They form from thunderstorm when moist air and cool dry air masses meet to create instability in the atmosphere and due to change in wind speed and direction at high altitudes the air swirl horizontally to form a funnel that progress to touch the ground and become the Tornado.
Explanation
First,a large thunderstorm occurs in a cumulonimbus cloud. A change in wind direction and speed occurring at high altitude will result into the swirling of air horizontally. The rising air from the ground will push up on this swirling air and tips it over. A funnel made up of warm air starts to suck up more warm air from the ground and surroundings thus growing longer and it stretches to reach the ground. When the funnel touches the ground, the tornado is formed.
Most tornadoes form from thunderstorms. You need warm, moist air from the Gulf of Mexico and cool, dry air from Canada. When these two air masses meet, they create instability in the atmosphere. ... Most strong and violent tornadoes form within this area of strong rotation.
