Author: Guanfu · Juntian

  Every summer and autumn, the northwestern Pacific near China produces many violent storms known as typhoons. Some dissipate over the sea, while others make landfall and bring fierce winds and torrential rain. Where do these typhoons come from, where do they go, and why do they affect China so profoundly? Today, I will offer a brief introduction to typhoons.

  What is a typhoon? In essence, a typhoon is a tropical cyclone: as the name suggests, a vortex formed by rotating tropical air. Tropical cyclones can be divided by wind strength into tropical storms, severe tropical storms, typhoons, and other categories.

How Does a Typhoon Form?

  Three conditions are required for a typhoon to form.

  The first is moisture: the ocean provides an inexhaustible supply of water vapor.

  The second is heat. Water vapor can rise only when summer sunlight is strong. Over a tropical ocean, direct sunlight heats the sea surface to about 26.5°C or higher. At this temperature, seawater readily evaporates into water vapor. As evaporation accelerates and the vapor rises, surface pressure falls. Air from every direction—at relatively high pressure—flows toward the center—at relatively low pressure—forming a tropical low-pressure center. Warm, moist air continues to rise, while surrounding air flows toward the low, creating wind.

  How, then, do the typhoon’s eye and spiral shape form? This brings us to the third condition: dynamics.

  The rising warm air climbs higher, cools, and condenses, releasing heat in the process. Pressure falls still further. As air converges on the low-pressure center, the Coriolis force gradually turns it into a rotating circulation. In the Northern Hemisphere, cyclones converge counterclockwise toward the center; in the Southern Hemisphere, they converge clockwise. As the typhoon strengthens, a ring-shaped wall of intense convective clouds forms near its center. Conservation of angular momentum, descending air, and other factors gradually reduce cloud cover and wind speed in the central region, producing the typhoon’s eye.

  A typhoon begins as a tropical depression. From the initial low-pressure circulation to a maximum sustained wind near the center at Force 8 generally takes about two days. A slowly developing depression needs three or four days; a fast one, only a few hours. During the development stage, the typhoon continually absorbs energy, its central pressure falls, and its wind speed rises. After landfall, surface friction and an inadequate energy supply gradually weaken the typhoon until it disappears. This process constitutes a typhoon’s life cycle.

How Large Is a Typhoon?

  Horizontally, a typhoon consists, from the center outward, of the eye region, eyewall region, and spiral rainbands—in other words, an inner, middle, and outer ring.

  1. The inner ring is the eye

  The circular or elliptical eye lies at the typhoon’s center. Because the centrifugal force generated by the cyclone pulls air away from the center, winds in the eye are actually the weakest in the storm. It is the typhoon’s “haven of peace,” with a radius of about 5–30 kilometers.

  2. The middle ring is the eyewall

  Conditions here are severe. Convection, wind, and rain are at their strongest and most destructive, and the typhoon’s devastating winds and downpours are concentrated in this region. Its radius is generally tens to more than one hundred kilometers.

  3. The outer ring contains the spiral rainbands

  The typhoon’s influence gradually weakens here. The radius can reach several hundred or even more than a thousand kilometers. Vertically, a typhoon is generally about 12–18 kilometers high.

Where Does the Name “Typhoon” Come From?

  Is a “typhoon” a wind that blows from Taiwan?

  No. The prevailing view is that the English word “typhoon” may be a transliteration of the Cantonese Chinese pronunciation “toi fong,” which later entered Mandarin vocabulary. There are many explanations, however, for how the Cantonese word itself arose.

Why Are There No Typhoons at the Equator or the Poles?

  Even when the equatorial region has enough moisture and heat, the Coriolis force there is too weak. Air can rise, but the Coriolis force is insufficient to turn the flow into a vortex.

  At the poles, the seawater is too cold and not enough evaporates. A typhoon can arise only through the combined action of moisture, heat, and dynamic conditions.

Why Does a Typhoon Move Toward Land after It Forms?

  The secret is the subtropical high. A typhoon lies below it and is steered forward by the airflow along its edge.

Why Does a Typhoon Always Bring Wind and Rain When It Makes Landfall?

  Most of China has either a subtropical monsoon climate or a temperate continental climate. The South China Sea summer monsoon acts like a pump, carrying warm, moist air from the ocean toward land. When a moving typhoon encounters that monsoon, it gathers up this warm, moist air. The typhoon itself was also born over a tropical ocean and is rich in water vapor, so it brings a great deal of typhoon rain when it reaches land.

  A typhoon also produces rain while it is over the ocean; we simply pay less attention to it there. After landfall, the storm no longer receives moisture from the ocean and is also affected by surface friction. Once wind and rain have released its energy, it gradually disappears.

The Dangers of Typhoons

  A typhoon is a typical form of hazardous weather. It can bring destructive winds, extreme rainfall, storm surges, and a series of other disasters that severely affect human production and daily life. Fierce winds possess enormous destructive power: they can destroy homes, buildings, and elevated facilities and sever power and communications lines. Extreme rainfall frequently causes flooding and readily triggers debris flows, landslides, and other geological disasters. Storm surges can overturn vessels at sea, breach seawalls, and send seawater flooding inland. Every year, landfalling typhoons cause major economic losses in China.

  Typhoons can also bring abundant rainfall, however, easing the midsummer drought in the middle and lower reaches of the Yangtze River to some extent and temporarily relieving extreme heat.

  Satellite technology can now monitor typhoons effectively, but observation and warnings alone are far from enough. When a typhoon approaches, close doors and windows securely, prepare supplies, avoid going outside wherever possible, and stay away from hazardous objects such as billboards and large trees.