as the depth of liquid water accumulated over a stated period. A rain gauge collects precipitation through a known opening. Observers read the water level, while automated gauges can record accumulation and rate. Frozen precipitation can be collected and melted to measure its liquid-water equivalent. For example, a liquid-water equivalent of $10\ \text{mm}$ represents a layer of water $10\ \text{mm}$ deep over a level surface, whether the precipitation fell as rain or snow.

Snowfall depth and snow depth are separate measurements. Snowfall depth is the depth of newly fallen snow on a representative, level surface, often measured with a snow board. Snow depth is the snow remaining on the ground at the time of observation. They are not interchangeable: snow depth varies with snow density, settling, melting, and wind, so a fixed snow-to-water conversion is only an approximation.

Hail size is commonly described by its diameter. A rain gauge’s melted reading alone does not describe hailstone size or accumulation.