Every fact in the three rainforest 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 facts are simplified or less certain: sources give different ranges for rainfall and temperature because rainforests differ from place to place: NASA gives 2,000–10,000 mm of rain a year and average daily temperatures of 20–25 °C [1], National Geographic 1,800–2,500 mm and 18–30 °C [3], and Britannica about 30 °C by day and 20 °C at night [2]. The share of Earth's surface covered by rainforests (2–6%) and the claim that more than half of all plant and animal species live in them are estimates [3]; numbers of species and trees in the Amazon are also estimates. Layer heights are typical values: Britannica places the canopy at 30–50 m [2], NASA mentions giant trees of 75 m or more [1] and National Geographic emergent trees of up to 60 m [3]. The finding that the Amazon starts its own wet season comes from satellite measurements of water vapour and is one study's conclusion [5]. Transpiration figures (an oak tree, a field of maize) are USGS examples, not rainforest measurements [6].
All sources
[1] Mission: Biomes – Tropical rainforest
NASA Earth Observatory (Mission: Biomes) · Reliability: High
Type: National space and Earth science agency (education page)
Tropical rainforests lie between the Tropic of Cancer and the Tropic of Capricorn. Average daily temperatures are 20–25 °C and it never freezes. A typical year has 2,000 to 10,000 mm of rain. Example: Manaus, Brazil, averages about 2,323 mm a year, about 308–310 mm in the wettest months (March, April) and 68 mm in August; monthly mean temperatures are about 27–28 °C. Layers: a canopy of giant trees 75 m or more tall whose dense leaves block much of the sunlight; an understory of vines, smaller trees, ferns and palms; a forest floor of wet leaf litter where few plants grow because of low light. Leaf litter decomposes rapidly in the wet, warm conditions, returning nutrients to the soil. Many houseplants come from the rainforest.
Rainforests are lush forests of tall, broad-leaved trees, usually in wet tropical regions near the Equator: mainly South and Central America, West and Central Africa, Indonesia, parts of Southeast Asia and tropical Australia. Temperatures are usually about 30 °C by day and 20 °C at night. The canopy extends across the treetops at 30–50 m; below is a thick understory of small trees, lianas and epiphytes; the forest floor is bare except for a thin layer of humus and fallen leaves. Most animals live among the leaves and branches. Most rainforest soils are permanently moist and not very fertile; organic matter breaks down quickly and nutrients are taken up rapidly by roots and fungi. Rainforests continue to be cleared.
National Geographic Society (Education resource library) · Reliability: Medium-high
Type: Educational non-profit (encyclopedic entry)
Tropical rainforests receive roughly 180–250 cm of rain a year and have temperatures of 18–30 °C; temperate rainforests, mostly in coastal mountain areas, receive at least 140 cm. Rainforests cover just 2–6% of Earth's surface, but more than half of the world's plant and animal species live in them. Emergent trees reach up to 60 m; the canopy is a layer about 6 m thick that blocks wind, rain and sunlight; understory plants have large leaves to catch the little light; the forest floor is the darkest layer. Shallow roots absorb nutrients released by decomposers. Clearing land for agriculture has greatly harmed rainforests. The Amazon has around 40,000 plant species, nearly 1,300 bird species, 2.5 million insect species and an estimated 400 billion trees.
Smithsonian Tropical Research Institute · Reliability: High
Type: Research institute (education page)
Four layers from top to bottom: emergent, canopy, understory and forest floor; the canopy cuts off most of the light and wind from below. Emergent trees stick up above the canopy and face strong winds, intense sun, heavy rain and big temperature swings; they have small, tough, waxy leaves that limit water loss, and many have wind-spread pollen and winged seeds. Many emergent-layer animals fly or glide because the top branches cannot hold much weight; examples are the harpy eagle (a top predator hunting monkeys, sloths and snakes), the pygmy glider and morpho butterflies, whose blue colour can be seen from low-flying planes.
Type: National space and Earth science agency (research news)
Water released by plants contributes much of the rain that falls on the Amazon. Satellite measurements of deuterium (heavy hydrogen) in water vapour showed that, at the end of the dry season, the clouds building over the southern Amazon form from water rising from the forest itself; this primes the atmosphere so the rainy season starts in mid-October, before winds bring ocean moisture. Clearing and burning could disrupt this; in some observations the wet season has started up to a month later since the 1970s, and if drying continues parts of the rainforest could turn into savanna.
U.S. Geological Survey (Water Science School) · Reliability: High
Type: National geological survey (education page)
Transpiration is the release of water vapour from plant leaves, through openings called stomata, after plants take up water from the soil. A large oak tree can transpire about 151,000 litres (40,000 gallons) a year; an acre of maize gives off about 11,400–15,100 litres a day. Transpiration increases with temperature and with wind and decreases when the air is more humid; sunlight also affects it. A plastic bag wrapped around leaves collects transpired water as it condenses.
U.S. Geological Survey (Water Science School) · Reliability: High
Type: National geological survey (education page)
Tie a plastic bag around a bunch of leaves and wait a day: the air in the bag becomes saturated and a significant amount of water collects in the bottom of the bag. Plants lose water through their leaves all the time, an invisible process that the bag makes visible.