Question

Estimate the total charge q on the cloud when the breakdown of the surrounding air is reached

Answer 1

 First, you must breakdown electric field:
E = 9x10^9 q / R^2
where R is the radius of the cloud
q is the required charge. 

So this will come up to a solution of: 
q = E R^2 / 9x10^9
= 3 x10^6 x (500)^2 / 9 x10^9 

= 83.33 C.

The answer is 83.33 C.