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Tropical rainforests lie between the tropics, with average daily temperatures of about 20–25 °C and 2,000–10,000 mm of rain a year. Sources disagree in detail: National Geographic gives 1,800–2,500 mm and 18–30 °C. Comparing ranges like these, and asking how each was measured, is part of judging evidence.

Rainfall in millimetres is a depth: 1 mm on 1 m² is 1 litre. Manaus in Brazil averages about 2,323 mm a year, so every square metre receives about 2,323 litres, and a hectare (10,000 m²) about 23 million litres. Its wettest months have about 310 mm and its driest (August) 68 mm.

Rainforests cover only about 2–6% of Earth's surface yet hold more than half of all plant and animal species; the Amazon alone has an estimated 400 billion trees and 2.5 million insect species. The vertical structure (emergent layer, canopy at 30–50 m, understory, forest floor) creates many different habitats with very different light, wind and humidity.

Experiment: Measuring transpiration rate by mass loss

1. Water all plants equally an hour before. Cover each pot and the soil surface with plastic so water can only leave through the plant.
2. Weigh each plant and record the mass, temperature and time.
3. Place one plant in still air and one in front of the fan on low (same light and temperature). A third plant with all leaves removed can act as a control.
4. Weigh each plant again after 24 hours (and 48 hours if possible).
5. Estimate the total leaf area by tracing a few leaves on squared paper and multiplying by the number of leaves.
6. Calculate the rate of water loss in g per hour and in g per hour per 100 cm² of leaf. Compare still and moving air, and estimate the uncertainty from the balance and the leaf area estimate.

Plant	Mass at start (g)	Mass after 24 h (g)	Water lost per hour (g/h)	Per 100 cm² leaf (g/h)
Still air	Prediction
Fan	Prediction
Leafless control	Prediction

Questions

1. A rainforest receives 2,500 mm of rain a year. Calculate the volume of rain, in litres and in m³, that falls on one hectare in a year.

2. A sealed potted plant drops from 412.6 g to 398.2 g in 24 hours. Its leaf area is 360 cm². Calculate the transpiration rate in g/h and in g/h per 100 cm² of leaf.

3. NASA gives 2,000–10,000 mm of rain a year for rainforests, National Geographic 1,800–2,500 mm. Suggest two reasons why the ranges differ.

4. Explain the feedback between deforestation and rainfall in the Amazon.

Sources for every fact on this sheet: echoesof.science/school/rainforests/sources/

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