Source: http://www.slate.com/articles/health_and...

Lightning is a natural electrical discharge—but scientists are still scratching their heads trying to figure out what triggers it. Renowned Russian physicist Alexandr Gurevich tells Katia Moskvitch about his theory, which really is out of this world

What don't we know about lightning?The main problem is that we don't know how a thundercloud gets the spark needed to initiate a lightning bolt. The biggest mystery is that the electric field in thunderclouds is not very large. Years of experimental measurements from aeroplanes and air balloons have shown that the field is about 10 times smaller than what is needed to initiate lightning. It is not clear how a lightning bolt is born, but the idea is that something has to "seed" it first.