Clouds are visible masses of tiny liquid water droplets or ice crystals suspended in the atmosphere. They form when water vapor cools and condenses around microscopic particles like dust or salt. Precipitation can, and often does, form high up in the atmosphere, sometimes far above the main cloud layer visible from the ground, addressing the misconception that precipitation only begins at the cloud base.
The Mechanics of Precipitation
For water droplets or ice crystals to fall as precipitation, they must grow large enough to overcome the upward drag of air currents within the cloud. Simply condensing from vapor is not enough; a typical cloud droplet is about 100 times smaller than a raindrop and remains suspended. Two main processes facilitate this growth into a falling particle, depending on the cloud’s temperature.
In warm clouds, where temperatures are above freezing, the Collision-Coalescence process dominates. Larger droplets fall faster than smaller ones, colliding with and absorbing them as they descend, rapidly increasing the droplet’s mass until it can fall out of the cloud.
In cold clouds, where temperatures are below freezing, the Bergeron Process, also known as the ice-crystal process, is more efficient. This process relies on the coexistence of supercooled water droplets and ice crystals. Since the saturation vapor pressure is lower over ice than over liquid water, water vapor rapidly moves from the liquid droplets to deposit onto the ice crystals. This causes the ice to grow quickly at the expense of the surrounding liquid. These ice crystals eventually become heavy enough to fall, often melting into rain before reaching the surface.
Precipitation That Never Reaches the Ground
Precipitation frequently begins high above the ground, but the atmosphere below the cloud often prevents it from completing its descent. The phenomenon is known as Virga, a Latin word meaning “rod” or “branch,” which describes the wispy streaks of precipitation seen hanging beneath a cloud. Virga forms when the air beneath the cloud is extremely dry and sometimes warm. As liquid raindrops fall into this low-humidity environment, they evaporate, turning back into water vapor. If the precipitation is in the form of ice or snow, it can sublimate, changing directly from a solid to a gas.
High-altitude clouds, such as Cirrus, are composed entirely of ice crystals and constantly produce precipitation. However, the extremely cold and dry air below the cirrus causes these crystals to sublimate almost immediately.
The Upper Limit of the Water Cycle
The practical ceiling for significant precipitation and weather phenomena is the Tropopause, the boundary between the Troposphere and the Stratosphere. The Troposphere is the lowest layer of the atmosphere and contains nearly all of the atmosphere’s water vapor. Weather processes like cloud formation and rain are confined to this layer.
The Tropopause acts as a physical boundary because the temperature trend reverses here: temperature decreases in the Troposphere but increases in the Stratosphere. This temperature inversion prevents the vertical movement of air, effectively capping the upward motion of water vapor. Water vapor that reaches the Tropopause is condensed out in the “cold trap,” limiting moisture entry into the Stratosphere.