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Lightning!

Atmospheric instability is quite common and occurs when warm air is surrounded by cooler air.  The natural difference in density between the two causes the warm air to move upwards relative to the denser cool air.  When the rising warm air is humid, at some elevation it cools to the dew point (becomes saturated) and moisture begins to condense out to form a cloud.  Air temperature for dry air decreases around 5.5 degrees F for every 1000 foot rise in elevation while saturated air cools less (averaging around 3 degrees F per 1000 feet) with increasing height.  Thus, the rising moist air of the cloud remains warmer than the surrounding dry air and so the instability remains in place.

Eventually the temperature reaches the freezing point and the condensed moisture begins to either form small ice particles or remains in a super-cooled liquid state.  As the ice particles are dragged aloft by the upward moving air, more and more moisture droplets condense out of the air onto the growing ice particle.  At some stage of the ice particle growth it succumbs to the force of gravity and begins to fall as precipitation. 

Meanwhile an upper atmospheric disturbance called a divergence creates a driving force that accelerates the upward flow of air even more than the normal density driven instability.  This forced upward flow causes an increase in friction between ascending ice particles and the larger bits of frozen ice descending as hail.

Fueled by the rising warm and moist air continually fed at the base, the thundercloud grows vertically to tremendous heights sometimes reaching to the top of the troposphere where it ceases to grow vertically.  At the tropopause, the upper most discontinuity between the troposphere and the stratosphere, a temperature inversion occurs whereby temperatures actually begin to rise with increasing height and this puts an abrupt halt to the instability condition.  Here, at the tropopause or wherever the instability extinguishes, the thundercloud spreads laterally instead of upward and forms the classic anvil shaped top. 

Ascending and descending ice particles may recycle several times before growing to a sufficient size to overcome the upward force of the rising air and fall to the ground as a torrent of precipitation (rain or hail).  The friction between the ascending and descending ice and hail creates a condition whereby electrons are stripped from the upward flowing frozen moisture creating small positively charged ice particles that collect in the upper regions of the cloud and negatively charged hail in the lower region of the cloud. 

Ultimately, a tremendous charge distribution builds up within the cloud.  The negatively charged cloud bottom forces a positive charge build-up on the ground.  In a short time a very high potential difference forms between the top and bottom of the cloud as well as between the bottom of the cloud and the ground as shown in the diagram below.  Air is a pretty good electrical insulator but it only has a finite ability to resist this potential so when the voltage builds to about 915,000 volts per foot, the insulating properties of air breaks down.  When this happens, charge attempts to transfer through the ionizing air in the form of stepped leaders and ultimately lightning appears in an effort to neutralize the potential difference. 

Charge Distribution Immediately Before a Lightning Strike
(from The Complete Weather Source Vol.2) 

In cloud to ground lightning, an invisible stepped leader forms first which is sort of the precursor to what we perceive as lightning.  It is a zig-zaggey affair of invisible electrons responding to the call from the build up of positive charge at the ground level.  It moves toward the earth at about 1000 to 3000 feet per millisecond in an unpredictable path determined by how the air is ionizing in front of it. It forks and branches accordingly. As the stepped leader approaches to within 300 feet of the earth’s surface, lightning (still invisible) strikes out from the surface to meet it.  This ground based lightning is referred to as the “return stroke”.  Most, but not all of the time, it originates from a tall object like a building or tree where positive charges have accumulated as shown in the accompanying diagram

When contact is made between the return stroke and the stepped leader, the lightning path is created and a tremendous surge of electrons flow down his path from the cloud to the ground.  Even so, this is not the lightning we see.  The visible flash we identify with a lightning bolt is the subsequent flow of positive ions back up this path.  It generates tremendous heat, literally hotter than the surface of the sun, and explodes the air in its path.  This heated, violent expansion of gas is what creates the visible bright flash of lightning.  The viscous expansion of air creates a sound wave that we experience as thunder.

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