十万个为什么·科学启蒙30篇（4）
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Why Thunder Follows Lightning—But Seems Delayed
为什么打雷往往先于闪电被感知到（科普）
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Light travels nearly a million times faster than sound—about 300,000 km/s versus just 340 m/s in air.
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So even though lightning and thunder happen simultaneously, light reaches you almost instantly while sound takes time.
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Counting seconds between flash and boom lets you estimate distance: five seconds ≈ one mile (or three seconds ≈ one kilometer).
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Thunder rumbles because sound arrives from different parts of the lightning channel at slightly different times.
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Temperature and wind layers in the atmosphere bend sound waves, sometimes making thunder inaudible even close by.
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High-frequency tones fade faster over distance, leaving low rumbles to travel farther—hence the ‘rolling’ quality.
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If you see lightning but hear no thunder, the strike was likely over 25 km away or blocked by terrain or buildings.
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Safety rule: when thunder roars, go indoors—even if the storm seems distant—because lightning can strike ahead of rain.