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Answer: Precipitation happens when cloud droplets grow heavy enough to fall. Clouds contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as precipitation, but they are large enough to form visible clouds.

How a raindrop forms

• First, tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus.
• Water droplets may grow as a result of additional condensation of water vapor when the particles collide.
• If enough collisions produce a droplet with a fall velocity that exceeds the cloud updraft speed, it falls out of the cloud as precipitation.
• This is not a trivial task, since millions of cloud droplets are required to produce a single raindrop.

Most of the condensed water in clouds does not fall as precipitation because its fall speed is not large enough to overcome the updrafts that support the clouds.

Why it matters

Rainfall is the main way that the water in the skies comes down to Earth, where it fills our lakes and rivers, recharges the underground aquifers, and provides drinks to plants and animals.

How much falls

The amount of precipitation that falls around the world may range from less than 0.1 inch per year in some deserts to more than 900 inches per year in the tropics. One of the driest spots on Earth is Iquique, Chile, where no rain fell for 14 years. The world's wettest spot, Mawsynram, in Northeastern India, experienced a mean of 494 inches of rain a year during the years 1989-2010.

Claims

• id: C1 claim: Water is continually evaporating and condensing in the sky. source: S1 evidence: "Water is continually evaporating and condensing in the sky."
• id: C2 claim: Cloud droplets are too small to fall as precipitation. source: S1 evidence: "These droplets are way too small to fall as precipitation"
• id: C3 claim: Rain and snow are key elements in Earth's water cycle. source: S2 evidence: "Rain and snow are key elements in the Earth's water cycle"

Sources

• [S1] Precipitation and the Water Cycle. USGS. https://www.usgs.gov/special-topics/water-science-school/science/precipitation-and-water-cycle . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Rain and Precipitation. USGS. https://www.usgs.gov/special-topics/water-science-school/science/rain-and-precipitation . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_cqgcf0w4,
  author = {{iinventedlight}},
  title = {{Precipitation: how cloud droplets become rain}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/precipitation/},
  note = {Permanent ID iil:note:cqgcf0w4. License: CC-BY-4.0.},
}

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