Predict it yourself
Forecasters commit to numbers before reality arrives. Your turn — no peeking before you've made your call.
Exercise A — the July 2012 monster
A CME leaves the Sun at ~2,500–3,000 km/s — one of the fastest ever recorded. Earth's orbit is about 150 million km out. Your call: roughly how many hours from launch to Earth's distance? (Distance ÷ speed. Ignore deceleration for the estimate.)
Reveal what happened
At 2,500 km/s: 150,000,000 ÷ 2,500 = 60,000 seconds ≈ 17 hours. The real event crossed to 1 AU in under a day and slammed into the STEREO-A spacecraft, which measured a Carrington-class storm. Earth had been at that exact spot in its orbit about nine days earlier.
The uncomfortable lesson: the most dangerous CMEs give the least warning. A three-day forecast window is a luxury of slow storms.
Exercise B — the St. Patrick's Day surprise
March 2015: a thoroughly ordinary CME leaves the Sun — moderate speed, taking a leisurely ~2 days to arrive. Nothing about the launch looks special. Your call: what storm level would you forecast — minor (G1), moderate (G2), strong (G3) or worse?
Reveal what happened
It drove a G4 (severe) storm, Kp ≈ 8 — the biggest storm of the entire solar cycle to that point. The launch data couldn't have told you: what mattered was the CME's internal magnetic field, which pointed strongly southward for hours and poured energy into Earth's field. That orientation is essentially unmeasurable until the CME reaches the L1 spacecraft, an hour before arrival.
The lesson every forecaster lives with: arrival time is predictable days out; storm strength often isn't known until the last hour. Two different forecasts, two different confidence levels.
Exercise C — the storm lab: dial your own CME
This isn't a toy calculator: the sliders below run the same drag-based physics model this site uses for its real forecasts. Launch a CME and watch what the solar wind does to it.