Lightning Facts

You can build a static charge in low humidity conditions by rubbing a balloon against your hair, or dragging your feet across a motel carpet, etc. Materials with weakly bound electrons will transfer them to a material of tightly bound electrons when they are rubbed together.  But the spark you typically feel is only 1% of what it would take to kill you.  The most you can get is 30% of a lethal jolt, which will not kill you  -- but I promise you it will leave a lasting memory. 


Lightening occurs in Florida mostly in the summer months.  All of Florida has a lot of lightening, but the peninsula has the most lightening and also the most rainfall.


Ice in a thunderstorm exists in two forms. In one form, super cooled liquid cloud droplets freeze on contact with ice, forming a conglomerate of really tiny balls of ice.  This is called riming.   As riming continues, and the piece of ice gets bigger, it is called graupel. (Bigger still, the same frozen ball is called a hail stone). 


In the other process, water vapor molecules in the air move to, and attach themselves to a particular vacant space on a hexagonal crystal of ice. These hexagons fit together very nicely and make bigger and bigger ice crystals. As this continues, the ice crystal grows and can take the form of a rod, a plate or the familiar, ubiquitous, and beautiful snowflake.


No one knows the details of how lightening forms in thunderstorms, but it clearly has to do with the difference in the surface electro-chemistry between these two forms of ice.  When these two different forms of ice collide, there is a flow of electrons from the graupel (frozen drops) to the ice crystal as long as the temperature is warmer than -20 deg. C. At temperatures    below -20 deg. C (the reversal temperature), the electrons flow the other way so that the heavier graupel accumulates negative charge and the ice crystals become positively charged.  The lighter crystals rise in the updraft and the heaver graupel falls, separating the charge. So the thunderstorm starts out being negatively charged at the top and then as it grows, it becomes positively charged at the top and the negative charge accumulates near the bottom.  


In a practical sense, only the electrons move so that when something is positively charged, it has lost electrons.  Even though the earth has a net negative charge (thanks to all the lightening strikes) the earth below the thunderstorm becomes positively charged since, by induction, the negative charges on the earth leave beneath the storm as the negatively charged storm base repels the negative charges on the ground. 


Thanks to Ben Franklin, current flow is defined as the direction a positive test charged would move.  So it is in the opposite direction of the flow of electrons. Backwards.


As the potential difference grows, eventually it reaches the breakdown potential, and, just like when you get a static discharge, you get a lightening bolt.  It is complicated, as the spark goes in a series of steps, looking for the shortest electrical path, which isn’t necessarily the shortest geometric path. It does this in a succession of steps until it finds the perfect path close to the ground and then “hooks up”. The electrons flow in spurts down the ionized channel at 60,000 miles per second or 1/3 the speed of light, and eventually the illumination move from the ground up, much as a line of cars react to a stop-light turning green.  This is quickly repeated as the region of the discharge expands throughout the cloud. The lightning bolt may contain a current load is as high as 230,000 amps and have over a billion volts. No wonder lightening kills.


We hear the thunder from the powerful lightening bolts between cloud and ground. But 90% of the lightening is going on within the cloud with path lengths from centimeters (or less) to meters and everything in-between. These can only be detected by special equipment.


Next – how to protect yourself and things from lightning.