打雷:
Thunder is the sound made by lightning.Depending on the nature of the lightning and distance of the listener,it can range from a sharp,loud crack to a long,low rumble (brontide).The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning.In turn,this expansion of air creates a sonic shock wave which produces the sound of thunder.
The cause of thunder has been the subject of centuries of speculation and scientific inquiry.The first recorded theory is attributed to the Greek philosopher Aristotle in the third century BC,and an early speculation was that it was caused by the collision of clouds.Subsequently,numerous other theories have been proposed.By the mid-19th century,the accepted theory was that lightning produced a vacuum.In the 20th century a consensus evolved that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel.In a fraction of a second the air is heated to a temperature approaching 28,000 °C (50,000 °F).This heating causes it to expand outward,plowing into the surrounding cooler air at a speed faster than sound would travel in that cooler air.The outward-moving pulse that results is a shock wave,similar in principle to the shock wave formed by an explosion,or at the front of a supersonic aircraft.More recently,this consensus has been eroded by the observation that measured overpressures in simulated lightning are greater than what could be achieved by the amount of heating found.Alternative proposals rely on electrodynamic effects of the massive current acting on the plasma in the bolt of lightning.
閃電：
Lightning is an atmospheric discharge of electricity,which typically occurs during thunderstorms,and sometimes during volcanic eruptions or dust storms.In the atmospheric electrical discharge,a leader of a bolt of lightning can travel at speeds of 60,000 m/s,and can reach temperatures approaching 30,000 °C (54,000 °F),hot enough to fuse silica sand into petrified lightning,known scientifically as glass channels or fulgurites which are normally hollow and can extend some distance into the ground.There are some 16 million lightning storms in the world every year.For an American,the chance of being struck by lightning is approximately 1 in 576,000 and the chance of actually being killed by lightning is approximately 1 in 2,320,000.
Lightning can also occur within the ash clouds from volcanic eruptions,or can be caused by violent forest fires which generate sufficient dust to create a static charge.
How lightning initially forms is still a matter of debate.Scientists have studied root causes ranging from atmospheric perturbations (wind,humidity,friction,and atmospheric pressure) to the impact of solar wind and accumulation of charged solar particles.Ice inside a cloud is thought to be a key element in lightning development,and may cause a forcible separation of positive and negative charges within the cloud,thus assisting in the formation of lightning.