Look at any synoptic chart: the wind never flows straight into a low, it circles — anticlockwise in the NH, clockwise in the SH. The same effect spins tropical storms in opposite directions across the equator. The cause is the Coriolis force — the apparent sideways push that emerges from describing motion on a rotating Earth.
Parcels released at the equator, both heading straight to their nearest pole through inertial space.
NH parcel deflects right of motion (eastward).
SH parcel deflects left of motion (eastward).
The dashed grey line shows the parcel's straight-line path if Earth weren't rotating. Deflection grows with latitude (sin φ) — zero at the equator, maximum near the poles.
What the Oxford ATPL says
Coriolis Force, (CF), is the force caused by the rotation of the earth. It acts 90° to the wind direction causing air to turn to the right or veer in the Northern Hemisphere and to the left or back in the Southern hemisphere. CF is maximum at the poles and minimum at the Equator.
The Coriolis force is not a true force but is an explanation of the effect the rotation of the earth has on a free moving body not in contact with the earth.
CF = 2 Ω ρ V sin φ
— Oxford ATPL, Meteorology, Ch.10 Winds
Why it happens
Earth rotates west to east. The ground at the equator races east at ~1 670 km/h; at 30° latitude it slows to ~1 450 km/h; at 60° to ~835 km/h. An air parcel carries the eastward speed of the latitude it started at. As it drifts north, the ground beneath it turns slower — the parcel outruns the surface and appears to drift east. That eastward drift is the rightward Coriolis deflection in the NH (left in the SH).
Buys Ballot's law and the geostrophic wind
In the Northern Hemisphere, with your back to the wind, the low pressure is on your left. In the Southern Hemisphere, with your back to the wind, the low pressure is on your right.
— Oxford ATPL, Meteorology, Ch.10 Winds
The rule is the consequence of two forces balancing:
- Pressure-gradient force (PGF) points from high to low, perpendicular to the isobars.
- Coriolis force points 90° to the wind — right of motion in NH (left in SH) — and grows with wind speed.
Take a NH parcel at rest with the low to its north:
- PGF pushes it north toward the low.
- Coriolis (right of motion) deflects it east.
- The parcel keeps turning right until Coriolis points exactly south, cancelling PGF. Wind then blows due east — parallel to the isobars, low on the left.
That is the geostrophic wind. Flip to SH, Coriolis reverses, the same setup balances with wind blowing west and the low on the right.
Surface wind — friction modifies the rule
Geostrophic balance applies above the friction layer (~2 000 ft AGL). At the surface, friction (a drag opposite the wind) joins the balance. The wind no longer runs parallel to the isobars — it tips toward the low, crossing the isobars at about 30° over land and 10° over sea. Surface wind therefore spirals into a low and out of a high.
The latitude factor
The strength of the Coriolis force scales with the sine of latitude:
- Equator: sin 0° = 0 — no deflection.
- Poles: sin 90° = 1 — maximum deflection.
- Within ~5° of the equator, Coriolis is too weak for tropical revolving storms to form.
Common mistakes
- Treating Coriolis as a real push. It is an apparent force from the rotating reference frame, not a physical push.
- Applying it to stationary objects. Coriolis acts only on motion. A parked aircraft feels none.
- Confusing NH and SH. Deflection is mirror-image: NH right, SH left. Around a low → anticlockwise NH, clockwise SH; around a high → the opposite.
- Believing it spins your bath water. Coriolis is far too weak at sink scale.
Why it matters
Every ATPL meteorology paper tests Coriolis through Buys Ballot, the direction of flow around lows and highs, and the tropical-storm no-formation zone near the equator.