Every aurora app, including Moose, leads with a single number: the Kp index. It's the closest thing space weather has to a headline temperature. But "Kp 5" means something very different depending on where you're standing, and treating every point on the scale as equally far apart is the single most common mistake people make chasing a forecast.
What Kp actually measures
Kp is a planetary index — the "p" stands for planetarisch — built from magnetometer readings at roughly 13 observatories spread around the world. Each station measures how much the local magnetic field is being disturbed compared to a normal, quiet day, converts that into a local index, and NOAA's Space Weather Prediction Center combines all of them into one global number from 0 to 9, updated every three hours.
It isn't a measurement of the aurora itself — no camera or brightness sensor is involved. It's a measurement of how hard the whole magnetosphere is being shaken, which correlates closely with how far from the poles the aurora oval expands.
The scale isn't linear
Kp is a quasi-logarithmic scale. The jump from Kp 3 to Kp 4 reflects a much smaller real change in geomagnetic activity than the jump from Kp 7 to Kp 8. In practice, that means the difference between "quiet" and "unsettled" barely moves the needle, while the difference between "storm" and "major storm" can be the entire gap between seeing nothing and seeing color reach latitudes that almost never do.
NOAA's own G-scale (G1 through G5) is just Kp 5 through 9 relabeled for public storm alerts — the same number, described the way a hurricane category describes wind speed.
Why "now" and "forecast" often disagree
Kp is measured after the fact — each 3-hour value is finalized only once that window has passed. A forecast Kp, by contrast, is a prediction based on solar wind measurements from a satellite parked about 1.5 million km from Earth, roughly an hour upstream. That means a forecast for six hours from now carries real uncertainty, while a forecast for the next twenty minutes is about as good as space weather prediction gets. This is exactly why a sudden Bz swing can matter more in the moment than whatever Kp is currently reading — Kp hasn't caught up yet.
What this means for your own latitude
The aurora oval isn't a fixed ring — it's a band that expands toward the equator as Kp rises and contracts back toward the poles as it falls. If you live at a latitude where Kp 5 reliably brings color overhead, waiting for Kp 7 mostly means missing the two or three nights a month when Kp 5 was already enough. Conversely, if your latitude typically needs Kp 7 or 8, treating every Kp 5 alert as a reason to drive two hours will wear out its welcome fast.
That's the exact reasoning behind Moose's alert threshold slider — it's deliberately adjustable rather than fixed, because "worth going outside" genuinely means a different Kp number in Tromsø than it does in Edinburgh.
