The question of whether all rain begins as snow is common, and the answer is a nuanced “no,” though the majority of rain in many parts of the world does start as ice. Precipitation is any form of water falling from the atmosphere, governed by two main processes that depend heavily on cloud temperature. While most rain in temperate and polar regions originates as ice crystals high in the clouds, significant rainfall, particularly in tropical areas, forms without ever involving a frozen phase. The final form of precipitation—be it rain, snow, or ice—is determined by the temperature profile of the atmosphere below the cloud.
Rain Formation via Ice Crystals
The predominant mechanism for rainfall in mid-latitudes is the Bergeron process, sometimes called the “cold rain” process. This process relies on the co-existence of supercooled water and ice crystals within a mixed-phase cloud. Supercooled water droplets are liquid water that remains unfrozen below the typical freezing point of 0°C, often down to -40°C.
The saturation vapor pressure is lower over ice than it is over liquid water. This difference drives a process where ice crystals grow rapidly at the expense of the surrounding supercooled droplets. Water vapor molecules move from the liquid droplets toward the ice crystals, causing the droplets to evaporate while the ice crystals gain mass through deposition.
As the ice crystals grow heavier, they begin to fall from the cloud. On their way down, they may grow larger by colliding with and freezing onto supercooled water droplets, a process called riming. If the atmospheric temperature below the cloud remains entirely below freezing, the ice crystals reach the ground as snow. Conversely, if the falling ice crystals encounter air consistently above 0°C, they melt completely and arrive at the surface as rain.
Rain Formation Without Ice
Rain can also form through the collision-coalescence process, the primary source of precipitation in warmer, tropical regions. This “warm rain” process occurs in clouds where the temperature is entirely above freezing, meaning no ice crystals are involved. It requires clouds with a high liquid water content and a wide range of droplet sizes.
The process begins when a few larger-than-average droplets, often formed on giant condensation nuclei like large sea salt particles, begin to fall faster than their smaller neighbors. As these large collector drops descend, they collide with and absorb the numerous smaller, slower-moving droplets in their path. This phenomenon is called coalescence.
Each successful collision and coalescence rapidly increases the mass of the collector drop, which increases its fall speed and the likelihood of further collisions. Strong updrafts within the cloud help keep these growing droplets suspended longer, allowing them more time to sweep up smaller droplets. Eventually, the droplet becomes too large for the air resistance to support, reaching up to 5 millimeters before breaking apart, and falls to the ground as warm rain.
Why Precipitation Changes Form
The ultimate form of precipitation that reaches the ground is determined not by how it formed, but by the vertical temperature profile of the lower atmosphere beneath the cloud. If the precipitation starts as an ice crystal via the Bergeron process, its fate depends on the presence and depth of the melting layer, the region of the atmosphere above 0°C.
For precipitation to fall as snow, the air column from the cloud base to the ground must be consistently at or below freezing. If falling snow enters a warm layer and melts completely into a raindrop, but then passes through a shallow, sub-freezing layer near the surface, the rain can become supercooled. This supercooled rain freezes instantly upon striking cold surfaces, leading to a dangerous condition known as freezing rain.
Sleet, or ice pellets, occurs when the melted raindrop falls through a much deeper layer of sub-freezing air. This deep cold layer allows the raindrop enough time to re-freeze completely into a small pellet of ice before reaching the ground. Thus, the interplay of warm and cold layers dictates whether the initial ice crystal arrives as snow, melts into rain, or undergoes a refreezing cycle to create sleet or freezing rain.