These are some great questions!

So remember that electric field lines point always in the direction that a test charge (a positive charge by convention) would move in when placed in the vicinity of another charge.


According to the question stem we have the following:

Clouds: - - - - - - - (collection of negatively charged electrons)


Ground: + + + + + + + (relative lack of electrons)


Essentially, a significant potential difference has been created by the separation of the charges. Potential energy helps moves things around, in this case the electrons. Since the bottom of the clouds are full of negative charges, the clouds have a low voltage (potential) compared to the ground which has a higher potential (positive). As a result, as you go from the ground to the clouds, voltage decreases.

Hope this helps!