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heat lightning
Heat lightning is the name used for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not have accompanying sounds of thunder. This happens because the lightning occurs very far away and the sound dissipates before it reaches the observer. The term is a little misleading because it has nothing to do with the heat of the lightning itself. At night, it is possible to see the flashes of lightning from very far distances, up to 100 miles, but the sound doesn't carry that far.〔http://atmo.tamu.edu/weather-and-climate/weather-whys/666-heat-lightning〕 Lightning results from the discharge of negative ions created from the friction of ice and water particles bumping into each other at the bottom of a cloud. Heat lightning can be an early warning sign that thunderstorms are approaching. In Florida, heat lightning is often seen out over the water at night, the remnants of storms that formed during the day along a sea breeze front coming in from the opposite coast.
Heat lightning is not to be confused with electrically-induced luminosity actually generated at mesospheric altitudes above thunderstorm systems (and likewise visible at exceedingly great ranges), a phenomenon known as "sprites".
==Atmosphere==
The movement of sound in the atmosphere depends on the properties of the air, such as temperature and density. Because temperature and density change with height, the sound of thunder is refracted through the troposphere. This refraction results in spaces through which the thunder does not propagate. The sound of thunder often reflects off the Earth's surface. The rumbling sound is partly due to these reflections. This reflection and refraction leaves voids where thunder cannot be heard.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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