Tornadoes and their parent thunderstorms are among the most intensely studied hazardous weather phenomena. The vast majority of tornado research today is conducted in the US, where tornadoes occur more frequently than anywhere else on Earth. Theoretical contributions, computer simulations, and field observations, such as those from the 1994–95 Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) and subsequent projects, like the recently completed VORTEX2, have revealed a great deal. In this article we draw on roughly a half century of prior work to summarize the latest understanding of how tornadoes form, and we discuss where gaps in our understanding remain.

Atmospheric convection is the relatively small-scale upward and downward movement of air resulting from an imbalance between the vertical pressure-gradient force and the gravitational force. Most of the time, the two forces are nearly in balance, and vertical accelerations are very small, with air moving predominantly horizontally. But when small-scale pockets of air become cooler and denser or warmer and less dense than their surroundings, the forces can become out of balance. In fluid dynamics, we call such a pocket of air a parcel: an imaginary fluid element of arbitrary size, much smaller than the characteristic scale of the variability of its environment but large enough to avoid the complexities associated with the molecular nature of fluids.

The resulting net force on a given parcel of air is the familiar buoyancy force, which depends on the difference between the density of the parcel and that of its surroundings, with larger differences resulting in larger accelerations. It’s a bit more complicated because the pressure field can also be perturbed and change the force balance. We call such a departure of pressure from a reference state of hydrostatic balance the perturbation pressure.

On sunny days convection is ubiquitous in the atmosphere’s boundary layer (typically the lowest 1–2 km), as any air traveler sensitive to the bumps experienced in low-altitude flight can attest. Boundary-layer convection is driven by the heating of air as it comes in contact with the warm ground. The right conditions can trigger so-called deep moist convection, with large vertical displacements and accelerations of air; in extreme cases the displacement approaches 20 km and the acceleration 0.1 g, where g is the gravitational acceleration. Those numbers hint at the enormous amounts of energy that drive thunderstorms. Box 2 explains how that energy can be quantified.

What we know and don’t know about tornado formation

at http://scitation.aip.org/content/aip/magazine/physicstoday/article/67/9/10.1063/PT.3.2514