Watch how the Coriolis effect transforms a pressure low into a rotating cyclone. Particles deflect right in the Northern Hemisphere (anticlockwise spin) and left in the Southern (clockwise).
The Coriolis force is a fictitious force in a rotating reference frame: F = −2m(ω × v). In the Northern Hemisphere, moving air deflects to the right, creating anticlockwise rotation around low-pressure centres. In the Southern Hemisphere, the deflection is reversed.
Toggle between Northern and Southern Hemisphere to see cyclone rotation reverse. Switch between cyclone (low pressure) and anticyclone (high pressure) modes. Adjust particle count and observe the emerging spiral pattern.
The Coriolis effect is negligible for small-scale phenomena (bathtub drains) but dominant at planetary scales. A category 5 hurricane contains the energy equivalent of a 10-megaton nuclear bomb exploding every 20 minutes.
This simulation shows how the Coriolis effect turns a simple pressure low into a spinning cyclone. Roughly 300 particles are drawn toward a central low by a pressure-gradient force that falls off with distance, while the deflecting term F = −2Ω × v bends their paths sideways. The result is the familiar spiral: anticlockwise around lows in the Northern Hemisphere and clockwise in the Southern. It is a clear, visual way to see why rotation, not suction, sets a storm's swirl.
Each particle feels an inward pressure-gradient force that scales like 1/r² toward the centre, plus a Coriolis acceleration proportional to its velocity and the hemisphere sign (+1 north, −1 south). A small drag factor of 0.97 mimics friction. Together these produce inward spirals for a low (cyclone) and outward spirals for a high (anticyclone), matching the real F = −2Ω × v rotating-frame physics with Earth's Ω of 7.27×10⁻⁵ rad/s shown on screen.
The Coriolis slider scales the deflecting force from 0 to 3×, and the Inflow slider scales the pressure-gradient pull from 0.1 to 3×. Toggle NH (CCW) or SH (CW) to flip the hemisphere, and pick Cyclone (low pressure), Anticyclone (high pressure), or No Coriolis to compare. The readouts track spin direction, hemisphere, particle count and average speed.
The Coriolis force is far too weak to control which way a bathtub or toilet drains; those swirls are set by basin shape and residual motion. It only dominates over distances of tens to hundreds of kilometres, which is why large weather systems, not sinks, obey it.
It is an apparent sideways deflection of moving objects when viewed from a rotating reference frame such as the spinning Earth. Mathematically it appears as the term F = −2Ω × v, where Ω is the planet's rotation rate. Nothing actually pushes the air sideways; the deflection arises because the ground beneath it is itself turning.
Air rushing inward toward a low-pressure centre is deflected to the right in the Northern Hemisphere and to the left in the Southern. That consistent sideways nudge curves every inflowing stream the same way, producing anticlockwise rotation in the north and clockwise rotation in the south. Flipping the hemisphere button in the simulation reverses the sign and the spin.
The Coriolis slider multiplies the deflecting acceleration from 0 to 3×, so at zero the particles fall straight into the centre and at 3× they orbit tightly. The Inflow slider multiplies the pressure-gradient force from 0.1 to 3×, controlling how strongly particles are pulled toward the low. The balance between the two sets how open or tight the resulting spiral becomes.
It uses the correct rotating-frame deflection F = −2Ω × v and a realistic inward pressure-gradient force, so the qualitative behaviour, spiral direction, hemisphere reversal and the difference between lows and highs, is faithful. It is a simplified two-dimensional toy: it omits the vertical structure, moisture, latitude variation of the Coriolis parameter and full Navier–Stokes fluid dynamics, so it is for intuition rather than forecasting.
A cyclone forms around low pressure: the pressure-gradient force points inward, drawing particles toward the centre where they spiral in. An anticyclone forms around high pressure: the force points outward, so particles spiral away from the centre, and the Coriolis deflection makes them turn the opposite way to a cyclone in the same hemisphere. The No Coriolis mode removes the deflection entirely, leaving only straight radial motion.
This simulation models how Earth's rotation generates the Coriolis force that bends the paths of air masses flowing into low-pressure centres, causing them to spiral into rotating cyclones. Each particle in the simulation experiences an inward pressure-gradient force (F proportional to 1/r squared) and a deflecting Coriolis acceleration (F = -2 * omega x v), where omega is Earth's angular velocity of 7.27 x 10 to the power of -5 radians per second. The interplay of these two forces produces the characteristic counterclockwise spiral of Northern Hemisphere cyclones and the clockwise spiral of Southern Hemisphere ones.
Cyclones and the Coriolis effect govern some of the most destructive and economically significant weather events on Earth, from Atlantic hurricanes and Pacific typhoons to extratropical storms that shape mid-latitude climates. Understanding the physics behind their rotation is fundamental to numerical weather prediction, storm-track forecasting, and the design of global atmospheric circulation models.
The Coriolis force is a fictitious (pseudo) force that appears when motion is described from within a rotating reference frame such as the spinning Earth. Because the surface of the Earth moves eastward faster at the equator than at the poles, an air parcel travelling poleward lags behind the ground beneath it, appearing to curve. The mathematical form is F = -2m(omega x v), where omega is the planet's rotation vector and v is the parcel's velocity relative to the rotating frame. No actual sideways push exists in an inertial frame; the deflection is purely a consequence of describing motion on a spinning sphere.
Click the Cyclone button to set the central pressure low and watch particles spiral inward. Then click Anticyclone to flip to a high-pressure centre: the pressure-gradient force reverses direction, pushing particles outward, and the Coriolis deflection creates the opposite spin direction in each hemisphere. Use the No Coriolis button to remove the deflecting term entirely so you can see the purely radial inflow or outflow without any rotation, making the role of the Coriolis force immediately visible by contrast.
The Coriolis force only dominates at horizontal scales larger than roughly 100 kilometres and over time scales of hours to days, where its cumulative deflection overcomes the pressure-gradient force and inertia. Below these scales, such as in thunderstorm cells or kitchen sinks, surface tension, basin geometry, and residual fluid motion far outweigh any rotational deflection. This length-scale threshold is quantified by the Rossby number: when Ro = U / (f * L) is much less than 1, rotation governs the flow; when Ro is much greater than 1, rotation is negligible. For typical mid-latitude weather systems U is around 10 m/s, f is around 10 to the power of -4 s to the power of -1, and L must exceed 100 km for Ro to fall well below 1.
In two dimensions, the Coriolis acceleration is written as a_x = 2 * omega * sin(phi) * v_y and a_y = -2 * omega * sin(phi) * v_x, where phi is latitude and omega is Earth's angular velocity. The factor 2 * omega * sin(phi) is called the Coriolis parameter f and equals approximately 10 to the power of -4 s to the power of -1 at mid-latitudes. In the simulation the hemisphere sign (+1 for north, -1 for south) replaces the full latitude dependence, and the Coriolis slider scales the coefficient from 0 to 3 times its default value. The pressure-gradient force is modelled as a radial acceleration decaying exponentially with distance, and a drag factor of 0.97 per time step approximates atmospheric boundary-layer friction.
Tropical cyclones (called hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and simply cyclones in the Indian Ocean and South Pacific) are the clearest natural demonstrations. Hurricane Patricia in 2015 reached maximum sustained winds of 345 km/h, the strongest ever recorded in the Western Hemisphere, and its tight spiral eyewall perfectly illustrated the balance between the Coriolis deflection and the inward pressure-gradient force. Extratropical cyclones affecting Western Europe, such as the Great Storm of 1987 and Storm Ciaran in 2023, show the same anticlockwise rotation at larger scales and weaker intensity over the North Atlantic.
Correct: the Coriolis force is far too weak to determine the drain direction of a bathtub or toilet. At basin scales of roughly half a metre, the Rossby number exceeds 10,000, meaning inertia dominates completely. The direction water drains depends overwhelmingly on how the tub was filled, any residual motion, asymmetry in the drain, and the shape of the basin. Controlled laboratory demonstrations that do show Coriolis-driven rotation require very large, carefully filled tanks left undisturbed for 24 hours or more before the plug is pulled, which is never the case with a normal bathtub. The popular myth persists because the conclusion sounds plausible but the magnitude of the effect is negligible at human scales.
The French engineer and mathematician Gaspard-Gustave de Coriolis published the definitive mathematical treatment in his 1835 paper "Sur les equations du mouvement relatif des systemes de corps" (On the equations of relative motion of a system of bodies). He derived the fictitious forces that appear in rotating reference frames while studying the efficiency of water wheels and machinery, not meteorology. The meteorological application came later: William Ferrel applied the concept to atmospheric circulation in 1856, explaining the trade winds and the rotation of mid-latitude storms in what became known as Ferrel's Law. The force was not named after Coriolis until the late 19th century.
The jet streams, fast ribbons of wind at 8 to 15 km altitude encircling the planet, arise from the same geostrophic balance between the Coriolis force and pressure gradients that governs surface cyclones. Ocean gyres such as the North Atlantic Subtropical Gyre and the Kuroshio Current system are large-scale ocean analogues driven by the same force. The Hadley, Ferrel, and Polar circulation cells that structure global climate all depend on Coriolis deflection to turn poleward-moving air eastward. Related simulations to explore include jet-stream, atmospheric-front, and cloud-formation pages on this site.
Numerical weather prediction models such as ECMWF's IFS and NOAA's GFS explicitly solve the rotating-fluid equations including the Coriolis term on global grids with horizontal resolution as fine as 9 km, enabling 10-day forecasts with skill. Offshore structures, wind farms, and shipping routes are designed using return-period statistics of cyclone intensities derived from models validated against these physics. Industrial cyclone separators, used to remove particulates from gas streams in cement plants, power stations, and pharmaceutical manufacturing, are named after the same phenomenon because they exploit centrifugal and inertial effects analogous to those in atmospheric vortices, though they operate without the Coriolis force itself.
Rapid intensification of tropical cyclones, where wind speeds increase by more than 56 km/h in 24 hours, remains poorly predicted because it depends on fine-scale ocean-atmosphere coupling, convective organization, and boundary-layer processes not fully resolved in operational models. Climate change research is investigating whether a warmer ocean will produce more category 4 and 5 storms globally, even if total cyclone frequency decreases. The influence of the Coriolis effect at high latitudes on polar vortex disruptions linked to extreme winter weather events is an active area of study. On planetary scales, researchers studying the atmospheres of Jupiter, Saturn, and exoplanets apply the same Coriolis physics to interpret the banded cloud structures and giant persistent vortices observed by spacecraft.