Basically, typhoon is formed by earth’s rotation. The air over the tropical seas is warmer due to the heat of sunlight. Therefore, more sea water is evaporated that makes the air over the tropical seas warm and wet and expands due to its higher temperature, which will in turn reduces its density. This combined with the low wind force in the equatorial region provide a perfect recipe for air upward motion. When the air rises, cooler air from the surroundings will flow in to fill in the space; later this refilling air will be heated and lifted, gradually forming a circulation of air. This process, called convection, will cause the whole column of air to become lighter and lower in density, creating a low pressure system called tropical depression.

  Since the sun shines directly to north of the Equator during summer, the southeast trade wind from the Southern Hemisphere will be transformed into a southwest monsoon when they pass across to the Northern Hemisphere. When a southwest monsoon meets with a northeast trade wind in the Northern Hemisphere, the two will converge and bring the air upward, enhancing the convection effect. Due to the difference of the wind directions and the nature of the southwest monsoon and the northeast trade wind, they will disturb each other when they meet and create a vortex.

  This kind of disturbance and the convection of the tropical depression complement each other to further deepen the already existing low pressure vortex. In other words, they accelerate the speed of air flow in the surrounding area and the typhoon is initially formed.