About percent of all tornadoes in the united states develop from the month of may normally has the greatest number of tornadoes while the most violent tornadoes seem to occur in when vertical wind shear tends to be present as well as when horizontal and vertical temperature and moisture contrasts are greatest. although tornadoes have occurred at all times of the day and night, they are most frequent in the when the surface air is most unstable; they are least frequent in when the atmosphere is most stable. a. 75, march to june, march, the over night, late night b. 70, march to july, april, late afternoon, early morning before sunrise c. 70, march to july, april, late morning, early morning before sunrise d. 75, march to august, june, the over night, early afternoon