Every fact in the three desert 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/sĀ² 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: there is no single definition of a desert. The USGS uses a rainfall classification (arid = less than 250 mm a year) and says about one-third of Earth's land is desert [1], while National Geographic uses less than 25 cm of rain a year and says deserts cover more than one-fifth of the land [3]; the difference comes from definitions, and whether polar deserts are counted. NASA's day and night temperatures (38 Ā°C and about ā4 Ā°C) are averages for hot deserts, not values for every desert [2]. The figures from the US National Park Service (90% of the day's heat radiated at night, 40% of possible sunshine in humid lands, evaporation of 70ā160 inches a year) describe American deserts and are rounded [4]. The temperature conversions to Ā°C and the mm conversions are our own arithmetic. In the experiments, rolled-up paper towels are only a model of a cactus stem: real plants control water loss in more ways than shape.
All sources
[1] Deserts: Geology and Resources ā What is a desert?
U.S. Geological Survey (General Interest Publication) Ā· Reliability: High
Type: National geological survey (education publication)
Approximately one-third of Earth's land surface is desert. Deserts may be hot or cold, and may be regions of sand or vast areas of rocks and gravel with occasional plants. Classification by Peveril Meigs (1953): extremely arid lands have at least 12 consecutive months without rainfall; arid lands have less than 250 mm of rain a year; semiarid lands have 250ā500 mm and are generally called steppes, not deserts.
Type: National space and Earth science agency (education page)
Daytime temperatures average 38 Ā°C; night-time temperatures average about ā3.9 Ā°C, because dry air lets heat escape rapidly at night. Deserts get about 250 mm of rain a year, the least of all biomes; most get less than 300 mm, about 10% of a rainforest's rain. Typical plants are cacti, small bushes and short grasses. Cacti store water in their stems and use it very slowly; bushes conserve water by growing few leaves or by having large root systems; perennials go dormant and grow when water is available; annuals (ephemerals) complete their whole life cycle in a few weeks during rain. Example: Cairo, Egypt, averages 28 mm of rain a year; coolest month January (14 Ā°C), hottest July and August (28 Ā°C).
A desert receives less than 25 cm of rain a year; the driest parts of Chile's Atacama Desert get less than 2 mm. Deserts cover more than one-fifth of Earth's land and are on every continent; only about 20% of deserts are covered by sand; more than one billion people live in desert regions. The polar deserts of the Antarctic and Arctic are the largest deserts; the Sahara is the largest hot desert and can reach 50 Ā°C by day. Camels can go weeks without water and their nostrils and eyelashes guard against sand; the fennec fox hunts at night; the desert tortoise spends much of its time underground; the Namib desert beetle harvests fog for water; some plants grow long roots to reach deep water; cacti store water. Desertification is usually caused by deforestation and human demands on semiarid land, not by drought alone.
Many deserts lie under high-pressure systems with little cloud, so at least 90% of the Sun's rays reach the surface, while humid lands receive only about 40% of possible solar radiation. Around 30Ā° north and south (the Horse Latitudes) the air is dry because air rising at the equator has lost much of its moisture. Rain shadow: moist air rises over mountains, cools and drops its rain there, so little reaches the desert behind. At night about 90% of the day's heat radiates back to the sky, compared with about 50% in moister climates; daily temperature ranges are often 50 Ā°F or more. Evaporation in American deserts is 70ā160 inches a year. Death Valley has reached 134 Ā°F.
Type: Natural history museum (education fact sheet)
Kangaroo rats can survive without ever drinking water, getting the moisture they need from their seed diet. They do not sweat or pant to cool down, because that would lose water. They live in burrows they dig themselves, often at the base of a shrub, sleep underground during the day and come out to feed at night, when it is cooler.
The cactus family is very prominent in the deserts of the Americas but absent elsewhere; fleshy, cactus-like Euphorbia species are conspicuous in deserts of southern and eastern Africa. Spiny spinifex grasses are widespread in Australia; perennial grasses occur in most deserts. Many desert birds are nomadic, moving to areas where rain has recently fallen; seed-eating birds need to drink and sometimes fly long distances to find water, while birds of prey can get water from their prey.