In a desert where it almost never rains, where do living things get their water?
― They strain it out of fog flowing through the air like an invisible river
Some deserts in the world get less than a millimetre of rain a year. And yet insects and plants still live there. The water doesn't come from rain falling out of the sky — it comes from fog drifting in sideways off the sea. Fog droplets are too tiny to fall very far, so living things push their bodies out into the wind and "strain" the drifting droplets out of the air.
Walk outside on a deep foggy morning, and even though it isn't raining, your hair and the surface of your jacket can end up damp. Even under an umbrella, your face and fringe still get wet.
Spiderwebs sometimes glint like pearl necklaces, but only in the morning. That's because countless fog droplets have stuck to the fine threads.
Some living things rely on this "you get wet just by walking around" mechanism for their very survival. The stage for this is the Atacama Desert in South America and the Namib Desert in southwestern Africa.
Only two reasons let creatures pull water out of fog
Fog droplets are far smaller than raindrops and stay almost suspended in the air. Waiting above won't get you any. Instead, they're carried in sideways, one after another, on the wind.
Air flows around thick objects, and droplets tend to swerve along with it. But against a thin thread or spine, droplets can't swerve in time and collide with it. Desert creatures' bodies are shaped to take advantage of exactly this.
First, let's look at just how little fog actually "falls."
Fog isn't water that falls from the sky — it's water that flows in sideways
Some coastal towns in the Atacama Desert are said to average around just one millimetre of rain a year. Many parts of Japan get around 1,500 millimetres a year — a difference of more than a thousandfold. If living things depended on rain alone, they simply couldn't survive.
Yet along the coasts of these deserts, fog born over the cold sea often drifts in from night into morning. In the parts of the Namib Desert near the coast, some areas are reported to see fog anywhere from several dozen to over a hundred days a year.
Fog droplets are mostly somewhere between 1/100 and 1/50 of a millimetre across — only about a hundredth the size of a raindrop. At that size, air resistance overwhelms them, and they're said to fall at only around 1 centimetre per second. Even a slight breeze carries them sideways far further than they ever fall.
In other words, fog isn't water that "falls" to the ground. It's water that "flows" sideways along with the air. On a deep foggy day, it's as if an invisible river is flowing just above the ground, from the sea toward the land. The only way to draw water from this river is to stick something out into the flow and let the droplets collide with it.
The thinner the spine or thread, the more fog droplets collide with it
Take a look at Figure 1. When wind hits an object, the air changes direction just in front of it and swerves aside. Light fog droplets usually swerve right along with the air.
What matters here is the object's thickness. Around a thick pillar like the one on the left, the air starts curving well before it arrives. Droplets get plenty of "lead time" to start curving too, and they keep pace with the airflow. Around a thin thread like the one on the right, the air only curves sharply right at the last moment. The droplets' momentum keeps them from curving in time, so they carry straight on and collide with the thread.
Desert creatures put this "thinner objects catch more droplets" property to use through the shape of their bodies.
In the Namib Desert, there's a beetle that climbs to the crest of a dune on a foggy morning, lowers its head into the wind, and raises its rear end high. Fog droplets landing on its back and legs gather into drops, then run down the slope of its back to its mouth. This "fog-basking" behaviour was first reported in the 1970s.
The Atacama Desert also has plants that have almost no roots and simply roll across the sand. Their leaf surfaces are covered in fine, hair-like scales thought to catch fog droplets and draw them in. Cactus spines and the thin leaves of certain grasses are also thought to act as "combs" that catch fog droplets.
People have copied this trick. In the foothills of deserts in Chile and Peru, large nets woven from fine thread have been set up facing the wind to collect drinking water from fog. It's reported that a single square metre of net can gather several litres of water a day.
How much water a net collects changes a great deal with wind strength. Try it yourself in Figure 2.
Double the wind, and twice as much fog passes through the net, so twice as much water is collected. In practice, though, if the wind gets too strong, droplets are said to punch straight through the net, or the net itself can get damaged. Note that the figures in the diagram are rough estimates assuming a fixed fog density and net performance.
Dew on grass forms when water vapour in the air freshly condenses into droplets on a cooled leaf surface. Water collected from fog, by contrast, is water that was already droplets in the air, caught by collision. Desert creatures are thought to use both mechanisms, depending on the situation.
A study published in 2001 found that the backs of Namib Desert beetles are lined with small, water-attracting bumps alongside water-repelling grooves. The bumps gather droplets and grow them larger, and the repelling parts then let them roll off and be carried away. This idea has also been used in developing materials that collect water from fog.
Summary
Fog droplets are too small to fall to the ground the way rain does. Instead, they're carried in constantly from the side on the wind. Desert creatures push thin legs, spines, or hair-like leaves out into the wind, letting these droplets collide with them and get caught. The thinner the object, the less able droplets are to swerve away with the air, and the more of them collide.
Desert water doesn't fall from the sky.
It's fog flowing sideways through the air, strained out by thin bodies.
The mechanism by which night-time cooling creates water is covered in Why does morning dew form?, and the desert's extreme swings between day and night heat are covered in Why are deserts hot by day but cold at night?. For strategies on storing water inside the body, see also the article on cacti and the article on camels.
- On a foggy morning, set up a thin thread (fishing line or kite string, thinner than yarn) and a thick rod (such as a plastic-wrap tube) side by side somewhere the wind passes through, and leave them for about 10 minutes.
- Run your finger over the surface of the thread and the rod, and compare how much water has collected on each. Relative to its surface area, the thread should be far wetter.
- On a day without fog, make a fine mist with a spray bottle and fan a crosswind with a hand fan to compare the two the same way.
The reason spiderwebs only glint in the morning is the same mechanism. If you spot one, look closely at where along the thread the droplets line up.
Want to know more? ― Terms, formulas, and textbook linksWe've marked which level each section belongs to, from junior-high science to university specialist courses
- JHSCovered in junior-high school science
- HSCovered in high-school "Basic Physics" or "Biology"/"Earth Science"
- HS+Advanced high-school content, or textbook sidebar material
- UnivNot covered in high school — university-level specialist content (fluid dynamics, aerosol science, ecology)
- ResearchNot yet settled even at university level — an active area of research
JHSTerminology: this phenomenon has names
- Fog: a state where fine water droplets float in the air near the ground, cutting visibility below one kilometre.
- Sea fog (advection fog): fog that forms when warm, moist air flows over a cold sea and is cooled. The fog in the Atacama and Namib deserts is of this type.
- Fog-basking: behaviour in which beetles and similar creatures face into the fog with their bodies turned toward the wind, and drink the fog droplets that land on them as water.
JHSHSChecking with a formula: how much water collects on a fog net overnight
The water a net collects is a fraction of "the water contained in the air passing through the net" — specifically, the fraction that gets caught. Here we use the same values as in Figure 2 to work out how much water collects on a 1-square-metre net over one night.
| In symbols | W = L × v × A × η × t |
| In words | Water collected = fog water per cubic metre of air × wind speed × net area × capture fraction × time |
| Where this comes from | The volume of air passing through the net (wind speed × area × time), multiplied by the amount of water that air contains. This is the basic idea behind flow rate: "amount flowing in = concentration × flow speed × cross-sectional area." |
| Symbol | Meaning and unit |
|---|---|
| W | Amount of water collected (grams) |
| L | Fog water content. Weight of water droplets per cubic metre of air (grams per cubic metre) |
| v | Wind speed (metres per second) |
| A | Net area (square metres) |
| η | Fraction of fog droplets the net catches (0–1, no unit) |
| t | Duration of the fog (seconds) |
| Fog water content L (typical for dense fog, said to be around 0.05–0.5) | 0.2 g/m³ |
| Wind speed v | 3 m/s |
| Net area A | 1 m² |
| Capture fraction η (said to be around 20% for a fine-thread net) | 0.2 |
| Duration of fog t | 6 hours (1 hour = 3,600 seconds) |
| Rainfall for comparison (typical annual rainfall for the Atacama coast) | About 0.8 mm/year (about 800 g per square metre) |
| Fog water passing through 1 m² of net per second (L × v × A) | 0.2 × 3 = 0.6 g |
| Fog water passing through per hour | 0.6 × 3600 = 2,160 g |
| Of that, the amount caught (× η) | 2,160 × 0.2 = 432 g |
| Water W after 6 hours of fog | 432 × 6 = 2,592 g |
| Compared with a year of rain | 2,592 ÷ 800 ≈ 3.2 times as much |
A single foggy night can leave about 2.6 litres of water on a 1-square-metre net — roughly three times the rain that falls on the same area of ground in a whole year. That said, the net faces the wind while rain falls on flat ground, so this isn't a perfectly fair comparison. Still, it shows that in some deserts, far more water "flows in" than ever "falls."
HSHS+Why don't fog droplets fall?
HSA falling object experiences gravity and air resistance, and once the two balance, it reaches a constant speed (terminal velocity). The smaller the droplet, the larger air resistance is relative to its weight, so its terminal velocity is lower.
HS+At the size of fog droplets, resistance follows "Stokes' law," where resistance is proportional to speed. In that case, terminal velocity is proportional to the square of the diameter. A droplet 0.02 mm across is said to fall at only about 1 cm per second — hundreds of times slower than a 2 mm raindrop (6–7 metres per second).
UnivThe number that decides whether a droplet collides: the Stokes number
When droplets in a flow collide with an obstacle, this is called "inertial impaction." Whether a droplet can keep up with a bend in the flow is captured by the dimensionless "Stokes number" — the ratio of the time a droplet takes to change speed (its relaxation time) to the time the flow takes to bend around the obstacle. The larger the droplet, the faster the wind, and the thinner the obstacle, the higher the Stokes number, and the less able the droplet is to turn in time, so it collides. The article's claim that "thinner objects catch more droplets" comes from the fact that obstacle thickness sits in the denominator of the Stokes number. In practice, a net's actual collection efficiency is estimated by multiplying this collision efficiency by the fraction of droplets that stick rather than bounce off, and the fraction of air that escapes sideways around the net rather than passing through it.
📖 For the derivation and further reading: Stokes number (Wikipedia, Japanese) ／ The mechanism on a desert beetle's back (Parker & Lawrence 2001)
ResearchWhat's still not fully understood
- Is a beetle's back actually used for collecting fog? Some researchers point out that species with bumpy, grooved backs may not actually adopt the fog-basking posture. Which species collect water, and with which body part, remains a live debate.
- What fraction of their water do desert creatures actually get from fog? Fog, morning dew, and moisture from food each contribute different amounts, and it's thought this varies enormously by species and location.
- Will sea fog keep arriving the same way in the future? Changes in sea surface temperature and upper-level winds could change how often coastal fog occurs, researchers warn. The effect on ecosystems that depend on fog is still not well understood.
In other words, the content of this article is also "the best explanation we have right now." The specifics of how each creature collects water, in particular, keep getting revised as observation continues.
Links to textbooks (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| JHS | Grade-2 science, "Weather and its changes" (fog, dew point) | What fog actually is, and how it differs from morning dew |
| HS | Basic Physics, "Air resistance and terminal velocity"; Earth Science, "Atmosphere and ocean" | Why fog droplets don't fall; how sea fog forms |
| HS+ | Advanced physics, "Stokes' law" | Terminal velocity being proportional to the square of diameter |
| Univ | Fluid dynamics, aerosol science (inertial impaction, Stokes number) | Why thinner objects catch more droplets |
| Research | Desert ecology, biomimetic materials research | The role of a beetle's back; future changes in fog |
| ― | Everyday connections | Why clothes get wet on foggy mornings; droplets on spiderwebs; making drinking water in desert villages |
- Parker, A. R. & Lawrence, C. R. (2001) Water capture by a desert beetle. Nature 414
- Hamilton, W. J. & Seely, M. K. (1976) Fog basking by the Namib Desert beetle, Onymacris unguicularis. Nature 262
- Schemenauer, R. S. & Cereceda, P. (1994) A proposed standard fog collector for use in high-elevation regions. Journal of Applied Meteorology 33
- Wikipedia, "Fog" (霧, Japanese)
- Wikipedia, "Stokes number" (ストークス数, Japanese)
※This article is a general-audience science explainer. The figures given are rough estimates meant to help illustrate the underlying mechanism. If you plan to drink water collected from fog, separate hygienic treatment is required.