Germanwatch

How tropical cyclones form

Vast amounts of water evaporate; humid air ascends spinning around an eye, creating a self-reinforcing process fueled by ever warmer humid air. Once tropical cyclones hit the shore, this supply is interrupted and the storm weakens. Highest wind speeds are reached directly around the eye at the so-called eyewall. Precipitation is most intense in that area as well. Huge storms can have several eyewalls.

Illustration: eskp.de/CC-BY (modified).

For a tropical cyclone to form, several conditions need to be fulfilled. Usually sea temperature has to reach 26.5°C down to 50 metres below sea level. The atmosphere needs to be unstable i.e. there has to be a significant difference in air temperature at different altitudes allowing for convection. In contrast, the (vertical) wind shear should be low, otherwise the energy will be scattered over an area too large to form a strong storm. This is the main reason why tropical cyclones are extremely rare in the Southern Atlantic Ocean (NASA, 2004). High humidity in the lower to middle levels of the troposphere – the lowest layer of the Earth's atmosphere and the one where most weather takes place – contributes to the convection of air masses. Furthermore, sufficient Coriolis force is needed. Last but not least, tropical cyclones do not form out of nowhere. They need at least some form of disturbance close to the surface with a horizontal inflow of air (convergence) and a certain degree of horizontal circulation (vorticity) (NOAA 2018).