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The water cycle tells us about the movement of water between the atmosphere and Earth's surface.
Evaporation is one part of this cycle.
Evaporation occurs when water molecules in liquid water escape into the air as water vapor, a gas.
Some of the water vapor in the air begins to condense and form clouds.
Condensation is the process by which molecules of water vapor
are cooled and change from a gas to a liquid.
The liquid water drops collect in the clouds until they are heavy enough to fall as precipitation.
Precipitation is any form of water that falls from the clouds onto Earth's surface.
It may fall as rain, sleet, snow, hail,
or freezing rain depending on the atmospheric conditions present.
The water that falls can soak in or run off the soil.
Some will move back into the oceans, lakes, and streams.
Different clouds form when water vapor condenses to become liquid water or ice crystals.
As air cools, the amount of water vapor a cloud can hold decreases.
Aerosols, which are tiny particles like dust, salt crystals, or smoke must be
in the air so the water vapor has a surface onto which it can condense.
The types of clouds that form depend on the atmospheric conditions.
Precipitation always comes from clouds,
but not all clouds produce precipitation because the droplets may not be heavy enough to fall.
The shape of raindrops isn't the teardrop shape most people picture.
Raindrops typically have a parachute or jellyfish shape,
with most of the water around the rim or edges of the drop, while the upper
part contains a thin layer of water.
When raindrops fall in precipitation, their size is influenced by what happens to them as they fall.
Droplets can grow by colliding or combining with others.
While they vary in size, raindrops are typically around five millimeters in diameter,
with the largest recorded between 8 to 10 millimeters in diameter.
Drops less than 0.5 millimeters in diameter are classified as mist or drizzle.
Another factor in drop size is the number and types of particles in the air.
Large drops form over oceans because there are fewer particles,
while smaller drops form in smoky air due to there being a great amount of particles.
To study how clouds may affect the Earth's climate, scientists
use data from the Cloud and the Earth's Radiant Energy System (CERES) project.
Students can get involved in a citizen science project with NASA's GLOBE program.
Learners will apply their knowledge of the water cycle and understand through
first-hand experimentation why multiple sources of data are necessary to study Earth Systems.
Find out more in the NASA's Real World: Measuring Raindrops Educator Guide.
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