Air

Closer to the sun, and colder

Why Is It Colder Up a Mountain?

Climbing takes you nearer the Sun and makes you colder, because air is warmed from the ground up and cools again as it rises and expands.

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It thins rather than ends

Where Does the Air Stop?

Half of all the air sits below five and a half kilometres, lower than several mountain ranges, and the space station still feels a whisper of it.

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Weighing the sky

How Does a Barometer Work?

A barometer weighs the column of air above it, and that weight drops before a storm and rises before fine weather — hours ahead of the sky.

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Light getting bounced

Why Is the Sky Blue?

Air molecules knock short blue waves sideways about ten times more readily than red ones, so blue arrives at your eyes from every patch of sky at once.

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An argument about very large numbers

Every Breath You Take Has Been Breathed Before

A single breath holds more molecules than the atmosphere holds breaths — so an old one, well mixed, is almost certainly inside the breath you just took.

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Reading the sky

Why Do Clouds Have Different Shapes?

A cloud's shape is a picture of the air beneath it: rising bubbles pile into heaps, gentle slabs spread into sheets, and height decides water or ice.

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Droplets too small to fall

How Does Something That Heavy Float?

A cloud weighs tonnes, but its water is split into droplets so tiny that air resistance almost cancels their fall and updraughts do the rest.

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Where the rain never arrives

Why Are Deserts Dry?

A desert is dry for one of three reasons: air that sinks instead of rising, a mountain range wringing the rain out first, or sheer distance from any sea.

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A wave made of sand

How Does a Sand Dune Move?

A dune crosses the desert without a single grain going the whole way: wind rolls sand up the gentle side and tips it down the steep one.

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Escaping one at a time

Where Does a Puddle Go?

No boiling required. The fastest-moving molecules at the surface simply escape into the air — which is also why the water left in the puddle gets cooler.

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A cloud on the ground

What Is Fog?

Fog is a cloud that formed at your height instead of a kilometre up, when air cooled past the point where it could still hold its water.

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A blanket that lets light in

How Does the Air Keep Earth Warm?

The light coming in and the heat going back out are different waves — some gases ignore sunlight and absorb infrared, worth 33 degrees.

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Air that will not leave

What Makes a Heatwave?

A heatwave is not stronger sunshine. It is a dome of sinking air blocking cloud and wind, so the same heat accumulates day after day instead of leaving.

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A heat engine over warm water

How Do Hurricanes Form?

A hurricane is a self-feeding heat engine. Warm sea evaporates, the water condensing high up releases its heat, and the storm spins itself stronger on it.

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A fast band steering the weather

What Is the River of Wind Up High?

A band only a few hundred kilometres wide, carrying winds past two hundred kilometres an hour, thrown into a west-to-east track by the planet's spin.

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A sea breeze the size of a continent

What Is a Monsoon?

A monsoon is a continent-sized sea breeze that reverses with the seasons — summer heat drags in wet ocean air, and winter cold pushes it back out again.

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The sky letting go

Where Does Rain Come From?

Rain is water that rose off the ground invisibly, cooled into droplets far too light to fall, and then merged with its neighbours until gravity won.

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An old rhyme, half right

Does a Red Sky Really Predict the Weather?

A red sunset means clean dry air off to the west. That predicts tomorrow only where the weather reliably arrives from the west — Britain included.

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Land and sea disagree

Why Is It Windy at the Beach?

The risen air spreads out high up, drifts back over the water and sinks again: land, sky, sea and round, a slow wheel turning all afternoon.

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Plants moving without moving

How Do Seeds Travel?

Landing under your parent means losing, and plants cannot walk, so seeds hitch rides on wind, water and animals — or launch themselves like a spring.

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Light taking the long way

Why Are Sunsets Orange?

Near the horizon a sunbeam crosses roughly forty times as much air as at noon, and almost all of its blue is scattered away before it ever reaches you.

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Air exploding

Why Does Thunder Rumble?

Lightning heats air so fast that it explodes outward. Thunder is that bang, and the rumble is one long bolt arriving in stages, nearest end first.

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A spin tipped upright

How Does a Tornado Form?

Winds at different heights roll the air into an invisible horizontal tube. A storm's updraught tips that tube upright and stretches it until it spins hard.

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Physics run forward

How Can Anyone Predict Tomorrow?

A forecast measures the atmosphere as it is now, cuts it into millions of boxes, and steps known physics forward until the errors grow too large to trust.

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Air evening things out

What Makes Wind Blow?

Every temperature difference becomes a pressure difference, and wind is crowded air pushing towards the emptier place. Earth's spin makes it spiral.

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Named for where it starts

Where Does the Wind Come From?

A north wind blows from the north, and it brings whatever the north was like. Naming wind for its origin is really naming the air that is arriving.

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Fruit rusting

Why Does a Cut Apple Turn Brown?

Cutting an apple lets an enzyme that was safely stored meet the oxygen in the air, and together they build a brown pigment over the exposed surface.

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A gas being made

Why Does Baking Soda Fizz?

Baking soda meets an acid and the two swap parts to make carbon dioxide. The fizz is a brand-new gas taking up far more room than the ingredients did.

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Air pushing back

Why Does a Balloon Stay Blown Up?

Air molecules inside are constantly hitting the rubber, and their combined pushing outward exactly balances the stretched rubber pulling in.

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A fungus breathing out

Why Does Bread Rise?

Yeast is alive: it eats the sugar in the dough and breathes out carbon dioxide, and a stretchy gluten net catches the gas instead of letting it escape.

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The laziest possible shape

Why Are Bubbles Always Round?

The soap film pulls itself as small as it can, and the smallest surface that will hold a given volume of air is a sphere. Roundness is not a choice.

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Faster sound, same vibration

Why Does Helium Make Your Voice Squeaky?

Sound travels far faster in helium, so the resonances of your unchanged throat leap upward. Your vocal cords carry on buzzing at exactly the same pitch.

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A muscle misfiring

What Causes Hiccups?

An irritated nerve jerks the diaphragm into an unplanned gulp, and about 35 milliseconds later the vocal cords snap shut on it. That snap is the sound.

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The string is the point

Why Does a Kite Stay Up on a String?

Cut the line and a kite drops rather than climbs, and on a still day you can fly one by walking backwards. A wing only needs air moving past it.

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Trading height for distance

Why Does a Paper Plane Glide?

A paper plane does not fly so much as fall very slowly forwards, pushing air downwards with its wings and trading height for distance the whole way.

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Air losing its grip on water

Why Do Windows Fog Up?

Warm air carries water you cannot see. Cold glass chills the air touching it until it can no longer hold that water, and the surplus appears as drops.

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Iron combining with oxygen

Why Does Metal Go Rusty?

Rust is iron chemically joining with oxygen and water to become something new and crumbly, and it flakes off to expose fresh metal underneath.

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Gas hiding in plain sight

Where Do the Bubbles in Fizzy Drinks Come From?

The gas was dissolved into the liquid under pressure. Opening the bottle releases it, and the surplus gathers on rough spots and rises as bubbles.

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Air pressure pushing from outside

How Does a Straw Work?

You never pull a drink upwards. You lower the pressure in the straw and the atmosphere pushes the drink up for you — which caps any straw at ten metres.

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Proteins forced to unfold

How Does Egg White Turn Into Foam?

Whisking unfolds the proteins in egg white so they wrap every air bubble in a strong film. Beat too long and those proteins clump and the foam collapses.

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Angled blades catching moving air

How Does a Windmill Turn?

Angled blades deflect moving air sideways and the air shoves back. The tip travels far faster than the root, so a blade is twisted along its length.

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Cooling and catching

Why Do We Yawn?

Yawning seems to cool the brain rather than fetch oxygen, and it spreads between people along the circuits that carry empathy — strangers catch it least.

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A defence aimed at nothing

Why Does Pollen Make Some People Sneeze?

An allergy is your immune system mounting a real defence against something harmless, and the symptoms are the defence, not the pollen.

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Fetching oxygen, dumping waste

Why Do We Breathe?

Breathing collects the oxygen that releases energy from food and dumps the carbon dioxide left over. The urge to breathe comes from the waste, not the air.

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Air that took a wrong turn

Where Does a Burp Come From?

Almost every burp is air you swallowed without noticing. Gas floats to the top of your stomach, and the way in is the only way out.

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A sudden gale, from inside

Why Do You Cough?

A cough is a pressure explosion: seal the airway, squeeze hard, then let go all at once, fast enough to scrape the whole tube clear.

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Demand outrunning supply

Why Does Exercise Make You Puff?

Working muscles burn oxygen far faster than resting ones, so breathing and heart rate climb to keep up. Puffing is the delivery system catching up.

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Vibration turned into signals

How Do You Hear?

Sound shakes your eardrum, three tiny bones amplify the motion, and a coiled tube sorts it by pitch before hair cells turn it all into nerve signals.

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Half a tennis court, folded up

Why Are Lungs Spongy Instead of Hollow?

A hollow bag would touch air only at its walls, so lungs split into millions of tiny sacs — around 70 square metres of surface, folded up.

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Too much, in the wrong place

What Makes Something Pollution?

Nothing is pollution by itself. It becomes pollution when more of it arrives somewhere than that place can break down or carry away.

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Catching actual molecules

How Does Smelling Work?

Around 400 receptor types, each waiting for a particular molecular shape, combine into trillions of smells — and into most of what you call taste.

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Blasting the nose clear

Why Do We Sneeze?

A sneeze is a reflex, not a decision: irritated nerves in your nose trigger a blast of air fast enough to clear them — and to spread germs remarkably well.

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Some of it — and the sea makes more

Do Trees Make the Air We Breathe?

Trees release oxygen, but ocean plankton make about half the world's supply — and a forest that has stopped growing gives back roughly what it consumes.

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Two flaps, flapping very fast

How Do You Make Your Voice?

Whisper and the folds never touch — it is only rushing air, shaped the same way, which is why a whisper is still perfectly understandable.

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Throwing air downwards

Why Does a Wing Hold a Plane Up?

A wing throws air downward and the air shoves back. Lift grows with the square of speed, so twice as fast is four times as much of it.

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Lighter than air, and too slow

Why Did Airships Stop Flying?

Airships float because their gas is lighter than air, but they were too slow and too big for weather, and aeroplanes overtook them.

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Carrying heat out, not cold in

How Does a Box Make Itself Cold?

There is no such substance as cold. A fridge squeezes a gas hot to dump heat out behind it, then lets it expand cold to soak up some more.

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A fan and a hot wire, in that order

What Is Inside a Hairdryer?

A hairdryer is a fan and a coil of thin wire — and hair dries because moving air carries water away, not because the air is hot.

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Stopping the cabin spinning

Why Does the Little Tail Rotor Matter?

Spinning the main blades twists the body the other way, so a small rotor on the tail pushes sideways to cancel the twist and hold the nose still.

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The same bag, holding less

Why Does Hot Air Lift a Balloon?

Heating the air drives some of it out of the open mouth, so what is left inside is about a fifth lighter than the air outside the balloon.

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A wing going round in circles

How Does a Propeller Pull a Plane?

A propeller blade is a wing bent into a circle. Instead of lifting upward it lifts forward, throwing air backward and never once stopping.

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Two pushes and a leftover

How Does a Boat Sail Into the Wind?

A sail turns wind into a mostly sideways push, the keel refuses to go sideways, and what is left over carries the boat forward, even upwind.

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A very small parachute pointed the wrong way

Why Do Umbrellas Turn Inside Out?

An umbrella inverts because wind flowing over the dome lifts it from outside, and the ribs only brace against downward force.

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The room does the pushing

Why Does Suction Pick Things Up?

Nothing sucks. The fan throws air out of the exhaust, the hose drops below room pressure, and the room's own air pours in carrying the dust.

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Pushed from below, never pulled

How Does Water Get Up to the Top Floor?

Nothing can suck water higher than about ten metres, so tall buildings push instead: a pump flings it out and a narrowing pipe presses it up.

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Three blades, and each one is a wing

How Does a Wind Turbine Work?

A turbine blade is not pushed by the wind, it is lifted sideways like a wing — and no turbine can take more than two-fifths of the wind's energy.

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Sky on fire, quietly

What Are the Northern Lights?

Earth's magnetic field turns the solar wind aside but funnels some of it down at the poles, where it lights up atoms 100 km overhead like a neon sign.

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Three centuries of weather

Jupiter's Storm That Never Stops

Jupiter's Great Red Spot is a storm wider than Earth that has been spinning for over 300 years. There is no ground beneath it to slow it down.

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Far out and furious

The Windiest Place in the Solar System

Neptune has the fastest winds in the solar system, around 2,000 km/h, despite receiving the least sunlight — and nobody fully explains why.

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Nothing to carry the sound

Why Is Space Silent?

Sound has to be handed from one particle to the next, and space has essentially no particles to do the handing, so nothing out there is heard.

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Not friction

Why Do Spacecraft Get So Hot Coming Back?

A capsule coming home at eight kilometres a second is not rubbed hot by the air. It squashes the air in front of it, and squashed air glows.

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The Sun is leaking

What Is the Solar Wind?

The Sun leaks a steady stream of particles that blows out past every planet. Earth's magnetic field is the reason almost none of it reaches us.

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Both, and neither

Is Space Hot or Freezing?

Space is neither hot nor freezing in any useful sense. Temperature needs particles, and there are almost none — objects there heat and cool by light alone.

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The air is doing it

Why Do Stars Twinkle but Planets Do Not?

The twinkling happens in our restless air, not in the star. A star is a single point of light for the air to bend; a planet is wide enough to average out.

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A blanket that will not lift

Why Is Venus Hotter Than Mercury?

Venus sits further from the Sun than Mercury yet runs far hotter, because its thick carbon dioxide blanket traps heat airless Mercury loses.

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An invisible ocean

Air Is Pressing on You

Ten tonnes of air is pressing on you right now and you feel nothing, because it presses equally from every side. Break that balance and you notice.

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You would not get stronger

What If Gravity Were Half as Strong?

Your jump would double and nothing about you would improve: bones weaken, trees overreach, and the planet slowly leaks its air away.

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How big the push is

What Makes a Sound Loud?

Loudness is how hard the air gets shoved; pitch is how often. The two are entirely independent, which is why a whisper and a shout can hit the same note.

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Air shoved out of the way

How Does a Parachute Slow You Down?

A parachute does not slow you by being large. It forces you to shove a huge amount of air aside, and the air pushes back until it balances your weight.

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Force, spread out

What Is Pressure?

The same push feels utterly different depending on the area it lands on. Shrink the area and you have a drawing pin; spread it out and you have a snowshoe.

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Air, pushed

How Does Sound Travel?

Sound is not a thing that flies. It is a squeeze handed from one air molecule to the next, passed all the way along to your eardrum.

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Squeeze and stretch

What Does a Sound Wave Look Like?

Each patch of air only shuffles back and forth a fraction of a millimetre, passing the squeeze along at 343 metres a second. Nothing travels with it.

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Empty space cannot push

What Does Nothing Actually Do?

Air strikes every square centimetre of you with about a kilogram of force, from all sides at once — which is why a can crumples and you do not.

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99.8 per cent nothing

The Solid That Is Almost Entirely Air

Aerogel is a gel with the liquid removed and the solid skeleton left standing — a feat of drying rather than chemistry, and the best insulator we can make.

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A sugar you cannot digest, but bacteria can

Why Do Beans Make You Windy?

Beans carry a sugar humans have no enzyme for, so it reaches the large intestine intact and the bacteria living there ferment it, giving off gas.

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Most of it is smell

Why Does Coffee Smell Better Than It Tastes?

Coffee's appeal is almost entirely aroma, because the tongue only reports bitter and sour and the nose reports hundreds of things.

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Mostly air, held in a shape

Why Is Foam So Light?

Packaging foam is 95 to 98 per cent air. Trapped air cannot circulate, so it insulates — and thousands of bubble walls buckling is what absorbs a knock.

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A candle, a mouse, and a jar

How We Found Out What Air Is

Sealing a candle and a mouse under a jar showed that air is a mixture, and that only one part of it keeps a flame or an animal alive.

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A lined pit that runs out of air

Where Does Rubbish Actually Go?

Digs into old tips turn up decades-old newspapers still readable and food scraps barely changed. A landfill is a store rather than a solution.

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The bits that did not get to burn

Why Does a Fire Make Smoke?

Smoke is fuel that never finished burning: tiny solid specks that failed to meet enough oxygen, carried off by the hot air rising above the fire.

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One plant, four drinks

Why Is Tea Different Colours?

Green, black, white and oolong all come off the same bush, separated only by how long the leaf was left to brown before it was dried.

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Smelling with your forehead

What Are an Insect's Antennae For?

An insect's antennae are not feelers but mostly its nose: pitted surfaces catching single scent particles, two of them giving direction.

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Everything about it is built for lightness

How Does a Bird Stay Up?

A bird bends passing air downwards off its curved wings, and almost every part of it — hollow bones, enormous chest muscles — pays for that.

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A nose that catches smells

Why Is a Dog's Nose Wet?

The bits of stuff that carry a smell stick to damp skin, so a wet nose holds on to what a dry one would let blow straight past.

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Four wings, four motors

How Does a Dragonfly Fly Backwards?

Each of a dragonfly's four wings has its own muscles, so it can aim the push anywhere — which is how it hovers, reverses and turns on the spot.

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One structure, many jobs

Why Do Birds Have Feathers?

A feather is a branching shaft whose barbs hook into one another to make a sheet — light and stiff for flight, tangled and loose for warmth.

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Taking oxygen out of water

How Do Fish Breathe Underwater?

Fish do not breathe water. They pull the oxygen dissolved in it through hugely folded gills, with blood running the opposite way to catch more of it.

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Everyone gets a tow except one

Why Do Geese Fly in a V?

Each bird rides the swirl of rising air spilling off the wingtip ahead, which is real work saved. They take turns at the front, where there is none.

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From somewhere it has never been

How Does a Pigeon Find Its Way Home?

A pigeon works out where it is from magnetism and smell, then takes a bearing from the sun — so a shifted body clock sends it confidently wrong.

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