With thunder， there's always lots to hear. Now there's also something to see. For the first time， scientists have precisely mapped the loud clap radiating from a lightning strike. This picture of thunder's origins could reveal the energies involved in powering some of nature's flashiest light shows.

Lightning strikes when an electric current flows from a negatively charged cloud to the ground. This rapidly heats and expands the surrounding air， creating sonic shock waves. We hear this as thunder.

Scientists have a basic understanding of the origins of thunder. Still， experts have lacked a detailed picture of the physics powering the loud cracks and low rumbles.

Maher Dayeh works at the Southwest Research Institute in San Antonio， Texas. As a heliophysicist， he studies the sun and its effects on the solar system， including Earth. He and his colleagues also study lightning — by making their own. These experts trigger the bolts by firing a small rocket into an electrically charged cloud. Trailing behind the rocket is a long， Kevlar-coated copper wire. The lightning travels along that wire to the ground.

For their new experiment， the scientists used 15 sensitive microphones laid out 95 meters （312 feet） from the strike zone. The team then precisely recorded the incoming sound waves. Those from higher elevations took longer to reach the microphones.

That allowed the scientists to map the acoustic （sound） signature of the lightning strike. That map revealed the strike with "surprising detail".

How loud a thunderclap sounds will depend on the peak electric current flowing through the lightning， the researchers found. Explains Dayeh， this discovery could one day allow scientists to use thunder to sound out the amount of energy powering a lightning strike.

用眼睛“看”雷声

雷声轰鸣，不绝于耳。但现在雷声也可用“眼”观。科学家初次用图精确地绘制出雷击所发出的巨大霹雳声。该雷声起源图能够揭示自然界最耀眼的闪光背后的驱动力量。

当电流从带负电荷的云团流向地面的时候，雷击就发生了。雷击使周围空气温度迅速升高并膨胀，产生冲击声波，冲击声波就是我们听到的雷声。

科学家们已经基本理解了雷声的起源。但他们还是缺乏一张能够详细描写产生这种巨大霹雳声和低沉轰鸣声的物理学动因的图。

Maher Dayeh就职于美国得克萨斯州圣安东尼奥市的西南研究所。作为一名太阳物理学家，他从事研究太阳以及太阳对太阳系的影响，包括对地球的影响。他和同事们也研究闪电，他们通过自己创造的闪电来一探闪电的究竟。这些专家将一枚小型火箭发射到负电荷云团来触发雷电，火箭的尾部系着一根长长的、用凯夫拉尔纤维进行过表面处理的铜线。闪电通过那根铜线传导到地面。

在一项新的实验中，科学家将15个高灵敏度的扩音器安置在离雷击区95米（312英尺）以外的地方。然后他们精确地记录了传导进来的声波。声波传导到扩音器的时间随着声波源高度的增加而增加。

这样，科学家就能绘制出雷击的声波标记图。这张图揭示的雷击的精细程度令人惊讶。

这些研究者发现，霹雳的响亮程度取决于穿越闪电的峰值电流。Dayeh解释道，这一发现有朝一日能够使科学家运用雷鸣来搞清楚驱动雷击所需能量的大小。……