<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD><TITLE>Lightning</TITLE><!-- Created with the CoffeeCup HTML Editor 2008 --><!-- http://www.coffeecup.com/ --><!-- Brewed on 7/9/2010 5:06:56 PM --> <META content="text/html; charset=unicode" http-equiv=Content-Type> <META name=GENERATOR content="MSHTML 8.00.6001.18928"></HEAD> <BODY><SPAN id=Leader_formation_and_the_return_stroke class=mw-headline>The diagram below and the&nbsp;explanation that follows are from <EM>Wikipedia</EM>. &nbsp;At the very end, they mention that photographs&nbsp;of&nbsp;non-connected positive streamers(that shoot up from the ground ) have been taken. By complete fluke, while I was filming lightning, there were unconnected streamers in my own backyard. They were invisible but became apparent when I slowed down the video. They became more obvious when I created&nbsp; a slow motion version of the film posted on <EM>Youtube</EM>. At first I thought I had one streamer that appeared and disappeared before the stepped leader from the clouds connected with a positive streamer elsewhere, but I now hypothesze that there were two separate unconnected positive streamers in my backyard. Is this why my dog Blues was barking like he had seen a devil?<BR>&nbsp;<BR><STRONG>Leader formation and the return stroke</STRONG></SPAN> <DIV class="thumb tleft"> <DIV style="WIDTH: 222px" class=thumbinner><A class=image href="/H:/wiki/File:Leaderlightnig.gif"></A><IMG style="Z-INDEX: 100; POSITION: absolute" border=0 alt="" src="Leaderlightnig.gif" width=543 height=367>&nbsp; <DIV class=thumbcaption> <DIV class=magnify><A class=internal title=Enlarge href="/wiki/File:Leaderlightnig.gif"><IMG alt="" src="http://bits.wikimedia.org/skins-1.5/common/images/magnify-clip.png" width=15 height=11></A></DIV>Illustration of a negative streamer (blue) meeting a positive counterpart (red) and the return stroke. Click to watch the animation.</DIV></DIV></DIV> <P><BR><BR><BR><BR>&nbsp;</P> <P><BR><BR>&nbsp;</P> <P>As a <A class=mw-redirect title=Thundercloud href="/wiki/Thundercloud"><FONT color=#0645ad>thundercloud</FONT></A> moves over the surface of the Earth, an <A title="Electric charge" href="/wiki/Electric_charge"><FONT color=#0645ad>electric charge</FONT></A> equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. The induced ground charge follows the movement of the cloud, remaining underneath it.</P> <P>An initial bipolar discharge, or path of <A class=mw-redirect title=Ionized href="/wiki/Ionized"><FONT color=#0645ad>ionized</FONT></A> air, starts from a negatively charged mixed water and ice region in the thundercloud. Discharge ionized channels are known as <I>leaders</I>. The negatively charged leaders, generally a "<A title="Stepped leader" href="/wiki/Stepped_leader"><FONT color=#0645ad>stepped leader</FONT></A>", proceed downward in a number of quick jumps (steps). 90 percent of the leaders exceed 45&nbsp;m (148&nbsp;ft) in length, with most in the order of 50 to 100&nbsp;m (164 to 492 feet).<SUP id=cite_ref-21 class=reference><A href="#cite_note-21"><FONT color=#0645ad><SPAN>[</SPAN>22<SPAN>]</SPAN></FONT></A></SUP> As it continues to descend, the stepped leader may branch into a number of paths.<SUP id=cite_ref-22 class=reference><A href="#cite_note-22"><FONT color=#0645ad><SPAN>[</SPAN>23<SPAN>]</SPAN></FONT></A></SUP> The progression of stepped leaders takes a comparatively long time (hundreds of <A title=Millisecond href="/wiki/Millisecond"><FONT color=#0645ad>milliseconds</FONT></A>) to approach the ground. This initial phase involves a relatively small <A title="Electric current" href="/wiki/Electric_current"><FONT color=#0645ad>electric current</FONT></A> (tens or hundreds of <A title=Ampere href="/wiki/Ampere"><FONT color=#0645ad>amperes</FONT></A>), and the leader is almost invisible when compared with the subsequent lightning channel.</P> <P>When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the <A title="Electric field" href="/wiki/Electric_field"><FONT color=#0645ad>electric field</FONT></A>. The electric field is strongest on ground-connected objects whose tops are closest to the base of the thundercloud, such as trees and tall buildings. If the electric field is strong enough, a conductive discharge (called a <A title="Positive streamer" href="/wiki/Positive_streamer"><FONT color=#0645ad>positive streamer</FONT></A>) can develop from these points. This was first theorized by <A class=new title="Heinz Kasemir (page does not exist)" href="/w/index.php?title=Heinz_Kasemir&action=edit&redlink=1"><FONT color=#ba0000>Heinz Kasemir</FONT></A>.<SUP id=cite_ref-23 class=reference><A href="#cite_note-23"><FONT color=#0645ad><SPAN>[</SPAN>24<SPAN>]</SPAN></FONT></A></SUP><SUP id=cite_ref-24 class=reference><A href="#cite_note-24"><FONT color=#0645ad><SPAN>[</SPAN>25<SPAN>]</SPAN></FONT></A></SUP> As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and forming the main discharge path. Photographs have been taken on which non-connected streamers are clearly visible.<SUP id=cite_ref-25 class=reference><A href="#cite_note-25"><FONT color=#0645ad><SPAN>[</SPAN>26<SPAN>]</SPAN></FONT></A></SUP></P></BODY></HTML>