Skip to content
aurorahour

How we calculate the aurora chance

Last updated: October 23, 2026

Every forecast on this site combines public data with simple, published rules. This page explains them, so you can judge each number yourself. All of it is an estimate: the aurora can be brighter or weaker than any forecast.

1. Geomagnetic latitude

The aurora forms around Earth's magnetic poles, not the geographic ones. We convert each place's coordinates to geomagnetic latitude with a tilted dipole model of Earth's field, with the north geomagnetic pole at 80.85° N, 72.68° W (the published value for 2025). The magnetic pole sits on the Canadian side, which is why the northern US is closer to the aurora than Europe at the same latitude.

2. The Kp each place needs

As the planetary Kp index rises, the edge of the auroral oval moves toward the equator. We use a straight-line fit of where the aurora is usually seen low on the poleward horizon by eye from a dark place: about 65° geomagnetic latitude at Kp 0, moving 2.2° toward the equator for each Kp step. A phone camera in night mode records faint aurora about one step earlier. Above Kp 9 means "extreme storms only": the Kp scale stops at 9, and only the strongest storms of a cycle bring the aurora that far.

3. Tonight, hour by hour

  • Kp: the NOAA SWPC forecast for each 3-hour block (UTC), mapped to local hours.
  • Darkness: the Sun's altitude for each hour, computed with Astronomy Engine. We count an hour as dark when the Sun is 12° or more below the horizon.
  • Moon: its altitude and how much of it is lit. A bright Moon hides faint aurora but not a strong display.
  • Clouds: the National Weather Service hourly sky cover forecast for the place (US only). Above 75% cover we show "Clouds in the way".
  • Chance words: forecast Kp at least half a step above the need: good chance; within about half a step: possible; within one and a half steps: low chance; otherwise unlikely.

4. How often it happens: our own count

For each place we went through the official Kp record from GFZ Potsdam, 3-hour block by 3-hour block, from 2006 to 2025. A night counts when, in at least one block during darkness at that place, Kp reached the level the place needs. Clouds are not included, because there is no cloud record that matches, so the real number of nights with a visible aurora is lower.

5. What we do not do

  • We do not promise a display. A forecast of "good chance" means the conditions line up, not that the aurora will appear.
  • We do not use the logos of NOAA, NASA, the National Weather Service or any agency, and we are not affiliated with them.
  • We do not track you. "Use my location" works only inside your browser.

Data and licenses

NOAA SWPC and National Weather Service data are in the public domain. The Kp archive is from GFZ Helmholtz Centre for Geosciences under CC BY 4.0 (Matzka, J., Stolle, C., Yamazaki, Y., Bronkalla, O. and Morschhauser, A., 2021, The geomagnetic Kp index and derived indices of geomagnetic activity, Space Weather). The full list is on the sources page. Found an error? Email [email protected].