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Three-cell circulation

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Field photograph or official scientific figure for teaching three cell. A second photograph for the projector script — not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CC

Why this lesson, and why the 3D

Three-cell circulation is a fifteen-minute projector lesson, not a homework packet and not a film with a quiz bolted on. Put this hook on the board before you dim the lights: One cell would be the textbook. Three cells are the planet we have. By the end of the slot a pupil should be able to toggle 1-cell vs 3-cell, name Hadley / Ferrel / Polar, and watch the ITCZ walk with the seasons. Curriculum tags in the header (GCSE weather; A-level; NGSS HS-ESS2-4; APES 4.5) are filters, not a flag. The room is worldwide; SI first; International English. Ages on the card (GCSE, A-level, NGSS MS, APES) are a suggested band, not a gate.

ITCZ and West African rains, the Florida wet season, and a UK westerly year are this walk. One cell would send rise at the equator and sink at the pole; London would sit in a steady polar northerly and Florida would never see a season in the rain. Three cells are the planet we have: Hadley in the tropics, Ferrel in the midlatitudes, Polar at high latitude. GCSE names the cells. A-level and APES need the ITCZ as a moving rainband, not a painted equator. If pupils think the equator is always the wettest place, fixed, they will misread a June Florida convective afternoon and a January UK chart in the same breath. Play a year. Watch the thermal equator walk. Then put the UK in the Ferrel story, not in Hadley.

The 3D is a named, inspectable object. Click the part, not a caption. If the model will not spin, the Why panel still carries the mechanism, the two photographs, and the film. Do not apologise for the model. Drive it. A silent spin teaches a screensaver. A named mesh that changes under a slider teaches the process.

Fifteen minutes on the projector

Toggle Three cells off. One cell: hot equator rise, pole sink. Ask why UK westerlies would not exist on that planet. Toggle Three cells on. Name Hadley, Ferrel, Polar. Play a year. The ITCZ walks with the subsolar latitude. The equator is not always the wettest place, fixed. Rain follows the ITCZ. That is the misconception. Hadley is the tropical loop. Ferrel is the midlatitude indirect cell. Polar is the high-latitude loop. Do not draw three equal cells. Do not put the UK in the Hadley cell. West African rains and the Florida wet season are this walk. Finish in Check.

Open the lab full screen (F). Labels on (L). Speak the first step before you touch a slider: One cell: hot equator rise, pole sink. Too simple for a spinning planet. Then the second: Three cells. Hadley, Ferrel, Polar. Play a year: ITCZ walks. Then the third: Hadley is the tropical loop. Ferrel is the midlatitude indirect cell. Polar is the high-latitude loop. ITCZ is the meeting of the trades.

The sliders that earn their keep are Three cells. Space plays; arrows nudge time; R resets. Parameters the URL can remember: three. If you only have eight minutes, skip the last step and finish in Check. Projector layout (P) is for the back row: one idea, large type, the model still live.

Toggle Three cells off for the 1-cell planet, then on. Space plays a year so the ITCZ walks. Arrow keys scrub the seasons; freeze on a solstice and ask where the rain went. L hides labels; make them name Hadley, Ferrel, Polar. R resets the year and the toggle. P is projector. This lab has a year clock: 1× 2× 4× if you need a faster walk. Share the URL on a June ITCZ over Florida’s latitude, then again in December, so the walk is a pair of links not a hand wave.

The in-site film is “Where Does Wind Come From? Crash Course Geography #8” (Crash Course Geography, about 11 min, Crash Course / Complexly on YouTube. Standard YouTube licence. Educational embed, autoplay off.). Autoplay is off. Play one minute if you need a second voice; do not let the film eat the slot. The 3D still has to move under your hand.

What the third row will get wrong

You will hear the misconception before you have finished the first sentence. Write the false claim on the board and leave it there until the model has contradicted it. Do not skip this. A fluent pupil can recite the hook and still believe the false sentence.

“The equator is always the wettest place, fixed.” — no. The rain follows the ITCZ, which walks with the seasons.

“The UK sits in the Hadley cell.” — no. Hadley is tropical. The UK’s westerlies live in the midlatitude Ferrel story, storm tracks included.

“The three cells are equal in size and equally clean.” — no. Hadley is the big thermally direct loop. Ferrel is indirect and stormy. Do not draw three matching doughnuts.

Pitfalls to keep in your pocket: Drawing three equal cells. Putting the UK in the Hadley cell. Teaching the ITCZ as a painted line on the equator. Selling Ferrel as a clean thermally direct loop. Do not let a pupil leave thinking the 3D is a photograph. It is a teaching model. Where vertical exaggeration exists, T collapses it and the caption says so. A labelled lie of scale is honest. An unlabelled one is a cartoon.

Two places, not a slogan

On a non-rotating planet a single thermally direct cell could run from equator to pole. Earth rotates, Coriolis breaks that loop, and the school model is three cells each side of the equator. Hadley: rise near the thermal equator (ITCZ), poleward aloft, sink in the subtropics, return as the trades. Ferrel: the midlatitude indirect cell, driven more by the cells and the eddies either side than by local heating alone; real midlatitudes are storm tracks, not a clean classroom loop. Polar: rise near 60°, sink at the pole. The ITCZ is where the trades meet and air rises; it walks with the subsolar latitude through the year, so rain follows heat. Florida’s wet season is that walk in one peninsula. West African rains are the same walk on another coast. The UK sits in westerlies under the Ferrel story, not in Hadley; do not draw three equal cells, and do not park London on the equator. Toggle Three cells off to show the failed 1-cell planet, then on again. Play a year so the ITCZ is a traveller, not a tattoo.

The first place to land is Florida wet season. The second is ITCZ and seasons. Same process, different rock or climate. If a pupil can only name one place, they have a postcard, not a mechanism. Atlas entries for those names are a click from the case list; send them back to the lab with one sentence they can say in Check.

Do not flatten the comparison into “this country versus that country.” Name the landform. Holderness is till, not chalk. Hawaii is a hotspot, not an island arc. Yosemite is a glacial trough, not a river canyon. The 3D is allowed to be schematic; the place names are not allowed to be sloppy.

Figures and the film

Field photograph: The ITCZ as a rain band from orbit. The ITCZ as a rain band from orbit. This is the classroom sense of three-cell: a named landform, process, or object. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC. Credit: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.

Mechanism photograph: Field photograph or official scientific figure for teaching three cell. Field photograph or official scientific figure for teaching three cell. A second photograph for the projector script — not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC. Credit: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.

Two photographs, two jobs. The field picture is the place. The mechanism picture is the process. Do not run the lesson on a single Apollo Earth disc. A carbon-cycle class is a forest and a seam, not a blue marble. An ENSO class is a Pacific temperature map, not Africa from Apollo 17.

Words they must be able to say

Say these out loud once while you click the matching mesh. If a word has no mesh, it still has a photograph in Terms.

Hadley cell: Tropical thermally direct cell. Rise near the ITCZ, sink in the subtropics. Ferrel cell: Midlatitude cell, indirect in the school model, driven by the cells and storms either side. Polar cell: High-latitude cell. Rise near 60°, sink at the pole. ITCZ: Belt where the trades meet and air rises. Walks with the seasons. Exam language also accepts “Intertropical convergence zone”. Trade winds: Surface return of the Hadley cell, toward the ITCZ, deflected by Coriolis. Subtropical high: Belt of sinking air on the poleward side of Hadley. Deserts often sit under it. Thermal equator: Latitude of strongest heating, not always 0°. The ITCZ follows it. Westerlies: Midlatitude west-to-east winds. The UK’s usual steering, under the Ferrel story.

Exam-language toggle (the Teacher tab) swaps UK/US pairs where they exist — levée/levee, arête/arete — so a mixed room is not punished for spelling.

Close the slot

If JavaScript or WebGL dies, the 2D fallback still shows the three loops. Teach from the steps: 1-cell failure, three names, ITCZ walks with the year. Check still asks where Hadley rises, why the ITCZ walks, why Ferrel is indirect, and why one cell fails. Print the teacher script. The glossary still names trades, subtropical high, and westerlies. You lose the globe tour, not the reason Florida wet season and UK westerlies are different cells. A board sketch of three loops can stand in for the year walk.

Finish in Check — ten questions, not a mood. The bank mixes definition, process, the misconception you just killed, a named place, and a read-the-model item (click the part; T). If a pupil can only recite the hook, they have not used the geometry. Wrong answers still show the one-line explain. Do not skip the explain.

Sources on the Teacher tab are the trail, not a bibliography to copy. NOAA circulation; Met Office tropical weather.

Link back to the live lab and keep the same slug. Do not invent a parallel worksheet that contradicts T, SI, or the named parts.

This note is worldwide. Curriculum tags (GCSE weather; A-level; NGSS HS-ESS2-4; APES 4.5) are filters, not a flag. A class in Nairobi, London, or Shanghai can use the same 3D. Ages (GCSE, A-level, NGSS MS, APES) are a suggested band. If a sentence only works in one exam board, rewrite it before you project it.

Write these three lines on the board before you dim the lights. One: One cell would be the textbook. Three cells are the planet we have. Two: the misconception “The equator is always the wettest place, fixed.”. Three: the named parts they must click. Then drive the model. Check is ten questions covering definition, process, the misconception, a place, and a read-the-model item. Do not skip the explain on a wrong answer.

Board, units, and what not to say

SI first. Metres, kilometres, degrees Celsius, metres per second. A conversion in brackets is courtesy, not a second system. Do not let a pupil leave thinking the 3D is a photograph. It is a teaching model. Where vertical exaggeration exists, T collapses it and the caption says so.

Do not say “teaching mesh”, “sprinkle”, or “thin free surface not a fat pipe” out loud. Say the landform. Seasons are ~23.44° of axial tilt, not Earth–Sun distance. The outer core is liquid; S-waves do not pass. Yosemite is a glacial U-trough. San Andreas is a transform. Hawaiʻi and Yellowstone are hotspots. Florida is not a glacial trough.

If the room is noisy, projector layout (P) is the back-row setting: one idea, large type, the model still live. Full screen (F). Labels (L). Space plays. Arrows nudge time. R resets. The URL can remember three. Send them home with the same link; there is no login.

In-site film

Where Does Wind Come From? Crash Course Geography #8. Crash Course Geography. Crash Course / Complexly on YouTube. Standard YouTube licence. Educational embed, autoplay off. Autoplay off.