Earth & sky

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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Raindrops as prisms

How Does a Rainbow Work?

Every raindrop bends sunlight, splits it and throws it back at a fixed 42-degree angle. The bow is a circle drawn on the sky opposite the Sun.

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Pressure finding a way out

Why Do Volcanoes Erupt?

Molten rock is lighter than the rock above it, so it rises. Dissolved gas expanding on the way up is what turns that rise into an eruption.

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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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The Moon pulling

Why Does the Sea Go In and Out?

The Moon pulls the ocean into two bulges on opposite sides of the planet, and Earth turns through both, which is why high tide comes round twice a day.

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A bump of one part in seven hundred

Shrink Earth to a Marble and You Could Not Feel Everest

The tallest mountain on Earth is a rounding error against the planet's radius, which is why a scale model of Earth would feel smooth in your hand.

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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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A weak layer buried inside

Why Does a Snowy Slope Suddenly Let Go?

An avalanche happens when a buried weak layer collapses under a rigid slab of snow, so the whole sheet lets go at once instead of sliding.

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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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Ice you cannot see

Why Is Black Ice So Dangerous?

Black ice is transparent and bubble-free, so it looks exactly like wet road while removing very nearly all of your grip. You see it once you are on it.

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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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Water dissolving stone

How Do Caves Form?

Rainwater is faintly acidic, and over ages it dissolves limestone into rooms and tunnels — then the same water drips back in and decorates them.

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One day, or thirty years

What's the Difference Between Weather and Climate?

Weather is what happened today; climate is what usually happens over decades — which is why one cold week proves nothing either way.

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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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Because it was one

Why Does the Map Look Like a Jigsaw?

South America and Africa fit together because they really were joined, and new sea floor forming between them has pushed them apart since.

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Where a river puts down its load

Why Do Rivers Fan Out at the Sea?

A river carries dirt a thousand kilometres and drops nearly all of it in the last few, choking its own channel and fanning out sideways.

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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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The temperature where air gives up

What Is the Dew Point?

The dew point is the temperature at which air is completely full of water. Chill anything below it and the invisible vapour has to become visible drops.

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How hard the air pushed

Why Is Some Rain Gentle and Some Rain Heavy?

Drizzle falls from a shallow cloud with no lift in it. A downpour falls from a tall one whose updraughts held the drops up until they had grown huge.

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Rain going missing

What Is a Drought?

A drought is not simply no rain. It is a long shortfall against what a place is built around — and dry ground bakes hard, which makes the shortfall worse.

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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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Rock finally slipping

Why Does the Ground Shake?

Earth's plates grind past one another, snag, and hold. The shaking is centuries of stored strain let go in seconds when the snag finally gives way.

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The line the sunlight hits squarely

What Is the Equator?

The equator is the line halfway between the poles, where sunlight lands most directly and days stay the same length all year.

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Patience with sandpaper

How Do Rivers Carve Canyons?

A river carries grit, the grit grinds rock, and enough time turns a stream into a canyon — helped along by water undercutting its own banks.

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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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When the ground stops drinking

Why Do Rivers Flood?

A flood is not extra rain — it is rain the ground could not swallow fast enough, running straight off into a channel that is already full.

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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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Rock replacing bone

How Does a Fossil Form?

A fossil is not the original bone. It is stone that grew into the bone's exact shape, and it takes fast burial plus real luck for any of them to exist.

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Air letting go of water

Why Is the Grass Wet in the Morning?

Grass cools overnight until the air touching it can no longer hold its water, which is left behind as dew, or as frost if it is cold enough.

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A kettle with a narrow neck

Why Does the Ground Shoot Boiling Water?

Water works down through cracks to rock heated by magma two kilometres below, then waits there hotter than boiling for something to let the pressure go.

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Ice that flows

How Do Glaciers Move?

Pile ice deep enough and it stops behaving like a solid: it deforms and creeps downhill under its own weight, carving U-shaped valleys as it travels.

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A saw made of grit

How Does a River Cut a Gorge?

Water alone barely scratches rock. A river cuts a gorge with the grit it drags along, going down faster than its hard walls can fall inwards.

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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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Soaked rock, not caverns

Where Is Water Underground?

There are no hidden lakes down there. Groundwater fills the microscopic pores between grains of rock like water in a sponge, and it moves very slowly.

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Ice going round again

How Does Hail Form?

A hailstone gains one layer per trip through a storm's updraught, thrown up and falling back until it is finally too heavy to be lifted again.

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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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Old shapes and young ones

Why Are Hills Round and Mountains Pointed?

A rounded hill is usually a very old mountain. Peaks stay sharp only while they are still rising; weather rounds off everything that stops.

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Atoms that tick

How We Know How Old a Rock Is

Certain atoms turn into other atoms at an utterly fixed rate, so the ratio between them in a rock says how long it has been sitting there.

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Boulders in the wrong place

How We Found Out There Were Ice Ages

Boulders the size of houses sat on ground no river could have brought them to — that, and scratches only ice makes, gave the ice ages away.

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Nobody has been below 12 km

How We Know What Is Inside the Earth

Earthquake waves travel through the planet, and one kind of wave never comes out the far side — which is how we know the core is liquid.

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When sweat stops working

Why Do Some Hot Days Feel Worse Than Others?

You cool down by evaporating sweat, and humid air is already close to its water limit. It will not take any more, so your main cooling system simply stops.

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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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One-ninth showing

Why Do Icebergs Float So Low?

Ice is only slightly less dense than seawater, so an iceberg floats with roughly eight-ninths of itself hidden below the surface and out of sight.

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You are already moving very fast

What If the Earth Stopped Spinning?

If Earth stopped turning, everything loose would keep going east at the speed of a rifle bullet — and the sea would slide off the middle of the planet.

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Gravity does not care about the ground

What If the Earth Were a Cube?

A cube-shaped Earth would have eight corners sticking out above the air, and all the water and air pooled into six round seas.

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Most of it would change nothing

What If All the Ice Melted?

Melting sea ice raises the sea by nothing at all. Everything depends on the ice sitting on land — and almost all of that is in one place.

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Four different ways

How Does a New Island Appear?

Islands appear four different ways — a volcano building up, coral growing, land cut off by rising seas, or the sea dropping — and the shape shows which.

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A bridge of land

What Is an Isthmus?

A strip of land a few kilometres wide is a bridge for everything that walks and a wall for everything that swims, and it redraws trade routes.

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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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Sea, with a wall in front of it

How Does a Lagoon Form?

A lagoon is a piece of sea fenced off by a bar or a reef, and the still warm water behind that fence becomes something else entirely.

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Building up and wearing down

Why Does Land Have Shapes At All?

Every hill and valley is a contest between forces shoving the land upwards and weather grinding it back down, settled by the rock underneath.

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A slope losing an argument

Why Do Hillsides Slide?

Every slope is a slow argument between gravity and friction, and a landslide is the moment water, undercutting or shaking makes friction lose.

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Two numbers for anywhere on Earth

How Do You Say Exactly Where You Are?

Any point on the planet can be named with two angles, and longitude was far harder than latitude: it had to wait for an accurate clock.

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Sorted by weight

What Is Inside the Earth?

The young Earth was molten, so heavy iron sank and light rock floated. Earthquake waves passing through confirm the layers are still arranged that way.

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A giant spark

What Is Lightning?

Lightning is the doorknob spark scaled up beyond imagining: a storm cloud builds millions of volts against the ground, and eventually the air gives way.

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Giving the strike an easy road

How Does a Lightning Rod Protect a House?

A lightning rod repels nothing. It offers a strike a sharp, easy metal path to the ground, so the current goes around the house instead of through it.

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Hot, and solid anyway

Why Is the Inside of the Earth Not Liquid?

The Earth's inside is hot enough to melt but held solid by pressure — so magma is made by dropping the pressure or adding water, not by adding heat.

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Atoms in formation

What Is a Mineral?

A mineral is a natural solid with a fixed recipe and atoms stacked in a strict repeating lattice, and that hidden lattice shapes the crystal outside.

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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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Crust crumpling

How Do Mountains Grow?

Mountains rise where colliding plates crumple the crust upward, and they last only as long as that uplift keeps ahead of the weather wearing them down.

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Salt arrives and never leaves

Why Is the Sea Salty?

Rivers carry dissolved salt down to the sea, and evaporation lifts only the water back out. That one-way door has left seawater 3.5 per cent salt.

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Land with sea on three sides

What Is a Peninsula?

Some are ridges of hard rock the sea never wore through, some are sandbars built by currents, and some are high ground the rising sea went around.

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Lifted, not built

Why Are Some Mountains Flat on Top?

A plateau is not a mountain that lost its peak — it is flat land lifted as one block, keeping its surface until rivers cut in from the edges.

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Too dry for forest, too wet for desert

Why Are Grasslands Treeless?

Grasslands sit between forest and desert on rainfall, and fire and grazing finish the job. All three keep an enormous belt of the world treeless.

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Ice you never see

Why Does Rain Begin as Snow?

Most rain starts as ice, because an ice crystal grows by stealing water vapour from the droplets around it — far faster than droplets can grow by bumping.

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Warm and windy beats damp and still

Why Do Puddles Dry Slower Some Days?

The same puddle can vanish by lunchtime or sit there all day, because molecules land back into it almost as fast as they escape.

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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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The air arrives empty

Why Is One Side of a Mountain Dry?

Air forced up a mountain cools and drops its rain on the way. It arrives on the far side dry and warm, and a desert is often the result.

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Warm, wet, and never switched off

Why Does So Much Live in a Rainforest?

A rainforest has no winter, so growth never pauses and life stacks into layers — though the richness sits in the trees, not in the thin soil.

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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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Everything that drains to one place

Where Does a River's Water Come From?

A river basin is all the land draining into one river, and every point on Earth belongs to exactly one — decided by ridges, not by borders.

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Erosion on the outside

Why Do Rivers Bend?

Water runs fastest round the outside of a bend, so it cuts that bank and drops sand on the inner one. Every bend therefore makes itself a little sharper.

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Nothing is permanent

Can a Rock Turn Into a Different Rock?

Weather grinds rock to grains, burial presses the grains back into stone, and deeper heat cooks that stone into something new without ever melting it.

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Grass, lakes and cattle, 6,000 years ago

The Sahara Turns Green, and Will Again

A slow wobble in Earth's orbit swings the African monsoon north every twenty-odd thousand years, and last time it did the Sahara held lakes and herders.

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Mountains, eventually

Where Does a Beach Come From?

Sand is what is left when rock has been broken down for ages and everything softer has washed away. Tropical sand is often ground-up coral instead.

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Water eats red light

Why Is the Sea Blue?

Ten metres down there is essentially no red light left at all, and a calm surface still mirrors the sky, which is why the colour varies so much.

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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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The ground is still catching up

Why Is the Hottest Month Not the Longest Day?

Midsummer is peak sunshine, but ground and sea keep taking in more heat than they lose, so the hottest weeks arrive some weeks afterwards.

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A page for every year

How Do Rocks End Up in Stripes?

Layered rock is a stack of old surfaces: material settles flat, gets buried and pressed, and each band records a different time.

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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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A flake's journey down

Why Does It Sometimes Rain and Snow at Once?

Almost all rain begins as snow. What lands on you is decided in the last kilometre, by the warm and cold layers the flake happens to fall through.

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Ice with air in it

Why Is Snow White and Fluffy?

A snowflake is ice grown into a six-armed crystal, and a snowdrift is mostly the air between them. The whiteness is countless surfaces bouncing light.

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Six arms, one journey

Why Is Every Snowflake Different?

Ice can only ever grow six ways, but the exact path a flake takes through the cloud — every temperature, every damp patch — makes its shape unrepeatable.

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Rock, remains and life

What Is Dirt Actually Made Of?

Half of good soil is empty space where roots find air and water, and one handful holds billions of bacteria and the fungal threads binding it.

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Light not getting through

Why Do Storm Clouds Look Dark?

Every bounce costs a little light, so through kilometres of droplets almost none reaches the base. The grey is a measure of how much water is overhead.

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Counting between flash and bang

How Far Away Is the Storm?

Lightning and thunder leave together, but light arrives instantly and sound takes three seconds a kilometre. Count the gap and divide by three.

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Wind piling up water

Why Does the Sea Rise in a Storm?

A funnel-shaped bay gives the water nowhere to escape, and the same surge arriving at high spring tide instead of low is what overtops a sea wall.

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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 wave that is all the way down

Why Are Tsunamis So Dangerous?

An ordinary wave is surface motion. A tsunami moves the entire depth of the ocean, so it carries far more water than its modest height at sea suggests.

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A loop with no end

Where Does Water Go?

The water in your glass has been round the same loop for billions of years: sea to sky to land and back. None of it is ever new.

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Energy moving, water staying

Do Waves Carry Water Along?

A wave carries energy across the sea while the water mostly circles on the spot. Waves break when the seabed drags on their base and the top runs ahead.

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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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Lines, triangles and half-circles

How Do You Read a Weather Map?

The lines join places of equal pressure. Crowded lines mean wind, highs mean settled air, lows mean rain, and fronts mark where air masses collide.

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Neither land nor water

Why Do Wetlands Matter?

A swamp looks like waste ground, but it soaks up floods, filters dirty water, and locks carbon into peat that only rots once it is drained.

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Fire as a gardener

Why Do Some Forests Need to Burn?

Some cones are glued shut with resin that only a fire can melt, so the seeds drop exactly when the ground has been cleared and made fertile.

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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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Losing your warm coat of air

Why Does Wind Make You Colder?

Your body heats a thin layer of air around your skin, and wind keeps stealing it. That is why you lose heat far faster than the thermometer suggests.

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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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