Why Is It Hot and Still Before a Typhoon Arrives? The Secret Behind Descending Airflows
Key SummaryTyphoons are renowned for bringing violent winds and torrential rain, so why do the one or two days before landfall often feature clear skies, stifling heat, and even eerie purple-red sunsets? The culprit is the typhoon's outer "subsiding airflow."
Why Is a Typhoon Like a Giant Air Conditioner?
The eye wall at the typhoon's center is an area of extremely intense rising airflow: warm, moist air spirals rapidly upward, condenses at high altitude, and releases latent heat (the source of the violent winds and rain, much like an indoor air conditioner unit). Once this air rises and flows outward at high altitude, it descends in the outer region roughly 500–1,000 kilometers from the typhoon's center, creating intense outer subsiding airflow—like a super-sized air conditioner blasting its "hot outdoor exhaust" directly onto surrounding cities.
Three Typical Signs of Subsiding Airflow
| Phenomenon | Scientific Explanation | Disaster-Prevention Warning |
|---|---|---|
| Extreme heat and humidity from subsidence-induced heating | As air descends from high altitude, it is compressed by the weight of the atmosphere, causing adiabatic warming (the temperature rises by about 6–10°C for every 1,000 meters of descent), while the descent also inhibits cloud formation and allows direct sunlight to reach the ground | The stronger the subsidence, the larger and more intense the typhoon's circulation system is likely to be |
| Light winds or even calm conditions, along with ozone pollution | Subsiding airflow traps near-surface air like a lid, resulting in low wind speeds and preventing pollutants from dispersing | This calm before the storm should never be mistaken as an all-clear; continue taking typhoon precautions |
| Spectacular purple-red "typhoon sunsets" | High-altitude cirrus clouds around the typhoon contain abundant ice crystals, which produce strong Rayleigh and Mie scattering when the evening sunlight passes through water vapor and dust | The folk saying "When fire-red clouds cover the sky, heavy rain will come quickly" indicates that the typhoon's outer cloud system has already arrived |
A Typical Case: Cities "Cooked" by a Typhoon
- When a super typhoon crosses the Bashi Channel or Taiwan, Guangdong, Fujian, and Jiangxi, located on its western side, are highly susceptible to the combined effects of the leeward slope and outer subsiding airflow (foehn warming plus subsidence-induced warming). Temperatures often soar to 37–40°C.
- Once the typhoon approaches further and enters the range of its 7-force wind circle, the subsiding airflow quickly gives way to spiral rainbands. Temperatures can plunge by more than 10°C within just a few hours, accompanied by violent winds and torrential rain.