The Severe Weather Bench
Two thunderstorms with the same instability produce very different outcomes; the one with wind shear rotates and lasts hours.
Phase 0 · The phenomenon
Same fuel, different storms
Left: an upright thunderstorm cloud with a single vertical updraft arrow and rain falling straight back down through the updraft. Right: a taller storm leaning to the right with a broad anvil, a curved rotating updraft arrow on one side and its rain shaft displaced to the other side, with a wall cloud beneath the updraft.
On a summer afternoon two thunderstorms grow in air with the same instability — the same amount of energy available to lift air. One rises, rains hard for half an hour and collapses in its own downdraft.
Two hundred kilometres away the winds change speed and direction with height. The second storm leans over, begins to rotate, drops golf-ball hail and keeps going for more than three hours, putting a town under a tornado warning.
In this lab you will take a storm apart on the bench: its engine, the hail it can hold up, the wind shear that organises it, and the warning time it leaves the people in its path.
Why do two thunderstorms with the same instability turn out so differently — and what does that mean for the people in their path?
What this lab works toward
HS-ESS3-1Explain how natural resources, natural hazards and climate change have shaped human activity.HS-ESS2-4Use a model to explain how changes in the energy flowing into and out of Earth's systems change climate.
Plain-English summaries. The course maps these performance expectations to this lab; the official wording is published at nextgenscience.org.
The stations
- Updraft EngineHow does a storm's energy (CAPE) set the speed of its updraft?
- Hail LiftHow fast must an updraft blow to keep a hailstone aloft?
- Wind ShearHow does wind shear decide whether a storm dies in minutes or lasts for hours?
- Warning Lead TimeHow do storm speed and detection distance decide the warning time a town gets?
Each station runs the same loop: predict before you see any data, investigate the model and record runs, reveal what the measurements show, then explain the gap between your prediction and the result. A claim-evidence-reasoning checkpoint and an exit ticket close the lab.
- Updraft speed comes from parcel theory (w = √(2 × CAPE)). Real updrafts mix in drier air and carry the weight of rain and ice, so the strongest observed cores are well below the theoretical value.
- Hailstones are treated as smooth spheres of solid ice falling through air of one density. Real hail is lumpy, partly air-filled and grows while it is suspended.
- Storm mode is read from one number — wind shear over the lowest 6 km. Forecasters also use the turning of the wind with height (helicity), moisture and the trigger for the storm.
- Warning lead time assumes the storm moves straight toward the town at a steady speed.