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Meteorology · Atmospheric circulation

The atmosphere is one big machine.

Sunlight strikes the equator harder than the poles. The atmosphere spends every second redistributing that heat — and the Earth's rotation bends every move into a curve. The result is three circulation cells per hemisphere, the trade winds, the westerlies, and two ribbons of fast air called jet streams. Switch between the surface you breathe and the jet stream that steers your weather.

Air temperature−55 °C30 °C
How the visualisation works

The velocity field is reconstructed from climatological positions and strengths of the trade winds, westerlies, polar easterlies, the ITCZ, both jet streams, and eight permanent pressure systems (Holton 2004, NOAA GFS, Copernicus ERA5 climatology). Jet meander follows a simple Rossby-wave parametrisation. It is not a live forecast — the goal is a faithful, readable picture of the annual mean. Coastlines are Natural Earth 110m outlines, drawn without fill because wind crosses continents freely. Particle colour is air temperature: each particle carries its own °C value, surface-layer values are latitude-based with a continental cold bias, jet-stream values are uniformly cold (~−50 °C). Fast particles mix slower with their surroundings — that is why the polar jet stays icy-blue all the way around.

How the machine works

Three cells, two jets, eight pressure systems.

  1. The three cells.

    At the equator, warm air rises. At thirty degrees north and south, it sinks — and the descending dry air carves out the subtropical deserts. The cell between equator and 30° was named by George Hadley in 1735. The cell from 30° to 60° is the Ferrel cell, and 60° to the pole is the polar cell. Each closes its own loop, and the Earth's rotation tilts every wind in each cell into the curves you see: trade winds blow from the east in the tropics, westerlies dominate the mid-latitudes, polar easterlies cap the high latitudes.

  2. Jet streams and Rossby waves.

    At the boundary between cells, where cold polar air meets warm subtropical air, the temperature contrast accelerates a narrow ribbon of wind at roughly ten kilometres altitude — the polar jet stream. It can reach 400 km/h. It does not run in a straight line. It meanders in long Rossby waves around the planet, and those meanders steer the storms below. Francis & Vavrus (2012) found Arctic amplification — the disproportionate warming of the Arctic — is making the jet wavier, slower, and more prone to stalling. A stalled jet means stuck weather: heatwaves that don't end, floods that don't move on.

  3. Where weather is born.

    Sinking air at 30° creates permanent high-pressure systems: the Azores High over the North Atlantic, the Pacific High off California, the South Atlantic St. Helena High. Rising air over warm oceans and cold land contrasts creates persistent lows: the Icelandic Low, the Aleutian Low. Each is a vortex you can see in the visualisation — clockwise around highs in the north, counter-clockwise around lows; reversed in the south because Coriolis flips. The Azores High decides whether Western Europe gets a dry summer; the Aleutian Low decides whether the Pacific Northwest gets soaked.

  4. What the wind carries.

    Sahara dust crosses the Atlantic and fertilises the Amazon rainforest with phosphorus (van der Does et al. 2020). Pollen, fungal spores, and bacteria ride trans-oceanic currents — global ecology is partly an aerial network. Methane from melting permafrost mixes within months hemisphere-wide. Microplastic fibres now circulate in stratospheric air samples. The same winds that move heat move what we put into the air, and the trip is fast: a Saharan dust storm can reach Florida in five days.

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