How forecasting works
Three instruments, in order of warning time:
- Coronagraphs (days of warning): space telescopes that blot out the Sun's disk so the faint CME shows. A CME aimed straight at us appears as a halo expanding around the whole Sun. This is where every arrival forecast starts.
- The drag model (the forecast): a fast CME plows into the slower solar wind ahead of it and decelerates, like a boat in water. Feed in launch speed and direction, and physics gives an arrival estimate. Honestly stated: it's good to about ±12 hours — validated against hundreds of real arrivals on our science page, misses included.
- The L1 tripwire (30–60 minutes): spacecraft sit 1.5 million km sunward of Earth. When a shock front sweeps past them, arrival is now measured, not modeled. That last hour is the only near-certain part of space weather forecasting.
▦ Open the tracker in monitoring mode ∑ The actual math & its error bars
The uncertainty IS the lesson. Why only ±12 hours? Because the inputs are imperfect: a halo CME's true speed is hard to measure head-on, and the solar wind it must push through changes daily. Forecasts aren't failed physics — they're honest physics with imperfect inputs. Very little in science education teaches that; a live forecast page does it every day.
Check yourself — Module 3
1. The L1 monitoring spacecraft give roughly how much certain warning?
L1 sits ~1.5 million km upstream. When a shock sweeps past it, arrival at Earth is measured, not modeled — about half an hour to an hour out.
2. Why is a storm's STRENGTH so much harder to predict than its arrival time?
Arrival is physics on speed and distance. Strength hinges on whether the CME's field points south — essentially invisible until the last hour.
3. Our published arrival-forecast accuracy is honestly about…
±12 hours, validated against hundreds of real arrivals — and that's the state of the field, not a shortcut. Imperfect inputs, honest physics.
Check yourself above, then continue — answers aren't stored anywhere.