What is a Typhoon? How Do Typhoons Form?

An analysis of the definition, formation conditions, and structure of typhoons.

What is a Typhoon

A typhoon is a strong tropical cyclone that occurs in the Northwest Pacific Ocean and the South China Sea. When the maximum sustained wind speed near the center of a tropical cyclone reaches 32.7 meters per second (Force 12) or higher, it is classified as a typhoon.

Four Conditions for Typhoon Formation

• Extensive Warm Ocean Surface: Sea surface temperatures must exceed 26.5°C, with a warm water layer depth of at least 60 meters, providing sufficient moisture and energy for the typhoon.
• Sufficient Coriolis Force: Typhoons generally form in sea areas above 5° latitude (North or South). Near the equator, the Coriolis force is too weak to initiate cyclonic rotation.
• Initial Disturbance: Pre-existing weather disturbances, such as convective cloud clusters within the Intertropical Convergence Zone (ITCZ), serve as the "embryo" for typhoon development.
• Weak Vertical Wind Shear: Small differences in wind direction and speed between upper and lower atmospheric levels allow heat to concentrate within a limited area, facilitating intensification.

Structure of a Typhoon

• Typhoon Eye: The central region, approximately 10–60 kilometers in diameter, characterized by clear skies and light winds.
• Eyewall: The ring of intense convection surrounding the eye, where wind and rain are most severe.
• Spiral Rainbands: Outer bands of rain clouds distributed in a spiral pattern.

Typhoon Classification Standards (China National Standard)