What it does
A cyclone is a huge storm spinning around a centre of very low pressure. This page shows one forming, seen from above. Air rushes toward a low over the sea, but the spinning Earth turns moving air sideways: to the right in the northern hemisphere and to the left in the southern. This is the Coriolis effect. So the air circles the centre, anticlockwise in the north and clockwise in the south.
Over a warm sea, water vapour rises, turns to cloud and gives out heat, which keeps air rising and pressure falling. The storm feeds itself and an eye forms. Cool the sea, switch off the spin or move to the equator and it fades.
It follows Class 8 Science, Pressure, Winds, Storms and Cyclones, and Class 11 Geography. Before this, wind from pressure shows why air moves from high to low pressure, and standard atmosphere calculator shows air pressure itself.
How to use it
- Press Start again to begin with still air around a weak low.
- Set the latitude: 15° N is about the Bay of Bengal; negative is south.
- Set the sea surface temperature.
- Untick Earth spins to see a planet with no Coriolis effect.
- Read the strength bar and which way the air circles. At Class 11 the page shows f = 2Ω sin φ.
Worked example
At 15° N, sea 29 °C, spin on, the air circles anticlockwise and within about 20 seconds the label reads Growing: a cyclone. At Class 11, f = 2 × 7.292 × 10⁻⁵ × sin 15° = 3.775 × 10⁻⁵ s⁻¹. At 15° S f is negative and the storm turns clockwise. At 0° f = 0 and the air runs straight into the low. Cool the sea to 25 °C and it fades, as below about 26.5 °C there is too little warm, moist air.
Tips
- The Coriolis effect is not a push; it is what straight motion looks like from a spinning Earth.
- Bay of Bengal cyclones are most common before and after the monsoon.
- In a cyclone warning, follow the official weather service.
FAQ
Do sinks drain the other way in the south? Not reliably; the effect is too weak at that size.
Are the strength numbers real? No, they are a cartoon.
Is anything stored? No.
Where the numbers come from
- Air flows toward low pressure; the Coriolis effect turns moving air to the right in the northern hemisphere and to the left in the southern, with Coriolis parameter f = 2Ω sin φ and Ω = 7.2921 × 10⁻⁵ rad/s (one turn per sidereal day): standard physics and geography.
- The rules of thumb that tropical cyclones need a sea of at least about 26.5 °C and form at least about 5° from the equator are commonly quoted in textbooks; real cyclones need more (a starting disturbance, moist air, steady winds aloft).
- The moving air and the storm's growth are a teaching cartoon written for this page, with the Coriolis effect scaled up so it can be seen. Checked by tests: f against hand values, the model spins anticlockwise in the north and clockwise in the south, does not circle without the spin or at the equator, and the Coriolis turn changes the direction of motion but not its speed.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-10.