Strikingly beautiful, lightning can take many forms.

Lightning is an electrical discharge through the atmosphere. Several forms of lightning may occur during a thunderstorm, and these discharges can cause additional optical spectacles at much higher altitudes.
 

Negative Discharge Lightning

Cloud to ground lightning is the most common form of lightning and the type of lightning we experience frequently. 

• This type of electrical discharge occurs when negative charges build up in lower parts of a thunderstorm. 
• In the middle section of a cumulonimbus cloud, rising ice crystals collide with a form of soft hail called graupel. 
• The result of these collisions makes the rising ice crystals positively charged and the falling soft hail negatively charged, creating a negative charge build up in the lower portions of the storm.
• Underneath the storm, the ground reacts to the negative charge by becoming positively charged.
   

Air conductivity

Air is a very poor conductor of electricity. Even when there are millions of volts of electrical potential present, air must first create a pathway for a lightning bolt to travel. The pathway is called a stepped leader.

Once the electrical charge difference between the air and ground is high enough, the air between the cumulonimbus cloud and the ground becomes ionized which reduces its resistance to electricity. 

Several pathways called stepped leaders start from the cloud and work their way towards the ground. These are commonly negatively charged. Differences in the air and the way the air ionizes makes a difference in how the pathways travel. 

When a particular stepped leader gets near the ground, a pathway from the ground streams up to meet it. A pathway is completed, and a negative discharge starts. The most rapid movement of negative charge is at first closest to the ground and works its way upward. We see this as the bright flash of lightning and is called a return stroke.

When you see lightning flash several times, it may have discharged 10 or more times, but it happens too fast for us to see.
 

Positive Discharge Lightning

A more dangerous form of cloud to ground lightning is positive discharge lightning. The phrase “a bolt from the blue” is likely from this type of discharge because it can strike the ground 10 to 20 miles from the storm.

This lightning originates in the upper reaches of a storm, which may be 10 miles or more above Earth.

• Positively charged ice crystals rise high into the storm. 
• Because the distance between this area of positive charge and the ground is much greater than for negatively charged lighting, there is a much greater build up of charge before the air can lose its resistance to electricity moving through it. 

Though positive discharge lightning is less common, it is very dangerous because it can strike seemingly out of nowhere and is much more powerful than negative discharges.
 

Lightning discharges can be long and long lasting

Research has shown that electrical discharges in a storm can be much more that a single flash. Some of the longest lightning has been measured well over 400 miles from end to end. In addition, some of the longest lasting discharges have been measured over 17 seconds from start to finish.
 

Lightning names and myths

Sheet lighting occurs when no bolt of lightning is visible; instead a general flash or “sheet” of light is seen. The bolt of lightning in this case is hidden by clouds or rain, so what is seen is the ambient light produced by the bolt.

Heat lightning are general flashes of light on the horizon during a warm night. In this case lightning is occurring far off in the distance from thunderstorms that cannot be seen or heard. It is not caused by a simple warm night.