eli5: How do tornadoes form?


Some of my co-worked are located in Tornado Alley and I’ve been wondering how tornadoes form and die. I have never seen one in person (and I don’t want to). How is is different form a dust devil? I saw a dust devil form across the street and stop once it hit my garage door where I was standing. I just froze and didn’t know what to do.

In: 3

On the large scale, tornados are formed from the collision of cold dry air funneled in from the Rocky Mountains, hot dry air from the deserts in the south west, and warm wet air from the Gulf of Mexico. Those three air masses hit each other, and it causes large, spinning thunderstorms.

As far as I’m aware though, we still don’t know the exact causes that turn a thunderstorm into a tornado. Fluids are weird, and tornados are chaotic enough that it becomes difficult to determine all the factors that go into formation.

I’ve seen a fair number in the UK (just funnel clouds) ie not touching the ground, often slanted (not straight up and down). I learned somewhere once that Tornadoes can form when a horizontal ‘rotor’ is churning in a thundercloud, warm air grating over cold air, one end of this rotor makes it to the ground and all hell breaks loose. Not gospel but s theory I heard, which correlates with all the slanted FC’s I’ve seen.

Tornadoes form when different large weather fronts meet each other. Cold air is more dense than warm air, so it wants to sink down, and less dense warm air wants to rise up. But, the ground absorbs a lot of sunlight, so it gets warm and warms up the air near it, while air cools as it rises up.

Broadly, this is what causes weather. The poles get less direct sunlight so they are cooler while the equator gets lots of direct sunlight and is warmer. Air warms at the equator and rises up, while air cools at the poles and sinks down. As the air cools around the poles, it sinks down and pushes warm air out of the way, towards the equator. But since the Earth has a lot of water and mountains and is spinning, the mixing of these fronts is very chaotic. When cold fronts move quickly, they shove themselves under the warm, moist fronts, forcing the warm air to rise rapidly, cool, and the moisture condenses out as rain.

Tornadoes happen when there is a third air front between them that is hot and dry. When all three fronts meet with the right timing, the cold air gets under the hot air, and then the warm, wet air gets rapidly shoved all the way on top of both. Normally, the warm air would cool down and there will be rain, maybe a storm, and the two fronts will mix with some wind. The hot air forms a *convection cap* between them, though, which traps the cold air low to the ground and prevents the warm front from mixing as it cools. Instead, as it cools, the warm front *tries* to sink down, squeezing the air around the ground that is warming and trying to go *up*. The rapidly cooling upper layer may even spill over, around the convection cap, but it can’t get in because the front against the ground is still there. This builds up pressure until a hole forms in the hot air layer.

That hole relieves the pressure. The air on the ground that is being squeezed and squeezed bursts through the hot air convection cap, blasting upwards with a lot of force. All of the air around that hole rushes in to fill that space. It’s like pulling the plug in a bathtub, all the water rushing towards the drain – except instead of gravity pulling water down, it’s the pressure forcing the air up through the hole. And, like in a bathtub, any little bit of angular momentum – any little bit of swirling – is conserved so that the air swirls and swirls around where the hole is, creating the whirl in the tub and a tornado in the air.