Why Does Lightning Occur?

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Separation of Charges
Lightning is the result of the separation of charges within the cloud, as is illustrated by the picture on the left.
There are two main theories as to why this occurs.  
1. The first is that the separation is caused by cosmic particles crashing into the particles in the atmosphere which leads to the electrons from these particles being knocked free and so there become positive and negative ions which then separate to the top and bottom of the cloud.  
2. The second theory is that it is ice particles and rain droplets that collide within the cloud which then leads to the separation of charges.
Whichever the method, the result is this obvious separation of charge.  At the base of the cloud all the negative charges start extending towards the ground and the positive charges that are attracted to the surface of the ground start extending towards the ground.  Objects at ground level  are now sending positive streamers to try to meet the downwards negative ones.
When the two streamers meet an upstroke occurs, that is a single weak upward discharge is released from the ground, which clears the path from ground to cloud.  This process is illustrated in the diagram above.
A fluctuation in the brightness of the lightning strike is caused by fluctuation in the voltage that is flowing.
The flash which occurs is caused by the extreme temperatures that occur due to the currents involved and the thunder is a shock wave that is produced as the air around the strike is caused to expand at such a rate, also due to the extreme temperature.

Lightning occurs more frequently during the summer months, because you need warm, moist air to create the right conditions. What is known is that raindrops are forced up into the cloud until some turn into ice ~ this normally happens around 20,000ft above sea level. There is some disagreement about how the charge is built up in the cloud, but the most popular explaination is that the ice crystals (being heavier) begin to drop, striking new forming ice crystal thus resulting in a small static charge. Once the charge has been built up current is released by burning a path in approximately 50ft sections called step leaders towards the ground, searching for a connetion. Once that connection on the ground is found the circuit is completed and current from the ground returns to the cloud along the path burnt into the air. The return current has a far greater luminesance which is the lightning you see. This all happens in about � a second.

Lightning strikes have been recorded up to 10 miles away from the storm. The associationed thunder is created by the return current super heating and the cooling of the air as it passes through. The rapid expansion and collapse results in an explosive force at the point of the lightning strike. As the explosive wave travels out from the strike point it becomes shock and sound waves.

It happens by the movement of positive and negative charges in the sky.

Static builds up from droplet collisions. A charge ladder is made, causing lightning when strong enough. Simple as.

Lightning occurs when Zeus get's mad

Lightning occurs when lightning occurs

It occurs because the stuff shoots stuff.

Lightning occurs when there is thunder