Thunderstorms and Tornadoes â Often appear as a vortex or funnel-shaped cloud of flying debris. Some are rope shaped and others are wedge shaped with more than one vortex. â Funnel-shape due to air pressure at the center being very low and air is sucked into the funnel expands and cools; water vapor in the air condenses, forming a funnelshaped cloud. If the air is drier or the air pressure inside the tornado is higher, it may consist of dust and debris creating a loud roaring sound. â They appear at the back edge of a thunderstorm and travel with the storm. â Tornadoes can travel in any direction and often move erratically. â Tornadoes can occur anywhere in the United States and at any time, however, they occur most often in Tornado Alley in the spring and summer.