Tornadoes often occur in areas where the air masses which have different temperature and humidity together. The temperature and humidity vary much. The collision was violent and more. The air condition must adjust the balance in "tornado funnel" is the area with the most severe clash of air masses. The cold, dry air from West blows toward the east. The warm moist air from the Gulf of Mexico blew up north. When met warm moist air to rise. Cold, dry air will float down. The cloud has covered the area. And if the humidity is high enough to cause rain or thunderstorm.