Author: Guanfu · Juntian

  Every summer and fall, violent storms known as typhoons form over the northwestern Pacific near China. Some dissipate at sea, while others make landfall with fierce winds and torrential rain. Where do typhoons come from? Where do they go? And why do they have such a powerful effect on China? Here is a short introduction.

  What exactly is a typhoon? It is a type of tropical cyclone: a rotating storm system that forms in the tropics. Tropical cyclones are classified by wind strength into tropical storms, severe tropical storms, typhoons, and other categories.

How Does a Typhoon Form?

  A typhoon needs three conditions to form.

  The first is moisture. The ocean provides a steady supply of water vapor.

  The second is heat. Strong summer sunlight allows water vapor to rise. Over a tropical ocean, direct sunlight warms the sea surface to about 26.5°C or higher, a temperature at which seawater readily evaporates. As evaporation accelerates and the water vapor rises, surface pressure falls. Air under relatively high pressure flows in from all directions toward the area of relatively low pressure, forming a tropical low-pressure center. Warm, moist air keeps rising, and the surrounding air flows toward the low, creating wind.

  How do the eye and spiral shape of a typhoon develop? That brings us to the third condition: atmospheric dynamics.

  As warm air rises, it cools and condenses, releasing heat along the way. The pressure drops still further. Air converging on the low-pressure center is gradually turned by the Coriolis force, creating a rotating circulation. In the Northern Hemisphere, cyclones spiral counterclockwise toward the center; in the Southern Hemisphere, they spiral clockwise. As the typhoon strengthens, a ring-shaped wall of powerful convective clouds forms near the center. Conservation of angular momentum, descending air, and other factors gradually reduce the cloud cover and wind speed at the center, producing the eye.

  A typhoon begins as a tropical depression. It generally takes about two days for an initial low-pressure circulation to develop maximum sustained winds near the center at Force 8 on the Beaufort scale. A slow-developing depression may need three or four days, while a fast one can take only a few hours. As the storm develops, it continually absorbs energy, its central pressure drops, and its wind speed rises. After landfall, surface friction and a dwindling energy supply gradually weaken it until it disappears. That is the basic life cycle of a typhoon.

How Big Is a Typhoon?

  Viewed horizontally, a typhoon has three main regions moving outward from its center: the eye, the eyewall, and the spiral rainbands. These form the inner, middle, and outer rings.

  1. The inner ring is the eye

  The circular or elliptical eye lies at the center of the typhoon. Because the cyclone’s centrifugal force pulls air away from the center, the eye actually has the storm’s weakest winds. 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 strongest and most destructive in the eyewall; this is where the typhoon’s devastating winds and downpours are concentrated. Its radius generally ranges from tens of kilometers to more than one hundred.

  3. The outer ring contains the spiral rainbands

  The typhoon’s effects gradually weaken in this region. Its radius can extend several hundred or even more than a thousand kilometers. From bottom to top, a typhoon is generally about 12–18 kilometers high.

Where Does the Name “Typhoon” Come From?

  Does “typhoon” mean a wind that blows from Taiwan?

  No. The prevailing view is that the English word “typhoon” may come from a transliteration of the Cantonese pronunciation “toi fong,” which later entered Mandarin. There are many theories, however, about where the Cantonese term itself came from.

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

  The equatorial region may have enough moisture and heat, but the Coriolis force there is too weak. Air can rise, yet the Coriolis force is not strong enough to turn the flow into a vortex.

  At the poles, seawater is too cold to produce enough evaporation. Typhoons can form only where moisture, heat, and the right dynamic conditions act together.

Why Does a Typhoon Move Toward Land after It Forms?

  The answer lies in the subtropical high. A typhoon sits below it and is steered forward by air flowing 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 the land. When a moving typhoon meets the monsoon, it draws in that air. The typhoon was itself born over a tropical ocean and already contains abundant water vapor, so it brings heavy rain when it reaches land.

  Typhoons also produce rain over the ocean; we simply pay less attention to it there. After landfall, the storm loses its supply of ocean moisture and encounters friction from the ground. As wind and rain release its energy, the typhoon gradually fades away.

The Dangers of Typhoons

  Typhoons are a classic form of hazardous weather. They can bring destructive winds, extreme rainfall, storm surges, and other disasters that seriously disrupt work and daily life. Fierce winds can destroy homes, other buildings, and elevated structures, as well as sever power and communications lines. Extreme rainfall often causes flooding and can trigger debris flows, landslides, and other geological disasters. Storm surges can overturn vessels at sea, breach seawalls, and drive seawater inland. Every year, landfalling typhoons cause major economic losses in China.

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

  Satellite technology now allows us to monitor typhoons effectively, but observation and warnings are not enough on their own. When a typhoon approaches, shut doors and windows securely, stock up on supplies, avoid going outside whenever possible, and keep away from hazards such as billboards and large trees.