For lightning flashes to earth, a downward leader grows step-by-step in a series of jerks from the cloud towards the earth. When the leader has got close to the earth within a few tens, to a few hundreds of metres, the electrical insulating strength of the air near the ground is exceeded.
A further “leader” discharge similar to the downward leader begins to grow towards the head of the downward leader: the upward leader.
Upward leader will be launched at points of greatest electric field intensity (see Fig.1) and can move in any direction towards the approaching downward leader. It is for this reason that lightning can strike the side of tall structures rather than at their highest point.