要保持多远的距离才不被雷击
How far must you be to prevent getting struck by lightning? <p> <br>Lightning is a form of visible electric discharge between rain clouds or between a rain cloud and the Earth. The discharge is seen as a brilliant arc and produces a sound wave that is heard as thunder. Most rain clouds are negatively charges at the base and positively charged at the top. When the electrical potential between two clouds or between a cloud and the Earth reaches a sufficiently high value, the air becomes ionised along a narrow path and lightning flash results. Lightning that strikes the Earth consists of one or more electrical discharges called strokes. The bright light that we see in a flash of lightning is called a return stroke. A typical lightning flash involves a potential difference between cloud and ground of several hundred million volts, with peak currents on the order of 20,000 amperes. <p>The flash of cloud-to-ground lightning is initiated by the neutralization of the small net-positive charge in the lowest region of the cloud. A cloud-to-ground flash comprises at least two strokes: a leader stroke and a return stroke. A leader stroke carrying a negative charge passes from cloud to ground. Occasionally, however, the leader stroke is from ground to cloud--especially with very high structures such as church steeples, multi-story buildings, or tall trees. The leader stroke is not very bright and is often stepped and has many branches extending out from the main channel. As it nears the ground, it induces an opposite charge, concentrated at the point to be struck, and a return stroke carrying a positive charge from ground to cloud is generated through the channel. The two strokes generally meet about 50 m above the ground. At this juncture, the cloud is short-circuited to the ground and a highly luminous return stroke of high current passes through the channel to the cloud. The entire process is very rapid; the leader stroke reaches the juncture point or the ground in about 20 milliseconds, and the return stroke reaches the cloud in about 70 microseconds. <p>Lightning usually strikes the nearest object. The general precaution against lightning is to avoid hilltops, beaches, small structures in open areas and open water. When you swim in open water, you head above water naturally becomes the closest object for lightning to strike. <p>Lightning protection systems control electrical discharges by directing them through a low-resistance path to the ground, avoiding passage through parts of a structure and reducing risk of fire or other damage. Air terminals (rods) fastened to the building intercept electric discharges, directing them through conductors to a grounding system and thence into the ground. <p>It is a common misconception that lightning rods silently discharge the electric charge in a thundercloud and thereby prevent lightning. Actually lightning only serve to route the lightning harmlessly to ground. In doing so they divert lightning when it is 10 to 100 metres away. A house protected by a single lightning rod has an assumed 45 degree-angle cone of protection. If the height of the rod were H metres, the base area of the effective cone would have a radius of H metres. Objects within this cone are assumed to be safe from a lightning strike. <p> 全是英文：<p>How far must you be to prevent getting struck by lightning? <p><br>Lightning is a form of visible electric discharge between rain clouds or between a rain cloud and the Earth. The discharge is seen as a brilliant arc and produces a sound wave that is heard as thunder. Most rain clouds are negatively charges at the base and positively charged at the top. When the electrical potential between two clouds or between a cloud and the Earth reaches a sufficiently high value, the air becomes ionised along a narrow path and lightning flash results. Lightning that strikes the Earth consists of one or more electrical discharges called strokes. The bright light that we see in a flash of lightning is called a return stroke. A typical lightning flash involves a potential difference between cloud and ground of several hundred million volts, with peak currents on the order of 20,000 amperes. <p>The flash of cloud-to-ground lightning is initiated by the neutralization of the small net-positive charge in the lowest region of the cloud. A cloud-to-ground flash comprises at least two strokes: a leader stroke and a return stroke. A leader stroke carrying a negative charge passes from cloud to ground. Occasionally, however, the leader stroke is from ground to cloud--especially with very high structures such as church steeples, multi-story buildings, or tall trees. The leader stroke is not very bright and is often stepped and has many branches extending out from the main channel. As it nears the ground, it induces an opposite charge, concentrated at the point to be struck, and a return stroke carrying a positive charge from ground to cloud is generated through the channel. The two strokes generally meet about 50 m above the ground. At this juncture, the cloud is short-circuited to the ground and a highly luminous return stroke of high current passes through the channel to the cloud. The entire process is very rapid; the leader stroke reaches the juncture point or the ground in about 20 milliseconds, and the return stroke reaches the cloud in about 70 microseconds. <p>Lightning usually strikes the nearest object. The general precaution against lightning is to avoid hilltops, beaches, small structures in open areas and open water. When you swim in open water, you head above water naturally becomes the closest object for lightning to strike. <p>Lightning protection systems control electrical discharges by directing them through a low-resistance path to the ground, avoiding passage through parts of a structure and reducing risk of fire or other damage. Air terminals (rods) fastened to the building intercept electric discharges, directing them through conductors to a grounding system and thence into the ground. <p>It is a common misconception that lightning rods silently discharge the electric charge in a thundercloud and thereby prevent lightning. Actually lightning only serve to route the lightning harmlessly to ground. In doing so they divert lightning when it is 10 to 100 metres away. A house protected by a single lightning rod has an assumed 45 degree-angle cone of protection. If the height of the rod were H metres, the base area of the effective cone would have a radius of H metres. Objects within this cone are assumed to be safe from a lightning strike. <p><br> 这鸡肠子说保护范围是按从屋顶45度角算，不是按俺们的滚球算，不过也差不太多。 这篇英文文章的内容是我们一般人都了解的，没有什么新东西。有些说法还与书籍中所说或我们的认识稍有不同。 雷神用这些鸡肠子来忽悠我们这些小学毕业的小朋友 <p>我都认得完..不过不知道什么意思</p> “<div id="textstyle_19757" style="FONT-SIZE: 10pt;">这鸡肠子说保护范围是按从屋顶45度角算，不是按俺们的滚球算，不过也差不太多。</div><p>”</p><p>公式计算结果好像不是这个样子的啊！</p><p>俺计算过的：</p><p>1.当避雷针高度在30m及以下的时候，其保护角度应该以0.5h分界，0.5h以上保护角大约为45度，0.5h以下为63度。</p><p>2.避雷针在（30，120]之间是0.5h以上保护角为arctg(5.5/sqrt(h));以下为arctg(11/sqrt(h)).这里的sqrt（h）表示对h开方；</p><p>3.当h>120m时，保护角仍旧以0.5h分界，0.5h以上为26度，0.5h以下为45度。</p><p></p> 那天我给翻译一下 <p>开个玩笑</p> <div class="quote"><b>以下是引用<i>XOZONG520</i>在2007-9-20 17:05:00的发言：</b><br/>雷神用这些鸡肠子来忽悠我们这些小学毕业的小朋友<img title="dvubb" alt="图片点击可在新窗口打开查看" src="http://www.asp.cn/bbs/Skins/Default/emot/em42.gif" align="middle" border="0"/></div><p>2004年，就是3年多前，某次和网友讨论滚球法，于是讨论到国外关于类似DL/T621-1997标准的计算有无相关说明文章时，发了此段。</p><p>1、你来的太晚；2、你不清楚前提；3、你随便说话；呵呵。</p>
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