# Geostrophic current On a rotating planet, large-scale ocean and atmosphere flow does not run from high pressure to low. It runs *along* the pressure contours, because [[Coriolis_force|Coriolis]] deflection balances the pressure gradient instead of opposing the motion. That balance is geostrophy, and it is why a [[Weather_forecasting|weather map]]'s wind arrows parallel the isobars rather than crossing them — the organising principle of [[Geophysical_fluid_dynamics]]. ## Microsim — p5.js Sidecar: `Geostrophic_current.p5.js` · route leaf: `microsim/p5js/Geostrophic_current__20260910T0400Z`. Built to the Betterfire Standard v0 (single `ARTICLE` constant, HUD title and Wikitube URL, control hints, parameter readout and equation). | Control | Does | |---------|------| | `slope slider` | sea-surface pressure gradient | | `f slider` | Coriolis parameter, zero at the equator | | `[p]` | show or hide the pressure contours | *What to watch:* Tilt the sea surface and watch the current refuse to run downhill. Crank Coriolis to zero and it immediately does — the balance is the only thing holding it sideways. ## A balance, not a force Start water moving down a pressure gradient and rotation deflects it sideways; it keeps turning until the deflection points exactly back up the gradient, and then nothing changes. The flow is now perpendicular to the force driving it — deeply counterintuitive, and completely standard above a few tens of kilometres — a [[Dimensionless_quantity|scale]] argument. The approximation fails near the equator, where the [[Coriolis_force]] vanishes, and in narrow or fast flows where acceleration matters, which is why equatorial and coastal dynamics need their own treatment. ## Reading currents from height Geostrophy is what makes satellite oceanography possible. Measure sea-surface height to a couple of centimetres and the slope gives the pressure gradient, which gives the current — no ship required. That is how the global [[Ocean_current]] field is now mapped continuously. The same reasoning inverted lets a research vessel compute currents from density profiles alone, the classical hydrographic method that built [[Thermohaline_circulation|deep circulation]] theory from [[Density]] and [[Properties_of_water|salinity]] alone. **Reads with:** *Introduction to Physical Oceanography (Robert H. Stewart, 2008)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/introduction-to-physical-oceanography) · [download](https://github.com/introocean/introocean-en/releases/tag/v20200229) · CC BY-NC-SA. Section 7 of the [[PORTAL_Thury_Hydrodynamics_Apex_Spine|Apex Spine]] book shelf. **On the spine:** [[Coriolis_force]] · [[Ekman_transport]] · [[Ocean_current]] · [[Geophysical_fluid_dynamics]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Geostrophic_current) : [Wikitube](https://en.wikitube.io/wiki/Geostrophic_current) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]]. --- *Book-section wave · 2026-09-10 · article + p5 microsim shipped together.*