Thunder

Template:Short description Template:Hatnote group Template:Listen Thunder is the sound caused by lightning.<ref name="NSSL">Template:Cite web</ref><ref name="FJFK: Thunder Facts">Template:Cite web</ref><ref name="NWS">Template:Cite web</ref> Depending upon the distance from and nature of the lightning, it can range from a long, low rumble to a sudden, loud crack. The sudden increase in temperature and hence pressure caused by the lightning produces rapid expansion of the air in the path of a lightning bolt.<ref>Template:Cite web</ref> In turn, this expansion of air creates a sonic shock wave, often referred to as a "thunderclap" or "peal of thunder". The scientific study of thunder is known as brontology and the irrational fear (phobia) of thunder is called brontophobia.

Etymology

The d in Modern English thunder (from earlier Old English þunor) is epenthetic, and is now found as well in Modern Dutch donder (cf. Middle Dutch donre; also Old Norse þorr, Old Frisian þuner, Old High German donar, all ultimately descended from Proto-Germanic *þunraz). In Latin the term was tonare "to thunder". The name of the Nordic god Thor comes from the Old Norse word for thunder.<ref>Template:Cite book</ref>

The shared Proto-Indo-European root is *tón-r̥ or *Template:PIE, also found in Gaulish Taranis.<ref>Matasovic, Ranko. Etymological Dictionary of Proto Celtic. Leiden, The Netherlands: Brill. 2009. p. 384. Template:ISBN</ref>

Cause

The cause of thunder has been the subject of centuries of speculation and scientific inquiry.<ref name = Vavreketaln.d.>Template:Cite web</ref> Early thinking was that it was made by deities, but the ancient Greek philosophers attributed it to natural causes, such as wind striking clouds (Anaximander, Aristotle) and movement of air within clouds (Democritus).<ref name="Heidorn1999">Template:Cite web</ref> The Roman philosopher Lucretius held it was from the sound of hail colliding within clouds.<ref name = Heidorn1999 /> By the mid-19th century, the accepted theory was that lightning produced a vacuum and that the collapse of that vacuum produced what is known as thunder.<ref name = Vavreketaln.d. />

Scientists have agreed since the 20th century that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel.<ref name="UmanVladimir A. Rakov2007">Template:Cite book,</ref><ref name = Heidorn1999 /> The temperature inside the lightning channel, measured by spectral analysis, varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about Template:Convert.<ref>Template:Cite book</ref> This heating causes a rapid outward expansion, impacting the surrounding cooler air at a speed faster than sound would otherwise travel. The resultant outward-moving pulse is a shock wave,<ref name="britannica">Template:Cite encyclopedia</ref> similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft. Near the source, the sound pressure level of thunder is usually 165 to 180 dB, but can exceed 200 dB in some cases.<ref>Template:Cite web</ref>

Experimental studies of simulated lightning have produced results largely consistent with this model, though there is continued debate about the precise physical mechanisms of the process.<ref>Template:Cite book</ref><ref name="UmanVladimir A. Rakov2007"/> Other causes have also been proposed, relying on electrodynamic effects of the enormous current acting on the plasma in the bolt of lightning.<ref>Template:Cite journal</ref>

Consequences

The shock wave in thunder is sufficient to cause property damage<ref name = Vavreketaln.d. /> and injury, such as internal contusion, to individuals nearby.<ref>Template:Cite book</ref> Thunder can rupture the eardrums of people nearby, leading to permanently impaired hearing.<ref name = Vavreketaln.d. /> Even if not, it can lead to temporary deafness.<ref name = Vavreketaln.d. />

Types

Vavrek et al. (n.d.) reported that the sounds of thunder fall into categories based on loudness, duration, and pitch.<ref name = Vavreketaln.d. /> Claps are loud sounds lasting 0.2 to 2 seconds and containing higher pitches. Peals are sounds changing in loudness and pitch. Rolls are irregular mixtures of loudness and pitches. Rumbles are less loud, last for longer (up to more than 30 seconds), and are of low pitch.<ref>Template:Cite web</ref>

Inversion thunder results when lightning strikes occur between the cloud and ground during a temperature inversion. The resulting thunder sounds have significantly greater acoustic energy than those produced from the same distance in non-inversion conditions. In a temperature inversion, the air near the ground is cooler than the air higher up. Inversions often happen when warm, moist air passes above a cold front. Within a temperature inversion, sound energy is prevented from dispersing vertically as it would in non-inversion conditions, and is thus concentrated in the near-ground layer.<ref>Template:Cite web</ref>

Cloud-to-ground lightning (CG) typically consists of two or more return strokes, from ground to cloud. Later return strokes have greater acoustic energy than the first.<ref>Template:Cite web</ref>

Perception

Template:Listen

The most noticeable aspect of lightning and thunder is that the lightning is seen before the thunder is heard. This is a consequence of the speed of light being much greater than the speed of sound. The speed of sound in dry air is approximately Template:Convert or Template:Convert at Template:Convert.<ref>Template:Cite book</ref>

This translates to approximately Template:Convert; saying "one thousand and one... one thousand and two..." is a useful method of counting the seconds from the perception of a given lightning flash to the perception of its thunder (which can be used to gauge the proximity of lightning for the sake of safety). To estimate the distance from the lightning strike, divide the counted seconds by five for miles,<ref>Template:Cite web</ref> or three for kilometers.

A very bright flash of lightning and an almost simultaneous sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike was very near.

Close-in lightning has been described first as a clicking or cloth-tearing sound, then a cannon shot sound or loud crack/snap, followed by continuous rumbling.<ref name = Vavreketaln.d. /> The early sounds are from the leader parts of lightning, then the near parts of the return stroke, then the distant parts of the return stroke.<ref name = Vavreketaln.d. />