A typhoon is a tropical low-pressure system that forms over tropical seas where the water is warmer than about 27 °C and whose maximum winds near the center reach 17 m/s or more.
Its fuel is the heat released when water vapor rising from the warm sea condenses into cloud, so it weakens quickly once it moves over land or cooler water.
Raising the wind speed in the diagram changes the strength category, and the picture shows the calm, clear eye, the eyewall just outside it where wind and rain are fiercest, and the spiral rainbands reaching outward.
In the Northern Hemisphere a typhoon turns counterclockwise seen from above, so on the right of its path its own winds add to its forward speed and it is more dangerous.
What Is a Typhoon?
🌀 A Giant Low Grown by a Warm Ocean
①It forms over tropical seas where the water is about 27 °C or warmer
②A tropical low whose maximum winds near the center reach 17 m/s or more is called a typhoon
③Heat released as water vapor condenses powers it, bringing strong winds and heavy rain
④In the Northern Hemisphere air spirals inward counterclockwise, seen from above
Structure of a Typhoon
33 m/s
seegongsik.com
The eye, eyewall and spiral rainbands of a typhoon seen from above
👁️ The Eye and the Eyewall
①Eye: a central area a few tens of kilometers across, where sinking air keeps the sky clear and the wind light
②Eyewall: the thick wall of cloud around the eye, with the strongest wind and rain
③Spiral rainbands: bands of cloud stretching outward and dropping rain
④Strength categories (Korea and Japan use similar limits): strong from 33 m/s, very strong from 44 m/s, violent from 54 m/s
How Typhoons Move and What They Do
🗺️ A Typhoon's Track
①It forms at low latitudes and is carried northwest by the trade winds
②At middle latitudes it meets the westerlies and turns northeast (recurvature)
③The right half of its path is the dangerous semicircle, where wind and forward speed add up; the left half is weaker
④It brings damaging winds, torrential rain, storm surges and power cuts
Summary
Typhoon
A tropical low with maximum winds near the center of 17 m/s or more
Forms over warm tropical seas and turns counterclockwise in the Northern Hemisphere
Pressure falls toward the center, and the wind is strongest at the eyewall
🎯 Common Test Points
①Forms over tropical seas warmer than about 27 °C
②Turns counterclockwise in the Northern Hemisphere and clockwise in the Southern
③The right half of its path is the dangerous semicircle
④The eye is clear and calm; the eyewall has the strongest wind and rain
⑤Over land or cool water its supply of water vapor drops and it weakens
Examples and Unit Review
Example 1
Where do typhoons form, which way do they turn in the Northern Hemisphere, and what wind speed makes a storm a typhoon?
1
A typhoon is a tropical low that forms over tropical seas about 27 °C or warmer, fed by water vapor from the sea.
2
It is called a typhoon when its maximum winds near the center reach 17 m/s or more, and in the Northern Hemisphere it turns counterclockwise seen from above.
▸ Over tropical seas about 27 °C or warmer; counterclockwise in the Northern Hemisphere; maximum winds of 17 m/s or more
In the Southern Hemisphere, tropical lows formed the same way turn clockwise.
Example 2
How does the weather differ between a typhoon's eye and its eyewall, and why is the right side of its path more dangerous?
1
In the eye, sinking air clears the clouds and the wind is light, while the eyewall gathers the strongest winds and heaviest rain.
2
On the right of the path, the direction of the typhoon's spinning winds matches the direction it is moving, so the two speeds add.
▸ The eye is clear with light wind; the eyewall has the fiercest wind and rain. The right half is worse because the forward speed adds to the spinning wind
As the eye passes, there is a short lull, then the far eyewall arrives and strong wind returns from the opposite direction.
Unit Test
Which statement about typhoons is correct?
①A typhoon forms over cold polar seas
②Winds are strongest in the eye of a typhoon
③In the Northern Hemisphere a typhoon rotates counterclockwise
④A typhoon grows stronger when it makes landfall
⑤Pressure rises toward the center of a typhoon
▸ ③ In the Northern Hemisphere a typhoon rotates counterclockwise
1
A typhoon forms over tropical seas at 27°C or higher (① wrong), and winds are weak in the eye (② wrong).
2
On land the supply of water vapor is cut off and a typhoon weakens (④ wrong), and pressure falls toward the center (⑤ wrong); in the Northern Hemisphere a typhoon rotates counterclockwise (③ correct).