WEBVTT

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The water cycle tells us about the movement of
water between the atmosphere and Earth's surface.

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Evaporation is one part of this cycle.

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Evaporation occurs when water molecules in liquid
water escape into the air as water vapor, a gas.

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Some of the water vapor in the air
begins to condense and form clouds.

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Condensation is the process by
which molecules of water vapor

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are cooled and change from a gas to a liquid.

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The liquid water drops collect in the clouds until
they are heavy enough to fall as precipitation.

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Precipitation is any form of water that
falls from the clouds onto Earth's surface.

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It may fall as rain, sleet, snow, hail,

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or freezing rain depending on the
atmospheric conditions present.

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The water that falls can
soak in or run off the soil.

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Some will move back into the
oceans, lakes, and streams.

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Different clouds form when water vapor condenses
to become liquid water or ice crystals.

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As air cools, the amount of water
vapor a cloud can hold decreases.

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Aerosols, which are tiny particles like
dust, salt crystals, or smoke must be

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in the air so the water vapor has a
surface onto which it can condense.

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The types of clouds that form depend
on the atmospheric conditions.

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Precipitation always comes from clouds,

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but not all clouds produce precipitation because
the droplets may not be heavy enough to fall.

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The shape of raindrops isn't the
teardrop shape most people picture.

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Raindrops typically have a
parachute or jellyfish shape,

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with most of the water around the rim
or edges of the drop, while the upper

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part contains a thin layer of water.

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When raindrops fall in precipitation,  their size is influenced by what happens to them as they fall.

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Droplets can grow by colliding
or combining with others.

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While they vary in size, raindrops
are typically around five millimeters in diameter,

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with the largest recorded
between 8 to 10 millimeters in diameter.

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Drops less than 0.5 millimeters in
diameter are classified as mist or drizzle.

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Another factor in drop size is the
number and types of particles in the air.

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Large drops form over oceans
because there are fewer particles,

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while smaller drops form in smoky air due
to there being a great amount of particles.

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To study how clouds may affect
the Earth's climate, scientists

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use data from the Cloud and the Earth's
Radiant Energy System (CERES) project.

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Students can get involved in a citizen
science project with NASA's GLOBE program.

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Learners will apply their knowledge of
the water cycle and understand through

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first-hand experimentation why multiple sources
of data are necessary to study Earth Systems.

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Find out more in the NASA's Real World:
Measuring Raindrops Educator Guide.

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