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Reading the weather · 5 min · 4 steps

Reading a typhoon forecast

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Goal. Read a typhoon track and forecast cone, and explain why slow, rainy typhoons are dangerous.

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Steps

  1. This is Typhoon Gaemi in July 2024, replayed from the international best-track archive (IBTrACS). The line shows where its centre went, and its colour shows the storm's strength. Typhoons are steered mostly by the winds around the Pacific high-pressure area.

  2. Predict

    Here is a forecast cone drawn from 22 July. What does the cone show?

    • How big the typhoon is
    • Where the centre of the storm will probably go
    • Where it will rain
    Show the answer

    Where the centre of the storm will probably go. Where the centre will probably go: in about two cases out of three the centre stays inside the cone.

    The cone widens with time because forecasts become less certain further ahead. This one is an illustration, built from the US National Hurricane Center's 2026 cone sizes and drawn along the path Gaemi really took. A real cone follows the forecast path, and the centre can still wander outside it.

  3. Gaemi slowed down and circled off Hualien, then made landfall at Nan'ao in Yilan around midnight on 24–25 July, Taiwan time. A slow storm is dangerous because its rain keeps falling on the same places.

  4. Typhoon Morakot in August 2009 was not especially windy, but it moved slowly and dropped about 3,000 mm of rain on the mountains of southern Taiwan in a few days. A landslide buried the village of Xiaolin. A typhoon's greatest danger is often its rain.

The common mistake

Myth

If my town is outside the cone, the typhoon will not affect me.

Fact. The cone only shows where the centre may go. Strong winds and heavy rain can reach hundreds of kilometres from the centre, and about one time in three the centre itself leaves the cone.

Check yourself

Question 1

What does a typhoon's forecast cone show?

  • The size of the storm
  • Where the centre will probably go
  • Where it will rain
Show the answer

Where the centre will probably go. The cone covers the likely positions of the centre; the storm's wind and rain spread much wider.

Question 2

Why does the cone get wider further ahead?

  • The storm grows bigger
  • Forecasts become less certain with time
  • The map is stretched
Show the answer

Forecasts become less certain with time. Small errors grow with time, so the possible positions spread out.

Question 3

What made Typhoon Morakot so deadly?

  • Record-breaking winds
  • Extreme rain from a slow-moving storm
  • A tsunami
Show the answer

Extreme rain from a slow-moving storm. Morakot's winds were moderate, but its slow movement brought about 3,000 mm of rain, floods and landslides.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
EMd-Vc-3 · Ib-Ⅳ-5
US Next Generation Science Standards
MS-ESS3-2
England national curriculum
GCSE Geography: weather hazards (tropical storms)