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📘 Rain begins with water vapor

Rain's raw material is water vapor, an invisible gas supplied by evaporation from oceans, lakes, soil, and plants.

4
lessons
~20 min
to learn
Adults
level
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What you’ll learn

  1. Vapor and dropletsTrace water vapor, nuclei, and suspended cloud droplets.Cooling and aerosol surfaces turn invisible vapor into many tiny suspended drops.
  2. Warm-rain growthExplain collision and coalescence in liquid clouds.Different fall speeds let drops collide and merge until gravity can carry them downward.
  3. Cold clouds and fallDescribe ice-process growth and editing during descent.Crystals and supercooled droplets grow aloft; warm or dry layers can melt or evaporate precipitation.
  4. Storm and surfaceConnect rain rate, storm organization, and the water cycle at the surface.Drop size and storm structure set intensity; rainfall returns water to land and ocean.

Questions this course answers

What starts cloud-droplet formation?

Cooling vapor condenses on aerosol particles that provide surfaces for droplets to form.

Why do larger cloud droplets collide with smaller ones?

Different fall speeds create relative motion that enables collisions.

What is coalescence?

Coalescence is the joining of droplets into a larger drop after collision.

How can snow become rain?

Ice precipitation melts when it passes through air above freezing.

Why might radar detect precipitation that never reaches the ground?

Evaporation below the cloud can produce virga before particles reach the surface.

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