What is a Tornado?
National Geographic
Tornados are movable, violent and destructive winds that look like funnels and rotate. A tornado moves violently and is in contact with the surface of the earth certain types of clouds or( is a dense, towering vertical cloud, forming from water vapor carried by powerful upward air currents) It is often referred to as a twister, whirlwind or cyclone
https://kids.nationalgeographic.com/science/article/tornado
What are cumulonimbus clouds and how do they form tornadoes?
Cumulonimbus clouds can form alone, in clusters, or along cold front lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes and hailstones. Cumulonimbus from overdeveloped cumulus develop as part of a supercell.(A supercell is a rotating updraft thunderstorm) 
https://www.universetoday.com/41646/cumulonimbus-cloud/
How are tornadoes formed?
Tornadoes form when warm, humid air runs into cold, dry air. The warm air rises through the colder air, causing an updraft. The updraft which makes the winds begin to rotate very sharply in speed or direction which causes tornadoes. Tornadoes can happen at any time of year, but they usually happen during a distinct season that begins in the beginning of spring for the states along the Gulf of Mexico. The season follows the jet stream—as it goes farther north, so does tornado activity. The month of May generally has more tornadoes than any other month, but April’s twisters are sometimes more violent than the ones that May has. Tornadoes tend to happen more later in the summer.
https://www.weatherwizkids.com/weather-tornado.htm
How does a tornado move, and eventually dissipate?
Inside the wall cloud, a funnel cloud forms and makes the air on the ground rotate; this eventually becomes a tornado. Tornados lose energy when the updrafts lose energy, and will slowly disappear.
https://www.groundzeroshelters.com/the-science-of-tornados
Where do tornadoes occur? What is “Tornado Alley?”
In the U.S. they are most common in the central plains of North America, east of the Rocky Mountains and west of the Appalachian Mountains. They occur mostly during the spring and summer; the tornado season comes early in the south and later in the north because spring comes later in the year as one moves northward.Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year.
https://www.foxweather.com/learn/tornado-alley-7-things-to-know
How do scientists research these storms on the ground? What are “Storm chasers”?
A storm chaser is a person who travels to locations where a tornado or other severe weather is forecast, as a hobby or in order to photograph or study it.
How are tornadoes rated/ranked?
Tornadoes are ranked on the Enhanced Fujita (EF) Scale, developed by a Japanese-American meteorologist who, got his start studying the damage caused by the atomic bomb in Hiroshima. The original Fujita scale was based on the amount of damage a tornado did based on wind speeds.
5 warning signs of a possible Tornado.
- a cloud of debris at ground level, even if a funnel, is not visible;
- a loud roar – like a freight train
- strange quiet occurring within or shortly after a thunderstorm;
- a change in the color of the sky;
- debris dropping from the sky.
How to stay safe?
Go to the basement or an inside room without windows on the lowest floor (bathroom, closet, center hallway). If possible, avoid sheltering in any room with windows. For added protection get under something sturdy (a heavy table or workbench). Cover your body with a blanket, sleeping bag or mattress.
How many people are killed each year?
In the United States over 80 people die and 1,500 injuries are caused by tornadoes each year. According to the National Oceanic Atmospheric Administration, the most tornado deaths are caused by people not following instructions on what to do the right way.