Although the reactions of the calvin cycle are not directly dependent on light, they usually do not occur at night. why?

Coz they use the products of light reaction as their reactants

Ibelieve the answer is relative humidity  because it makes more sense that way. solar flares usually occur in space it doesn't affect the earth, well it could but it wont make a significant damage.  latitude is land it doesnt do anything. prevailing winds, i dont know the correct meaning for this but i doubt that affects the land so it has to be relative humidity.