The previous section discussed flashes produced from downward-propagating negatively-charged leaders, commonly referred to as stepped leaders; however, about 10% of lightning flashes are positive flashes and are produced by downward-propagating, positively-charged leaders. While both positive and negative flashes are deadly, there are significant differences between the two in terms of their formation and behavior.
Positive leaders typically originate from the positive charged area in the upper part of the thunderstorm cloud. Normally, the ground is shielded from this upper positive charge by negative charges in the central part of the storm; however, when upper level winds are stronger than lower level winds and the storm becomes tilted, or when the anvil of the thunderstorm cloud spreads out ahead of or behind the updraft of the thunderstorm, the ground is no longer shielded from this upper charge. If charge differences between this upper level charge and the ground become too large, a downward-moving positively-charged leader can develop.
Because positive flashes often originate away from the main thunderstorm updraft, they often occur in areas some distance from where rain is occurring. Also, because the positive charge center is higher in the atmosphere and a much greater charge differential is needed to initiate a lightning flash, positive flashes occur much less frequently and are much larger distances between flashes. Consequently, many people are caught by surprise by positive flashes. Also positive flashes can precede the main part of the thunderstorm and the rain area by 5 to 10 miles or more.
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