What Kp is
Kp is NOAA's 0-to-9 measure of how disturbed Earth's magnetic field is, reported for each 3-hour block of the day (00–03 UTC, 03–06 UTC and so on). The bigger the Kp, the farther south the aurora reaches. NOAA publishes Kp in thirds and writes them as 5- (4⅔), 5o (5) and 5+ (5⅓); we write 4⅔, 5, 5⅓.
NOAA's storm scale G1 to G5 is tied to Kp. From NOAA's scale page, where aurora has been seen:
| Storm level | Kp | Aurora seen as low as (NOAA) |
|---|---|---|
| G1 minor | 5 | High latitudes (northern Michigan and Maine) |
| G2 moderate | 6 | New York and Idaho (typically 55° geomagnetic) |
| G3 strong | 7 | Illinois and Oregon (typically 50°) |
| G4 severe | 8, incl. 9- | Alabama and northern California (typically 45°) |
| G5 extreme | 9 | Florida and southern Texas (typically 40°) |
The Kp you need
NOAA's viewing tips: “At Kp = 0, the equatorward edge of the auroral oval is approximately 66 degrees [geomagnetic]. And it moves equatorward about 2 degrees for each level of Kp”, reaching 48° at Kp 9. NOAA adds that these relationships are approximate.
So we need your geomagnetic latitude: latitude measured from Earth's magnetic pole, not the geographic one. The magnetic pole sits over northern Canada, so places in the eastern US are magnetically farther south than western places at the same map latitude. We use the tilted-dipole part of the IGRF-14 world magnetic model (pole near 80.9°N, 72.8°W in late 2026). NOAA uses a more detailed “corrected” geomagnetic system that can differ by a few degrees, so we always say “about”.
Then:
- 66° or more: you live under the usual aurora zone. Any Kp can work on a clear, dark night.
- 48° to 66°: Kp needed = (66 − your geomagnetic latitude) ÷ 2, rounded up to the next third. Seattle (53.0°) → about 6⅔; Minneapolis (53.4°) → 6⅓; Bismarck (54.7°) → 5⅔; Boston (51.5°) → 7⅓; New York (49.9°) → 8⅓.
- 40° to 48°: only extreme storms (Kp 9) have reached this far, for example Denver (47.3°). Even at Kp 9 we say “possible”, not “good”.
- Below 40°: past where NOAA's strongest storm level typically reaches, for example Miami (34.8°) and Honolulu (21.8°). We say “very unlikely”.
NOAA's live model (OVATION)
NOAA's aurora model gives, for every 1° of latitude and longitude, the probability of visible aurora. It is a 30 to 90 minute forecast: the lead time is how long the solar wind takes to travel from a monitoring point between the Sun and Earth. When those measurements are bad, NOAA falls back to a Kp-driven version with no lead time. So it tells you about the next hour, not about tonight at midnight. That's why it's in “Right now”, with a dashed Live model badge, and doesn't change tonight's word.
The percentage we show is NOAA's own number for the grid cell over you: the only percentage on this site. NOAA notes the aurora can be seen from as far as 1,000 km away when it's bright, so for “where to look” we search NOAA's grid within 1,000 km for the strongest aurora and compute its compass direction and how high it should appear, assuming the lower edge of the aurora is about 110 km up.
The view line
NOAA's view line marks “the southern-most locations from which you may see the aurora on the northern horizon.” NOAA publishes it only as a picture, so we redraw it from the live model using the method NOAA's scientists published (Case, MacDonald & Viereck, 2016):
- For each longitude, find the highest aurora probability P in the northern hemisphere, and the most southern latitude φ where it occurs.
- View line = φ − (P ÷ 20 + 3) degrees.
- Smooth it by averaging 5 neighbouring longitudes.
This is our estimate of NOAA's line using NOAA's published method; NOAA's current software and its tonight map may differ, so we link NOAA's own map. The same study found the line is cautious: 62% of people who reported seeing aurora were south of it, by a median of 3.7° (about 400 km).
The verdict
| If… | Tonight's word |
|---|---|
| The sun never gets 12° below the horizon | Too bright |
| NOAA's Kp forecast didn't load | Can't tell yet |
| Below 40° geomagnetic | Very unlikely |
| 66° geomagnetic or more | Good chance |
| NOAA's highest Kp during your dark hours ≥ the Kp you need | Good chance (possible if only Kp 9 reaches you) |
| Up to 1 below the Kp you need | Possible |
| More than 1 below | Unlikely |
| Then, if clouds are on and above 70% | One step lower (never below very unlikely) |
“Dark hours” here means nautical darkness (sun 12° down), and Kp counts every 3-hour block that overlaps them. We never turn tonight into a percentage: a number like “73% tonight” would claim more precision than anyone has.
Darkness, moon and clouds
- Dark enough = sun's centre at least 12° below the horizon (end of nautical twilight). Darkest = 18° (astronomical). Sunset and sunrise use the sun's upper edge with normal refraction. We checked our times against the U.S. Naval Observatory for Seattle, Minneapolis, Fairbanks and Boston: all within a minute or two.
- “Tonight” runs from local noon to the next noon. After midnight, until the sky gets light again, tonight still means the night you're in.
- Moon: we show how much of it is lit and when it's above the horizon. If it's more than 75% lit and up for more than half of the darkest hours, we warn that it will wash out faint aurora. NOAA: “The full moon will also diminish the apparent brightness.”
- Clouds (optional): the National Weather Service's sky cover forecast for your area, averaged over the dark hours: up to 30% mostly clear, up to 70% partly cloudy, above that mostly cloudy (and the word drops one step).
- Times are always in your place's own time zone, with its abbreviation. If the clocks change during the night, both abbreviations are shown.
What we can't know
- Tonight's Kp is a forecast. It depends on when a solar storm cloud arrives and how its magnetic field is pointed, and can be off by several points either way.
- The view line is our reproduction of NOAA's published method, not NOAA's own line.
- Kp-to-latitude is approximate (NOAA says so), and our dipole latitude can differ from NOAA's by a few degrees.
- ZIP codes: we use the Census Bureau's ZIP areas (ZCTAs), which miss some PO-box-only ZIPs. Time zones near zone borders (parts of North Dakota, Kentucky, Indiana, Oregon/Idaho) can be wrong; that's why every time shows its zone abbreviation.
Privacy
- The site is plain static files. There is no server of ours that could see your place.
- NOAA's aurora files are the same download for everyone and carry no location. NOAA's map image loads only when you open it.
- ZIP and city lookups load one small list from this site, named by the ZIP's first three digits or the state, so your full ZIP never leaves your browser.
- “Use my location” stays in your browser and is rounded to about 1 km first.
- The cloud forecast is the one exception, and only if you turn it on: it asks the National Weather Service (api.weather.gov) for the forecast at your area, rounded to about 1 km.
- We remember your last place and your cloud choice in this browser only, with buttons to forget them. We count visits with Google Analytics without your place: see the about page.
Sources
Every fact above was checked on Oct 9, 2026 unless marked.