# Why does a vortex have so much suction power?

A vortex that's on the ground (tornado) can rip tiles off a roof of a house and basically suck in the roof tiles. On an aircraft that develops vortex lift the vortex sucks in air molecules and reduces the pressure on the wing. When there is a dust devil, it sucks in a lot of dust and throws it. So why does a vortex have so much suction power?

• Low pressure in a tornado, high pressure around it. Oct 8, 2015 at 2:45

If we consider the incompressible, 1-dimensional Euler equations (basically $F=ma$ but for a fluid without viscosity), the equation looks like:

$$\frac{\partial u}{\partial t} + u\frac{\partial u}{\partial x} = -\frac{1}{\rho}\frac{\partial p}{\partial x}$$

The left-hand side is the acceleration of a small lump of fluid and the right hand side is the force divided by the mass. Since we said this is inviscid, there is only the pressure gradient.

The negative sign in front of the pressure gradient means the acceleration vector is in the opposite direction of the pressure gradient vector. Gradient vectors go from low to high, and so the acceleration is in the direction of high pressure to low pressure.

The core of a vortex is, in general, a pressure minimum (although viscosity changes this slightly, not enough to talk about here). Therefore, there is a force from the outside of the vortex to the inside of the vortex; this is the suction force you are talking about. It is based entirely on how low the pressure can get inside the vortex relative to outside.

To get more suction, you need to make the pressure lower.

A vortex can have any amount of "suction power" from just more than zero (otherwise you'd have no vortex at all) to extreme (like the strongest tornadoes, or even more in artificial sources). The question of why a vortex has "so much" suction power, then, is related to the power of whatever is driving it. It cannot do more work (of which pushing and breaking buildings is a form of, in physics terms) than the input power, per conservation of energy.

A tornado has a lot of power because it mobilizes the energy of some very large masses of moving air, and concentrates that. It's a heat engine, operating between a hot and cold reservoir for which the difference in temperature may be only a few kelvins, but for which the masses of air involved are very large (cubic kms worth, equivalent to teragrams of mass). Thus there are similarly enormous amounts of potential energy available for conversion into mechanical work to drive the vortex. Initially, the tornado stars out as a much larger and more slowly-rotating vortex high in the atmosphere called a "mesocyclone". We are talking several tens of cubic km of air rotating in a vortex pattern. This rotation then becomes concentrated into a smaller area via effects involving precipitation, and because of conservation of angular momentum the rate of rotation increases rapidly (the "skater pulls in their arms" effect), creating winds often from 50-100 km/ks or more.

These high winds are very damaging because not only do they push but also pull and twist owing to the often relatively narrow size of the vortex on the ground as well as its movement thereover, and so while a building may be able to take some of this force from one direction only, when it becomes asymmetric like this it is easy to break because materials generally are not equally strong in all stressing modes. One may note that a hurricane, which also involves a vortex (the "eye wall", in effect an enormous tornado on the order of 100 km in size), tends to be actually less destructive on a per-building basis for a given wind speed. This is because the scale of the vortex is so large the winds are approximately linear at the size of a building, and thus you do not get this highly asymmetric and dynamic loading. Tornadoes, though, are often comparable to one or a few buildings in size, and loaded with turbulence.

If the air masses and temperature differences are not large, the vortex will be similarly small and weak. Such a phenomenon exists called a dust devil. Typically, they are not particularly dangerous, though a few can occasionally get strong enough to cause some mild damage like pulling the shingles off a roof or flinging light pieces of materiel like lawn chairs and tents. They result from more localized imbalances of temperature, such as in desert areas where a hot patch of ground is topped by cooler air, and have simpler flow patterns.

An even smaller and weaker vortex is seen commonly on a moderately windy day where localized areas of turbulence may occasionally stir bits of dust, leaves, and/or paper on the ground but produce no damage. So as one can then tell, the strength of a vortex varies across an entire gamut from harmless to catastrophic, and it all depends on what drives it, not that there is an inherent extremity to the phenomenon.

The low pressure inside the tornado against the high pressure outside the tornado creates a partial vacuum.

As the tornado spins, the centrifugal force increases and pushes air molecules away from the center of the tornado so that the concentration of air molecules inside the tornado is less that the concentration of air molecules outside the tornado. The faster the tornado spins, the greater the concentration gradient. Diffusion requires that the air molecules flow from a high concentration to a low concentration forcing air into the vortex. The speed at which the air molecules move into the vortex is dependent on the concentration gradient. These gradients are measured as the difference between the pressure outside the tornado and the pressure inside the vortex. The greater the difference, the faster the air rushes in, the greater the "suction".

In 2 words: CENTRIFUGAL FORCE. The spinning, like the impeller of a centrifugal pump, or vacuum cleaner fan, turns it into a giant vacuum cleaner. The only thing that keeps the tube of air going round in such a tight circle instead of flying apart under the force is the vacuum inside.

I couldn't find any data on the rotational speed of a tornado, but one about midway through the F6 category has a wind speed of about 560km/Hr. If it was a small 50m wide one, that equates to about 60rpm (not very impressive). The smallest ones are less that 10m wide. That equates to about 300rpm, but that is as seen on a Doppler radar from the outside, but as that mass of air is concentrated at the funnel, it will probably be travelling much faster, and the smaller the diameter the faster it spins for the same air speed. So the the funnel is probably doing 100s of rpm (maybe over 1000), and 1000s at the very tip. They have been known to suck the tar off a road. If you were standing there on the road ..............

They usually kill a few people and the woman in this car that was picked up thought she was going to be one of them. Her phone recorded a lot of screaming. https://www.youtube.com/watch?v=FG0jW-IBu6Y