ELI5: How do tornadoes form?

A tornado forms when warm, moist air crashes into cold, dry air, creating a giant storm that starts to spin like water going down a drain.

Imagine you are running a hot bath on a chilly winter morning. The bathroom gets full of warm, floating steam, while the hallway outside is cold. In the sky, the exact same thing happens, but on a massive scale.

Here is how that turns into a tornado:

• The Big Bump: Warm, sticky air from the ground floats up high, just like a hot air balloon. At the same time, heavy, cold air pushes down from the clouds. When these two different types of air bump into each other, they make a huge, grumpy storm cloud called a thunderhead.
• The Invisible Rolling Pin: High up in the sky, winds start blowing at different speeds and in different directions. Imagine putting a toy car between your hands and sliding one hand forward and the other backward; the wheels spin! The winds do the same thing to the air, creating a long, invisible tube of air that rolls horizontally, like a rolling pin.
• Standing Up Tall: The strong, warm air rising from the ground catches this rolling tube and pushes it upward. Now, instead of rolling on its side, the air spins upright, like a toy top spinning on the kitchen floor.
• The Swirling Funnel: The storm acts like a giant vacuum cleaner, sucking up more warm air and spinning faster and faster. If you have ever pulled the plug in a full bathtub and watched the water spin into a whirlpool, that is just what the air is doing in the sky. When this spinning funnel grows big and stretches all the way down to touch the ground, it officially becomes a tornado.