Question

Consider the Earth and a cloud layer 820 m above the Earth to be the plates of a parallel-plate capacitor. If the cloud layer has an area of 1.25 km2 = 1.25×106 m2, what is the capacitance?

Answer 1

so what's the formula for a parallel plate capacitor? If you can't remember, watch this excellent lecture where you will learn that among many other great things:

http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-7-capacitance-and-field-energy/