![]() The upward branching of such discharges makes them visually distinguishable from their “right-side-up” counterparts, giving the impression of a cloud-to-ground lightning flash that is upside down. These discharges often are initiated (or triggered) by tall structures or by towers on hilltops. Triggered lightningĪ small percentage of discharges between the cloud and ground are actually initiated at the ground and propagate upward to a charged region in the cloud. ![]() At distances of a few hundred metres, thunder begins with a sudden clap followed by a long rumble at larger distances, it begins with a rumble. The result is a series of sounds that are variously described as peaks, claps, rolls, and rumbles. The total thunder waveform comes from the entire lightning channel and includes the effects of channel branching and tortuosity, sound propagation in the atmosphere, and acoustic reflections from the local topography. Every three seconds correspond to one kilometre, and every five seconds correspond to one mile. The time delay between the bright flash of light and the arrival of the associated thunder can often be used to estimate the distance to a discharge. Since light travels at about 300,000 km (186,000 miles) per second and the speed of sound is only about 0.33 km (0.2 mile) per second, the light from a discharge will always be seen before the sound arrives at an observer. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century. ![]()
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