Grades 6–8 · Earth & Space
How can a thunderstorm form a tornado?
A tornado is a narrow, intensely rotating column of air connecting a thunderstorm to the ground. Some powerful tornadoes form in supercells, thunderstorms with a persistent rotating updraft. A tornado is much smaller than the storm circulation that surrounds it, and most supercells never produce one.
In the diagramA tornado needs interacting storm processesOne supercell pathway · most supercells do not form tornadoes1. Wind shearFaster wind aloftSlower wind below2. Rising airTilts and stretchesstorm rotation3. Near groundInteracting flowsNear-ground rotation may tighten into a tornado.Wind shear and a rotating storm do not guarantee a tornado.
Wind shear means that wind speed or direction changes with height or distance. In one common explanation, differences in wind with height supply rotation about a horizontal axis. Rising air in a thunderstorm can tilt this rotation and help stretch it into the storm’s rotating updraft. Warm, moist inflow can help sustain rising air, but merely bringing warm and cold air together does not explain tornado formation.
The final concentration of rotation near the ground depends on several interacting processes. Inflow, sinking air called downdrafts, temperature patterns and the strength and position of the updraft all matter. Air moving inward and upward can intensify an existing rotating circulation. The exact combination varies, and scientists continue to investigate why one storm produces a tornado while another does not.
The visible funnel is condensed water droplets, sometimes joined by dust and debris. Rotating air can reach the ground before a complete condensation funnel becomes visible. Other tornadoes form through pathways that do not begin with a supercell.
About this explanation
Starting knowledge: Understand rising and sinking air, wind direction and changes in speed with height.
Sources
- NOAA National Severe Storms Laboratory — Severe Weather 101: Tornado Types
- NOAA National Severe Storms Laboratory — Severe Weather 101: Tornado Basics