Types of Precipitation: Rain, Snow, Sleet, Hail, and More

How Precipitation Forms Inside Clouds

Cloud droplets are tiny, typically 10 to 20 micrometers in diameter, about 100 times smaller than a raindrop. A single raindrop contains roughly one million cloud droplets. Clouds can persist for long periods without producing precipitation because these minute droplets fall extremely slowly and are easily sustained by even weak updrafts. For precipitation to occur, cloud droplets must grow by a factor of about 100 in diameter and one million in volume, a process that requires specific mechanisms beyond simple condensation.

The collision-coalescence process dominates in warm clouds (those entirely above 0 degrees Celsius). Larger cloud droplets, which form on larger condensation nuclei or through random collisions, fall faster than smaller ones. As they descend through the cloud, they collide with and absorb smaller droplets in their path, growing progressively larger. The collection efficiency increases as the collector drop grows, because larger drops create a stronger wake that sweeps smaller droplets into them. This positive feedback allows raindrops to reach diameters of 1 to 5 millimeters within 20 to 30 minutes in a vigorously convective tropical cloud.

The Bergeron process (also called the ice crystal process) operates in mixed-phase clouds containing both supercooled liquid water droplets and ice crystals. Supercooled water is liquid water that exists below 0 degrees Celsius, a common condition in clouds between 0 and minus 40 degrees Celsius. The key physics is that the saturation vapor pressure over ice is lower than over liquid water at the same temperature. This means that air which is saturated with respect to a water droplet is supersaturated with respect to an ice crystal. Water vapor molecules migrate from the liquid droplets (which slowly evaporate) toward the ice crystals (which grow by deposition). The ice crystals grow at the expense of the surrounding water droplets, eventually becoming heavy enough to fall.

As ice crystals fall through the cloud, they may collide with supercooled water droplets (a process called riming) or with other ice crystals (aggregation). Riming produces graupel, small pellets of ice coated with frozen droplets. Aggregation produces snowflakes, as ice crystals stick together to form the intricate, branching structures we associate with snow. The largest snowflakes form near 0 degrees Celsius, where a thin film of liquid on the crystal surfaces acts as glue during collisions.

Rain

Rain is liquid precipitation with drop diameters of 0.5 millimeters or greater. Drops smaller than 0.5 millimeters are classified as drizzle. In temperate and polar regions, most rain actually begins as snow in the upper portions of clouds and melts as it falls through a layer of above-freezing air. In tropical regions, rain often forms entirely through the collision-coalescence process without an ice phase.

Raindrop size varies with the type of storm. Stratiform (widespread, steady) rain produces smaller, more uniform drops averaging 1 to 2 millimeters. Convective (showery, intense) rain produces a wider range of sizes, with drops frequently reaching 3 to 5 millimeters. Drops larger than about 5 millimeters become unstable and break apart into smaller fragments. The terminal velocity of a raindrop depends on its size: a 1-millimeter drop falls at about 4 meters per second, while a 5-millimeter drop falls at about 9 meters per second.

Rain intensity is measured in millimeters (or inches) of depth per hour. Light rain is less than 2.5 millimeters per hour, moderate rain is 2.5 to 7.6 millimeters per hour, and heavy rain exceeds 7.6 millimeters per hour. The heaviest rainfall rates in intense thunderstorms and tropical cyclones can exceed 100 millimeters per hour in extreme cases, though such rates rarely persist for more than a few minutes.

Snow

Snow forms when ice crystals grow through deposition and aggregation within clouds at temperatures below freezing and the entire column of air between cloud and ground remains at or below 0 degrees Celsius. Snow crystal shape depends primarily on temperature and humidity at the level where the crystal forms. Plates and stars form near minus 2 degrees, columns and needles near minus 5 degrees, and the classic six-armed dendrites near minus 15 degrees, where the branching growth rate is highest.

The snow-to-liquid ratio describes how many centimeters of snow produce one centimeter of water equivalent when melted. This ratio varies enormously depending on temperature. Cold, dry snow at minus 20 degrees may have a ratio of 20:1 or higher, meaning 20 centimeters of snow melts to just 1 centimeter of water. Warm, wet snow near 0 degrees may have a ratio of 5:1 or less. The commonly cited 10:1 ratio is a rough average that frequently does not apply to specific storms.

Lake-effect snow occurs when cold air passes over a large, relatively warm lake. The warm water heats and moistens the lowest layer of the atmosphere, destabilizing it and generating convective snow bands that deposit heavy snowfall on the downwind shore. Cities like Buffalo, New York and Marquette, Michigan routinely receive lake-effect snowfalls of 30 centimeters or more from individual events, with seasonal totals exceeding 250 centimeters.

Sleet and Freezing Rain

Sleet (ice pellets) and freezing rain both occur when a warm layer aloft causes snowflakes to melt into raindrops, but the temperature profile near the surface determines which form reaches the ground. The critical difference is the depth and coldness of the subfreezing surface layer.

Sleet forms when the subfreezing surface layer is deep enough (typically more than 1,000 meters) and cold enough for the raindrops to completely refreeze before reaching the ground. The result is small, translucent ice pellets that bounce when they hit surfaces. Sleet accumulations are generally minor because the pellets do not adhere to surfaces, but heavy sleet can make roads as slippery as glazing ice.

Freezing rain occurs when the subfreezing surface layer is too shallow or too marginally cold for the raindrops to refreeze in the air. The drops remain liquid until they contact a surface at or below 0 degrees Celsius, at which point they freeze on contact, creating a layer of glaze ice. Even a few millimeters of ice accumulation can make roads extremely dangerous, snap tree branches, and bring down power lines. Major ice storms that deposit 25 millimeters or more of ice can cause catastrophic damage to forests and electrical infrastructure over wide areas.

Hail

Hail forms exclusively in thunderstorms with strong updrafts that can suspend ice particles in the cloud long enough for them to grow through repeated cycles of riming. A hailstone begins as a small ice particle or frozen raindrop that is carried upward by the storm's updraft into regions rich in supercooled water droplets. As the particle moves through these zones, it collects layers of ice. If the updraft weakens or the stone grows too heavy, it falls to the ground.

Hailstone size depends primarily on updraft strength. Marginal updrafts produce pea-sized hail (about 6 millimeters). Strong supercell updrafts exceeding 30 meters per second can produce golf-ball-sized hail (about 45 millimeters). The largest hailstones on record, exceeding 15 centimeters in diameter and weighing nearly a kilogram, require exceptionally powerful updrafts of 50 meters per second or more. Cross-sections of large hailstones reveal alternating layers of clear and opaque ice, reflecting different growth conditions encountered during multiple cycles through the cloud.

Hail causes billions of dollars in damage annually worldwide, primarily to agriculture, automobiles, and roofing. The central United States receives the most frequent large hail globally, with the region from Texas to South Dakota sometimes called "Hail Alley." Hailstorms are most common during late afternoon in spring and early summer, when surface heating and atmospheric instability are greatest.

Measuring Precipitation

Standard rain gauges collect precipitation in a cylindrical container and measure the depth of accumulated water. The simplest design is a funnel atop a graduated cylinder, read manually. Tipping-bucket gauges automatically record each 0.25 millimeters of rainfall by counting the tips of a small calibrated bucket. Weighing gauges measure the total weight of collected precipitation, which works for both rain and snow without the need for melting.

Weather radar estimates precipitation over wide areas by measuring the intensity of microwave energy reflected back from raindrops and snowflakes. The returned signal strength (reflectivity) correlates with the size and number of precipitation particles, allowing estimation of rainfall rate. Dual-polarization radar improves these estimates by sending horizontal and vertical pulses, enabling the radar to distinguish between rain, snow, hail, and non-meteorological echoes like insects or birds.