The correct answer is C) surface temperature and temperature aloft are equal. When the surface temperature and temperature aloft are equal, there is little to no difference in temperature between the surface and the atmosphere above it. This results in minimal cloud formation, as the lack of temperature differential prevents the air from becoming saturated and condensing into visible clouds. The other options are incorrect because: A) High relative humidity actually promotes cloud formation, as it increases the amount of water vapor in the air. B) Low barometric pressure is associated with the development of storm systems and increased cloud cover, not minimal clouds. D) A large difference between surface and atmospheric temperatures drives the formation of convective clouds, not minimal cloud cover.