Capacitance and maximum charge HELP

[seant]

As a crude model for lightning, consider the ground to be one plate of a parallel-plate capacitor and a cloud at an altitude of 620 m to be the other plate. Assume the surface area of the cloud to be the same as the area of a square that is 0.40 km on a side.
(a) What is the capacitance of this capacitor?
nF
(b) How much charge can the cloud hold before the dielectric strength of the air is exceeded and a spark (lightning) results?
C

I used C=E0 A/D (8.85*10^-12 C^2/N*m^2)(.16km^2/620m) and conversions also, but to no avail can someone please help

 

[alphysicist]

Hi seant,

As a crude model for lightning, consider the ground to be one plate of a parallel-plate capacitor and a cloud at an altitude of 620 m to be the other plate. Assume the surface area of the cloud to be the same as the area of a square that is 0.40 km on a side.
(a) What is the capacitance of this capacitor?
nF
(b) How much charge can the cloud hold before the dielectric strength of the air is exceeded and a spark (lightning) results?
C

I used C=E0 A/D (8.85*10^-12 C^2/N*m^2)(.16km^2/620m) and conversions also, but to no avail can someone please help

What number did you get as your final answer, in nF?