but what if other way

1. Sunlight. Daily sunshine for every 2° cell from its latitude, the season and the axial tilt. Snow and sea ice reflect more of it.
2. Highs and lows. The planet’s spin sets how wide the tropical overturning cells are, and that places belts of high and low pressure. A faster spin packs in more belts. Hot land in summer makes extra lows (monsoons); frozen land in winter makes extra highs.
3. Wind. Air flows from high to low pressure but gets turned sideways by the spin (the Coriolis effect) and slowed by friction. This is the one place the direction of spin enters: flip it and every wind turns the other way around the same highs and lows. Trade winds blow from the west; mid-latitude winds blow from the east.
4. Heat and ocean. The wind carries warm or cold air inland. The same winds push the sea, and the model solves for the ocean gyres they drive (Stommel’s 1948 theory). The fast boundary current, today’s Gulf Stream or Kuroshio, forms on whichever side of an ocean the spin dictates. Winds blowing along coasts pull up cold water.
5. Rain. Water evaporates from sea and soil, rides the wind, piles up where air converges and rains out near saturation, especially when it is forced up a mountain. Downwind of the mountain is a rain shadow, so the rain shadows flip too.
6. Deep overturning. Salty water is dense and sinks. The model compares how much fresh water the Atlantic and the Pacific lose to evaporation. The saltier one gets the deep overturning, whose warm surface branch carries about 1 PW of heat to that ocean’s subpolar sea, as today’s Atlantic does for Europe.

Built from real topography (NOAA ETOPO5), 180 × 90 cells, one-day steps, about 2,200 lines of plain JavaScript. The two planets above are precomputed from a six-year run; the experiments run in a Web Worker on your device.

• It’s a toy. Real climate models resolve the atmosphere in 3D, with storms, clouds and a deep ocean. This one has one air layer, one ocean layer and a lot of approximations.
• Tuned to today, checked against the other way. Constants were adjusted until today’s Earth looked right: global mean ≈ 14 °C (real ≈ 14 °C), rain ≈ 2.8 mm/day (real ≈ 2.7), and Köppen zone shares mostly within a few points of the real map (tropical climates come out too scarce: 12% vs 19%). Nothing was tuned toward the reversed planet.
• Europe is too cold, even today. Western European winters run about 7–8 °C too cold, and London and Paris come out continental instead of oceanic. The westerlies off the Atlantic are too weak in winter. The change between the two planets is the point here, not the absolute values.
• The overturning doesn’t move. In the Max Planck run, the deep overturning moves to the Pacific when the spin flips. Here the Atlantic still evaporates more on the reversed planet, so the overturning stays put. The model has no river runoff (the Amazon and Congo freshen the Atlantic) and no feedback from the overturning’s own salt transport, either of which could tip it.
• Weak monsoons. The Indian monsoon barely appears, so South Asia is too dry on both planets.
• Known misses. The US Southeast is too dry on today’s Earth, and the model then has it getting wetter when the spin flips. Max Planck finds the opposite. Arabia also goes the wrong way.
• Coarse. At 2°, the Andes, Alps and many islands blur into their surroundings. A tap shows a regional climate, not a city’s.