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Where Wind Comes From — The Homeschool Factory

Where Wind Comes From

Weather · 10 min

Where Wind Comes From

On a baking hot beach the breeze comes IN off the cold water, towards the heat. If you think warm places blow air outward, the sea has been quietly disagreeing with you your whole life.

Nothing is blowing it

Wind feels like it is being sent. It arrives from a direction, it has a temper, and it stops.

But there is no fan. Nothing anywhere is pushing the air at you. And once you know what is really going on, you can stand on a beach on a hot afternoon and say which way the breeze will be blowing before you feel it.

First: air is heavy

This is the fact everything else hangs off, and almost nobody believes it the first time.

The air above you is pressing down with the weight of about ten tonnes on every square metre — roughly a family car's worth, on a patch of ground the size of a small doormat. You don't feel it, because it presses from all sides at once, including outward from inside you.

Now: that weight is not the same everywhere. Some patches of the world have a little more air stacked over them, some a little less. Weather forecasters measure it constantly and call it pressure.

And air, like anything else, gets shoved from where there is more of it towards where there is less.

Wind is not air being pushed along. It is air sliding sideways out of a heavy place into a lighter one — closer to water running downhill than to anything being blown.

Second: the Sun heats things unevenly

The Sun is beautifully even. What it lands on is not.

A dark car park, a green field, a wood, a lake, a snowfield, a strip of sea — all take the same sunshine and get to wildly different temperatures. Dark surfaces soak it up. Water shrugs most of it off and stirs the rest down deep.

So by mid-afternoon the ground is a patchwork of warm and cool. And the air sitting on each patch takes on that patch's temperature.

Here is the link. Warm air is lighter, so it rises. Where a patch of air lifts off the surface and heads upward, it stops pressing down as hard on the ground it left — so the pressure there drops slightly.

A warm patch is therefore not a place air is being blasted out of. It is a dip. A hole in the weight of the atmosphere. And everything around it starts sliding in.

Check yourself

Three in the afternoon. The sand is far too hot to stand on barefoot; the sea beside it is cold. You have not felt the breeze yet. Which way is it blowing, and why?

A sea breeze, built from nothing

Same beach, from sunrise. Press through and watch a wind assemble itself out of one temperature difference — no fan required at any point.

1 · Early morningno dip, no breeze

sand and sea are still about the same temperature, so the air over both is too

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Rising or sinking?

At the surface, rising air leaves low pressure and sinking air piles up high pressure. Sort these six, and read the reasons — the last two are why whole regions of the planet look the way they do.

Tap an item, then tap where it belongs

Air rising — low pressure below

Air sinking — high pressure below

How big do the dips get?

Ordinary sea-level pressure is about 1013 on the scale forecasters use.

The deepest dip ever measured was in the middle of Typhoon Tip in 1979: 870. The heaviest pile-up was over Siberia in the winter of 1968: 1083.8.

So the whole range the planet has ever managed is roughly 200 units wide — a bit over one part in ten. That is all it takes. Every wind you have ever felt, from a door slamming in a draught to a hurricane, comes out of differences smaller than that.

One more thing, and it's a big one, so it gets its own episode another day. Air sliding into a dip does not go straight in. The Earth is turning underneath it, and that turn bends every moving thing on the planet a little sideways — so the wind spirals around a low instead of diving into the middle. That bend is why storms on a satellite picture are always swirls.

Check yourself

Two in the morning on the same beach. The breeze has quietly turned around and is now blowing from the land out to sea. What changed?

Check yourself

Two weather maps. Both have a pressure dip of the same depth in the middle. On the first, the pressure changes gently over 500 kilometres. On the second, it changes by the same amount in only 100. Which one is windier, and why?

Four things to keep

01Air is heavy, and unevenly so. About ten tonnes pressing on every square metre, and never quite the same amount from one place to the next.
02The Sun heats patches, not the planet evenly. Dark ground, fields, water and snow all take the same sunshine and reach very different temperatures.
03A warm patch is a hole, not a source. Air rises off it and stops pressing down, so nearby air slides in underneath. That slide is the wind.
04Steepness sets the strength. The same pressure difference packed into a shorter distance gives a harder wind — which is what crowded lines on a weather map are telling you.

Keep going

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Where these figures come from

Standard sea-level pressure is 1013.25 hPa, which is 101,325 newtons per square metre. Dividing by g = 9.81 m/s^2 gives about 10,329 kg — roughly ten tonnes of air weight over every square metre of ground. (ICAO Standard Atmosphere; NOAA/NWS JetStream, 'Air Pressure'.) Wind is driven by the pressure gradient force: air accelerates from higher towards lower pressure, and the force is proportional to the pressure difference divided by the distance over which it occurs — which is why closely spaced isobars on a chart indicate stronger wind. (NOAA JetStream, 'The Wind: Origin and Local Effects'; UK Met Office, 'What causes wind?'.) Uneven surface heating drives local circulations: differing surfaces reach different temperatures under identical sunshine, warm air becomes less dense and rises, surface pressure falls beneath the rising air, and surrounding air flows in. (NOAA JetStream, 'Global and Local Winds'.) Sea breeze and land breeze: land heats and cools far faster than water, because water has a much higher specific heat capacity, mixes heat downward through depth, and loses heat by evaporation. By afternoon the land is warmer, air rises over it, and cooler marine air flows inland; after sunset the land cools below the water temperature and the circulation reverses. (NOAA JetStream, 'Sea and Land Breezes'; NWS.) The equatorial belt of rising air (the Intertropical Convergence Zone) and the descending branches of the Hadley circulation near 30 degrees north and south are why persistent low pressure sits along the equator and why many of the world's great deserts lie in the same latitude bands on both sides of it. (NOAA JetStream, 'Global Atmospheric Circulations'.) Pressure extremes: the lowest sea-level pressure ever measured was 870 hPa in Typhoon Tip on 12 October 1979; the highest was 1083.8 hPa at Agata, Siberia, on 31 December 1968. (World Meteorological Organization archive of weather and climate extremes; NOAA.) The Coriolis effect — the apparent deflection of moving air caused by Earth's rotation, which turns inflow into rotation around a low rather than straight into it — is named but deliberately not explained here. (NOAA JetStream, 'The Coriolis Force'.)