Every fact in the three water cycle lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the numbers are less certain or simplified: the shares of water (about 96% in the oceans, about 0.001% in the air) and the 90/10 split between evaporation and transpiration are rounded global estimates from the USGS. The yearly world rainfall of about 980 mm (NOAA) is an average over land and sea; real places get far more or far less. The latent heat of vaporisation of water depends on temperature: about 2256 kJ/kg at 100 °C and about 2428 kJ/kg at skin temperature (OpenStax). The lessons treat the cycle as four simple steps; in reality water can also skip steps, for example ice turning straight into vapour (sublimation).
All sources
[1] The Water Cycle
U.S. Geological Survey (USGS) Water Science School · Reliability: High
Type: Government science agency
States that oceans store 96% of all water on Earth; that the sun's energy causes liquid water to evaporate into water vapour; that clouds form by water condensing in the atmosphere; that water returns to the surface as precipitation; that water moves into the ground by infiltration and is stored as groundwater in aquifers; and that energy from the sun and gravity drive the movement of water.
U.S. Geological Survey (USGS) Water Science School · Reliability: High
Type: Government science agency, school page
Defines evaporation as the process that changes liquid water to water vapour; precipitation as water released from clouds as rain, freezing rain, sleet, snow or hail; describes runoff flowing downhill and infiltration of water into the ground; says over 96% of Earth's water is in the oceans.
U.S. Geological Survey (USGS) Water Science School · Reliability: High
Type: Government science agency
The atmosphere holds only about 0.001% of Earth's water; if it all fell at once it would cover the globe only about 2.5 cm deep; about 90% of the water in the atmosphere comes from evaporation from water bodies and about 10% from transpiration by plants; cooler temperatures higher up let the vapour condense into clouds.
Met Office (UK national weather service) · Reliability: High
Type: National weather service
The water cycle uses the energy of the sun; surface water heated by the sun evaporates into water vapour; plants release water vapour through their leaves (transpiration); ice can turn straight into vapour (sublimation); as air rises and cools the vapour condenses, often forming clouds; droplets merge into larger drops and fall as rain, sleet, snow or hail; water flows back into rivers and oceans or is stored as ice.
The world receives on average about 980 mm of precipitation a year; evaporation needs energy; when vapour condenses the extra energy is released as heat; about 99% of the water plants take up is transpired and only about 1% is used for growth; most water in rivers and lakes returns to the oceans; lakes without an outlet lose water only by evaporation, leaving salts behind.
[6] College Physics, 14.3 Phase Change and Latent Heat
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Gives Q = m·Lf and Q = m·Lv; it takes 2256 kJ to vaporise 1 kg of water at its boiling point; latent heat of fusion of water 334 kJ/kg; evaporating sweat at body temperature takes about 2428 kJ/kg, taken from the skin, which cools the body; enormous heat is released when water vapour condenses.