Why Do Tornadoes Produce Such Violent Winds?
― How a Wide Vortex in a Storm Cloud Is Squeezed Thin and Spins Faster
Inside a cumulonimbus cloud (a towering thundercloud, the kind that brings lightning) a slowly turning vortex can form that is several kilometres wide. Yet a tornado reaching down from it to the ground can carry some of the fastest winds ever recorded on Earth. It is the same vortex. Why does its speed change so much?
Sit on a spinning chair, like an office chair. Stretch your arms out wide and turn slowly. Now pull your arms in to your body, and you will feel the spin suddenly speed up.
All that changed was your shape: from "wide" with arms out to "narrow" with arms folded. Yet the same body starts spinning at a completely different speed. Something very similar happens inside a tornado.
Inside a well-developed cumulonimbus cloud, a relatively slow rotation (a mesocyclone) several kilometres wide can form.
When the vortex is stretched down towards the ground and its width shrinks to a few hundredths of what it was, a physical property called conservation of angular momentum is thought to make its rotation speed leap upward.
Let's look at this "wide vortex" and "faster when squeezed thin" mechanism step by step.
Why does a wide vortex form inside a storm cloud?
Inside a well-developed cumulonimbus cloud, the wind's direction and strength can differ with height. This difference is thought to create a "tube-shaped vortex" of air lying on its side. The strong updraft inside the cloud can lift this sideways tube and stand it upright. The result is a gentle vertical rotation several kilometres wide (a mesocyclone).
At this stage, the spin is still not very fast. The key part comes next.
Why does squeezing a vortex thin make it faster? Conservation of angular momentum
A spinning object has a quantity that measures its "amount of spin" (angular momentum). Unless some special outside force acts on it, this quantity stays roughly constant. Roughly speaking, it is set by "spin speed" × "vortex radius".
So if the vortex radius gets smaller, the spin speed has to rise by the same factor, or the angular momentum cannot be kept. When the updraft inside the storm cloud stretches the wide vortex upward and makes it thin, this relationship is thought to make the spin speed up all at once. The principle is the same as a figure skater who spins faster by pulling their arms in.
In reality, several processes are thought to act at the same time. For example, a downdraft near the ground (the RFD) wraps cold air around the base of the vortex. Simple conservation of angular momentum alone cannot explain the full strength of every tornado.
It is what happens when nature squeezes an existing wide vortex in an instant.
What you can check for yourself
- Sit on a spinning chair (such as an office chair) and stretch both arms out wide to the sides
- Have someone give you a gentle turn, or push off the floor with your feet, to start a slow spin
- While spinning, pull both arms in to your body
- Notice that the spin suddenly speeds up (for safety, do this in a wide space with no objects or people nearby)
Simply shrinking the "radius" by pulling your arms in makes you spin faster. This is the same relationship as a vortex squeezed thin into a tornado.
If you are outdoors, go at once to a windowless room inside a sturdy building. If you are already indoors, the basic advice is to move away from windows and outer walls and stay low. Cars, storage sheds, and prefab or temporary buildings are considered less safe than sturdy buildings, because of the danger from flying debris and collapse.
Summary
A tornado's ferocious wind does not come out of nowhere. It is thought to be the result of a gentle vortex several kilometres wide that was already inside the storm cloud being squeezed thin by the updraft, so that its spin speed leaps upward according to the conservation of angular momentum.
A wide, slow rotation, just by becoming narrow, turns into the fastest wind on Earth.
The way spin turns into speed can also be seen in how a cat rights itself in mid-air. We explain this in this article. A vortex speeding up as it is squeezed also shows up in flowing rivers. See also our article on the water rings that form below a step. And how hard that faster wind pushes on things, with the frightening arithmetic of "wind force grows with the square of wind speed", is explained in this article.
For those who want to know more ― terms, numbers and links to textbooksFrom middle-school science to active research topics, each item is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school "Physics", or material in textbook extensions and sidebars
- UniversityUniversity-level specialist subjects (meteorology and fluid dynamics), not taught in high school
- ResearchTopics researchers are still investigating, not taught even at university as settled fact
Middle schoolTerms: words used about tornadoes
- Cumulonimbus: a cloud that grows very tall, bringing lightning, heavy rain and gusts.
- Mesocyclone: a gentle rotation several kilometres wide that forms inside a cumulonimbus cloud.
- Angular momentum: a quantity that measures the "amount of spin" of a rotating object.
High schoolChecking with a formula: how does wind speed change if the vortex is squeezed to one tenth?
We use a simplified rule, that "radius" × "spin speed" stays roughly constant, to estimate how the wind speed changes when a vortex is squeezed.
Wind speed after squeezing ≒ original radius × original wind speed ÷ radius after squeezing
| Original radius | 2000 m (a typical mesocyclone) |
| Original wind speed | 5 m/s (a typical gentle rotation) |
| Radius after squeezing | 200 m (assuming it is squeezed to one tenth) |
* This is a rough estimate from a simplified model that ignores air friction and complex up-and-down motion. The wind speed of a real tornado is not thought to be set by this alone.
| Radius × wind speed (original) | 2000 × 5 ＝ 10000 |
| Wind speed after squeezing (m/s) | 10000 ÷ 200 ＝ 50 |
| Convert to km per hour (km/h) | 50 × 3.6 ＝ 180 |
| Result | In the simplified model, a wind speed of about 50 m/s (about 180 km/h) |
180 km/h means a wind close to the running speed of a Shinkansen bullet train, raging at ground level. Just shrinking the radius to one tenth gives this big a change in the calculation. In real strong tornadoes, the vortex may be squeezed even thinner or other processes may add to it, and wind speeds higher than this are thought to have been recorded.
High school+The physics of rotation, the same as in figure skating
The angular momentum of a rotating object is given by mass × speed × radius (strictly, the coefficient changes with how the mass is distributed and the shape of the rotation, but the basic idea is the same). A figure skater pulling in their arms to spin faster and a tornado vortex being squeezed and speeding up both follow the same rule: "make up for the smaller radius with more speed".
UniversityDynamical models of tornado formation in meteorology
In meteorology and fluid dynamics, several models have been proposed to explain why and where a vortex inside a mesocyclone is squeezed even thinner and becomes a tornado. In particular, researchers are studying how the downdraft that comes with a storm cloud (the RFD, or rear-flank downdraft) affects the temperature and humidity at the base of the vortex, as a key factor.
ResearchWhat is still unclear
- Which mesocyclones produce tornadoes and which do not, and how to tell them apart accurately in advance, is still an active research topic.
- The detailed process of tornado formation (tornadogenesis) is regarded as one of the famous unsolved problems in meteorology, and field-observation projects using mobile observation vehicles and Doppler radar are run in many countries.
- How climate change will alter the number of tornadoes, their strength, and the regions where they occur is also still being researched.
Even one violent phenomenon like a tornado holds a rich topic where meteorology and fluid dynamics meet, and research is still under way.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: weather / how clouds form | Basic terms: cumulonimbus, mesocyclone |
| High school | Basic Physics: rotational motion (advanced) | Proportional calculation of radius and wind speed |
| High school+ | Physics: angular momentum (advanced) | Link to figure skating |
| University | Meteorology, fluid dynamics | RFD and dynamical models of tornado formation |
| Research | Meteorology (active research) | Predicting tornado formation, effects of climate change |
- Explanations of cumulonimbus clouds, mesocyclones and tornado formation in meteorology textbooks.
- Explanations of conservation of angular momentum and vortex dynamics in fluid-dynamics materials.
- Explanations of tornado observation, advisories and safety measures from public bodies such as the Japan Meteorological Agency (気象庁).
- Reviews of field-observation research on tornado formation (tornadogenesis) in meteorology.
* The radius and wind-speed figures are rough values chosen to help explain the mechanism. The size and wind speed of real tornadoes are said to vary greatly from case to case.
* This article is a general-audience science explainer. The figures given are rough estimates to help you understand the mechanism. For actual weather decisions, follow the announcements and warnings of public bodies such as the Japan Meteorological Agency.