Question submitted by Patricia Lewis
Patricia - in light of the sad story that has come out of Ontario - I thought I would address this question on my blog.
Let me be very clear here:
THERE IS NO SUCH THING AS "RELATIVELY SAFE OR HARMLESS" LIGHTNING.
Lightning is an atmospheric discharge of electricty - but on a very large, very dangerous, potentially deadly scale. In the atmosphere an electrical discharge, a leader of a bolt of lightning, can travel at speeds of 60,000 m/s (130,000 mph), and can reach temperatures approaching 25,000 °C (54,000 °F), hotter than the surface of the sun and hot enough to fuse silica sand into glass channels.
90% of most cloud-to-ground strikes are considered "negative". The term negative is used to describe the polarity of the cloud base at the time of the strike. The only descriptive difference between the two kinds of lightning then (positive and negative) is the reversal of polarities in the cloud base. Normally the negative charge collects in the cloud base, with a corresponding net positive charge in the ground under the cloud. Lightning strikes originating from this configuration are therefore considered "negative" strikes.
An average bolt of negative lightning (the most common type of lightning) carries an electric current of 30 kiloamperes (kA), and transfers a charge of five coulombs and 500 MJ of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs.
An average bolt of "positive" lightning carries an electric current of 300 kA or about 10 times that of negative lightning.
Positive lightning, also known colloquially as "bolts from the blue", make up less than 5% of all lightning. They occur when the leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike tens of kilometres/miles from the cloud. At the time of writing this, it is thought that the people struck in Ontario on August 12th were indeed hit by a positive lightning bolt.
Remember to always take cover INDOORS during a thunderstorm. Never hide under a tree, and as long as you can hear thunder, the threat of a lightning stike close by remains.