The genesis of a tornado near LaGrange, Wyoming, on 5 June 2009. It was intercepted by the VORTEX2 project and is the best-observed tornadic storm in history. Video courtesy of Lyndon State University.
A nice place to start
I highly recommend that you start with this short, easy-reading, piece written by Dr. Yvette Richardson and me, published in The Conversation (click below). It’s formatted as five short answers to five questions about tornado formation, their parent storms, and how climate change might affect tornadoes in the future.
What I tell kids
Scientists are still studying exactly how tornadoes form. Although some of the details of tornado formation are still being investigated by scientists, we know the ingredients for tornadoes in a broad sense. In general, thunderstorms occur when warm, humid air near the ground is overlaid by relatively cold air aloft (in other words, temperatures cool rapidly with height). This variation of temperature with altitude results in what meteorologists refer to as “instability,” which gives energy to thunderstorms. Tornadoes can form within thunderstorms when instability is accompanied by what meteorologists refer to as “wind shear”—large changes in wind speed or direction with altitude. On tornado outbreak days, the temperature can decrease with height by nearly 30°F per mile (10°C per kilometer) over a depth of several miles. Moreover, wind speeds can change by over 50 mph over a similar depth. Both rising, spinning, warm air and sinking cool air are important for tornado formation.
Thirty-second explanation of how tornadoes form in supercell thunderstorms (applicable to virtually all tornado outbreaks)
Thunderstorms occur when warm, humid air near the surface lies beneath a deep layer of air in which the temperature decreases rapidly with height. Such an atmosphere is said to be “unstable,” which simply means that it contains ample energy to fuel thunderstorm updrafts. When an unstable atmosphere also is characterized by unusually high humidity near the surface and large changes in wind with height (“wind shear”), especially when considerable variation in wind is found over just the lowest few thousand feet of the atmosphere, tornado-producing supercell storms are possible.
FOR THOSE STILL YEARNING FOR MORE
I highly recommend the following publications, which appeared in Weatherwise, Physics Today, and the Bulletin of the American Meteorological Society:
Markowski, P., and Y. Richardson, 2013: How to make a tornado. Weatherwise, July/August 2013, 12–19.
SOME IMAGERY
Feel free to borrow the imagery below, as long as you reference this web site. There are a lot of bad illustrations of tornado formation. These are more accurate; hopefully you’ll also find them to be more understandable.
The vast majority of headline-making tornadoes are spawned by supercell thunderstorms, which are storms that are characterized by an updraft that rotates (counterclockwise rotation, if viewed from above, is most often observed, though some supercell updrafts rotate clockwise). Changes in wind speed and direction with altitude in the storm’s environment imply the presence of horizontal spin, similar to that of a football. As this spinning air is drawn into the storm’s updraft, the updraft rotates. Tornado formation is associated with a separate air stream, one that descends through a precipitation-driven downdraft and acquires horizontal spin by way of a horizontal variation of temperature along this air stream. If the air within this downdraft air stream is not too cold, this spinning air subsequently can be sucked upward by the supercell’s updraft, leading to a tornado.
Our present understanding of how a tornado develops in a supercell thunderstorm. (a) A tornadic supercell near Deer Trail, Colorado, intercepted by the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) on 10 June 2010 (photo by Bill Reid). The white arrows show the orientation of the vorticity vector, the yellow curved arrows indicate the sense of spin, and the red and blue lines indicate the paths taken by updraft and downdraft air parcels, respectively. In step 1 of tornadogenesis, the storm acquires large-scale rotation—a midlevel mesocyclone—by tilting the horizontal vorticity in winds entering the storm’s updraft. In step 2, buoyancy gradients due to relatively warm and cool air straddling either side of downdraft air parcels generate horizontal vorticity. That horizontal vorticity is then tilted upward by surrounding wind fields as the parcels descend. (b) A closeup of the region inside the dashed box in panel a. In step 3, conservation of angular momentum amplifies the now vertical vorticity as air converges toward the axis of rotation while being sucked upward by the strong mesocyclone above. (c) A nontornadic supercell with tornadogenesis failure in progress, intercepted by VORTEX2 near Panhandle, Texas, on 13 June 2009 (photo by Paul Markowski).
Wind shear creates the rotating updraft of supercell thunderstorms. Rotation next to the ground actually originates within a cool, descending airstream, however. Tornadoes form if the rotation within this cool air stream is greatly intensified. The intensification of the rotation occurs when the cool, rotating air is sucked upward into the overlying updraft of the supercell. The upward sucking is accompanied by an inward contraction of angular momentum, which increases the rotation in the same manner by which a figure skater spins faster. Photo by Bill Reid.