Tornado are the most powerful, unpredictable and destructive weather systems on Earth. A tornado generally occurs when high winds within a low pressure system (such as a thunderstorm) cause water vapor in the air to condense in to a condensation funnel cloud. Many less severe tornadoes are not even visible to the human eye. Major tornadoes usually become more visible when the strong winds within the funnel lift up dirt and debris off the Earth’s surface. Tornadoes are generally classified as either a land spout, a water spout or a gustnado.