Mid-latitude cyclones and typhoons, the main causes of heavy rain and strong winds near Japan, differ greatly in structure and formation mechanisms.
Basic Knowledge
Mid-latitude cyclone: Develops in the mid-latitudes and is accompanied by fronts. It develops using energy produced by the collision of warm and cold air. On weather maps, its features are spiral-shaped isobars and fronts extending in the southwest–southeast direction.
Typhoon: A tropical cyclone that develops over tropical seas (sea-surface temperature of 26℃ or higher) and reaches a maximum wind speed of 17.2m/s or higher. It is not accompanied by fronts, and its isobars form a vortex that is almost concentric.
Typhoon wind structure: The right side of the typhoon's direction of travel is particularly dangerous because the typhoon's own wind speed is added to its forward speed. The eye of the typhoon near the center is calm and clear, but the eyewall surrounding the eye has the strongest winds and rain.
Mid-latitude cyclone (A mid-latitude low-pressure system accompanied by fronts. Develops through the collision of warm and cold air)
Tropical cyclone (Forms over tropical seas. Uses water vapor from the sea surface as its energy source)
Typhoon (A tropical cyclone with a maximum wind speed of 17.2m/s or higher. North Pacific and South China Sea)
Eye of a typhoon (Calm region at the center. Maximum wind speed and rainfall occur around the eye)
Dangerous semicircle and navigable semicircle (Right side of the typhoon's path = dangerous semicircle; left side = navigable semicircle)
Extratropical transition (The transformation process in which a typhoon moves into the mid-latitudes and begins to develop fronts)
Further Study (Background and Significance)
About 25–26 typhoons form each year on average, and about 10–11 of them affect Japan. The area where they most often form is east of the Philippines to around the Mariana Islands. Typhoons move northwestward with the trade winds, then turn northeastward when they enter the westerly belt near Japan.
Because the energy source of a typhoon is the latent heat released by the condensation of water vapor, it weakens rapidly after making landfall and being cut off from the sea surface. It may also transform into a mid-latitude cyclone after merging with a front near Japan; at this time, the “typhoon-strength storm area” may expand.
Typhoon No. 19 of 2019 (Hagibis) reached M≒wind speed 60m/s and caused severe flood damage in Kanto and Tohoku. Linear rainbands (a phenomenon in which cumulonimbus clouds that form one after another line up in a band) have also caused many disasters in recent years.
- Typhoon = no fronts, concentric isobars, maximum wind speed of 17.2m/s or higher
- Mid-latitude cyclone = fronts present, warm air × cold air
- Typhoon energy source = latent heat of condensation of water vapor
- Eye of a typhoon = calm region at the center
- Right side of the direction of travel = dangerous semicircle
- Develops over sea-surface temperatures of 26℃ or higher
- Weakens after making landfall because it no longer receives water vapor
Points to Note (Common Confusions)
① A typhoon is not accompanied by fronts. It develops fronts when it transforms into a mid-latitude cyclone. ② The eye of a typhoon is calm, but the eyewall is the most dangerous. ③ The “dangerous semicircle” is the right side of the direction of travel (Northern Hemisphere). ④ The maximum wind speed of 17.2m/s is defined as the 10-minute average wind speed.
Practice
- State one major structural difference between a typhoon and a mid-latitude cyclone.
- What is the right side of a typhoon's direction of travel called?
- What is the energy source of a typhoon?