Under normal conditions air moves from pressure system to a pressure system

Under normal conditions air moves from higher pressure system to a lower pressure system
Air moves to lower pressure. This concept can be observed in the lungs and heart. When you trying to inhale, the muscle in ribs or diaphragm will pull the rib cage and decrease its pressure up to -1mm Hg. This will cause the air from atmosphere enter the lungs. 
In expiration, the muscle will relax and the air pressure inside lungs will become +1 mm Hg, makes the air pushed out.

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.