Clouds and Precipitation

Clouds are the visible signature of rising, cooling air, and the microscopic drama of droplets and ice crystals growing within them determines whether they merely drift by or release their water as rain, snow, or hail.

Definition

Clouds are visible suspensions of water droplets or ice crystals formed when moist air is cooled to saturation, and precipitation is the water that falls from them once particles grow large and heavy enough to overcome updrafts.

Scope

This topic covers the formation and classification of clouds, the nucleation of droplets and ice crystals on aerosols, and the microphysical processes, including collision-coalescence and the Bergeron-Findeisen ice process, by which cloud particles grow into precipitation.

Core questions

• How and on what particles do cloud droplets and ice crystals form?
• How are clouds classified by altitude and form?
• How do small cloud droplets grow into raindrops?
• What role do ice processes play in producing precipitation?

Key theories

Collision-coalescence (warm rain) process

In clouds warmer than freezing, larger droplets fall faster, collide with and absorb smaller ones, and grow by coalescence until they are heavy enough to fall as rain.

Bergeron-Findeisen ice process

In mixed-phase clouds the lower saturation vapor pressure over ice causes ice crystals to grow at the expense of supercooled droplets, an efficient pathway producing much of Earth's precipitation outside the tropics.

Mechanisms

Clouds form when rising, cooling air saturates and water vapor condenses onto cloud condensation nuclei to make droplets, or deposits onto ice nuclei to make crystals. These particles are too small to fall, so growth must occur: in warm clouds by collision and coalescence of droplets, and in cold or mixed-phase clouds by the Bergeron-Findeisen process, in which ice crystals grow rapidly while supercooled droplets evaporate. Once particles are large enough to overcome the updraft, they fall as rain, snow, sleet, or hail.

Clinical relevance

Understanding cloud and precipitation processes underlies quantitative precipitation forecasting, radar interpretation, aviation icing prediction, and weather-modification efforts such as cloud seeding, and it informs how clouds influence the planet's energy balance and climate.

History

Luke Howard's early-nineteenth-century cloud classification gave clouds their enduring names; in the twentieth century Bergeron and Findeisen identified the ice-crystal mechanism of precipitation, and detailed laboratory and field studies summarized in works such as Pruppacher and Klett established the modern microphysics of clouds and precipitation.

Key figures

• Luke Howard
• Tor Bergeron
• Walter Findeisen

Seminal works

• rogers1989
• pruppacher1997

Frequently asked questions

Why do some clouds rain and others do not?

A cloud only produces precipitation when its droplets or ice crystals grow large enough to fall against the updraft; this requires enough time, moisture, and the right microphysical processes, so many clouds simply evaporate without ever raining.

How do clouds get their names?

Clouds are classified by the system Luke Howard introduced in 1802, using Latin roots such as cumulus for heaped clouds, stratus for layered clouds, and cirrus for wispy high clouds, combined with prefixes for altitude and rain.