The minimum sea-surface temperature often cited for tropical cyclone formation is 26.5 °C. This threshold is a useful rule of thumb for the warm ocean water needed to fuel a tropical cyclone’s heat engine.
Warm water supplies evaporation, while condensation in towering thunderstorms releases latent heat. That heat lowers pressure and helps maintain the storm’s warm core. The traditional threshold generally assumes that water at roughly 26.5 °C extends through at least the upper 50 metres, rather than existing only as a thin surface layer.
The figure is not an absolute law. Tropical cyclones can sometimes form or persist over cooler water when unusually cold air aloft creates extra atmospheric instability. Conversely, water warmer than 26.5 °C does not guarantee development: sufficient moisture, a pre-existing disturbance, weak vertical wind shear, and enough distance from the equator are also important.
A common mix-up is treating 26.5 °C as the temperature required for every storm to begin. It is better understood as the conventional minimum for sustaining tropical development under typical atmospheric conditions, not a standalone forecast rule.