During a storm, the cumulonimbus cloud is electrically charged. The top of the cloud is postivly charged whereas its bottom is negativly charged. This mechanism of charges separation in the cloud is not very well understood but there are many possible explanations. The first dated from 1892 and even nowadays none is totally convincing.
For exemple, one of these says that it is the collision between particles ( ice’s crystals ) in the cloud that created the charging. The particles would take a negative charge, where as ice’s fragments extracted by friction then would be positvly charged. Ice , lighter, is at the top of the cloud whereas water droplets are at the bottom .
Whatever the real explanation is, even if this phenomenon is complicated and misunderstood, it leads to charges seperation. The charge at the top of the cloud, mainly made of light ice’s crystals, is globally positive, where as the top of the cloud , made of heavier water droplets, is globally negative.
Because the low part is negativly charged, the ground is positivly charged by influence.
During storms, we have an electrical tripole : the top of the cloud is positive, the bottom is negative and the ground is positive. If we take a look at what happens between the bottom of the cloud and the ground, we can compare that situation to a huge condensator made by air situated between the cloud and the ground.
Franklin's Lightning Rod 