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Reading the weather · 4 min · 4 steps

How radar sees rain

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Goal. Explain how weather radar measures rain, how it differs from a forecast, and why mountains make wet and dry sides.

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Steps

  1. Predict

    How does a weather radar find rain?

    • It photographs clouds from space
    • It sends out microwave pulses and listens for echoes from raindrops
    • It measures rain in buckets
    Show the answer

    It sends out microwave pulses and listens for echoes from raindrops. It sends out short microwave pulses and listens for the echoes that raindrops, snow and hail send back.

    Over Japan the rain layer shows real radar: rain measured minutes ago. Brighter colours mean a stronger echo and heavier rain. If no rain is falling there now, the map is simply empty.

  2. Each radar turns its dish every few minutes and times how long each echo takes to return, which gives the distance. A network of radars covers the whole country. Press play to watch the last two hours of rain move.

  3. Over Taiwan the app has no radar feed yet, so it shows the forecast model's rain instead, labelled Forecast model. Radar measures rain that is falling now; a model predicts rain that has not fallen yet.

  4. Mountains squeeze rain out of moist wind. In winter the north-east monsoon brings drizzle to Keelung and Yilan, on the windward side, while south-western Taiwan, in the mountains' shelter, stays dry. In summer the south-west monsoon wets the other side.

The common mistake

Myth

Weather radar shows clouds.

Fact. Radar sees raindrops, snowflakes and hail, not cloud droplets, which are far too small. A cloudy sky with no rain leaves the radar map empty; satellites are used to see clouds.

Check yourself

Question 1

What does a weather radar detect?

  • Cloud droplets
  • Raindrops, snowflakes and hail
  • The wind
Show the answer

Raindrops, snowflakes and hail. Only drops and ice big enough to reflect the microwave pulse strongly show up.

Question 2

Why is Keelung so wet in winter?

  • It is on the windward side of the north-east monsoon
  • It lies in the tropics
  • The sea is very warm there
Show the answer

It is on the windward side of the north-east monsoon. The moist north-east wind is forced up over the hills behind Keelung and drops its rain there.

Question 3

What does the label Forecast model on the rain layer mean?

  • Rain measured by radar
  • Rain predicted by a computer model
  • Rain measured by satellites
Show the answer

Rain predicted by a computer model. Where no radar is available, the layer shows how much rain the forecast model expects.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
Ib-Ⅳ-6
US Next Generation Science Standards
MS-ESS2-4 · MS-ESS2-5
England national curriculum
KS3 Geography: weather and climate