Why does lightning flash in the sky

There is moisture in the clouds . This moisture is in the form of very fine particles of 

water in the .clouds .there is friction between air and water particles .Eiectricity is

generated by friction and the water particles become charged .Some clusters of clouds .

are postive and some are negatively charged When positively and negatively charge 

clouds come close to each other ,they collide , producing very high power lightning.