Space weather tools that answer one question each.

These are the calculations that sit inside the Lumavik app, pulled out and made free on the web. They need no account, no download and no payment, they work on any device with a browser, and nothing you type into them is sent anywhere — the arithmetic runs on your machine, and the only network request any of them makes is to NOAA for public space-weather data.

We publish them for a straightforward reason. Aurora forecasting has a small number of genuinely useful calculations buried under a large amount of vague writing, and most of the pages that rank for these questions either hand you a single global number that does not apply to you or bury the answer inside a tour booking. A calculator cannot be vague. It either gives you a number you can check or it does not.

Which one you need

If you are outside right now and want to know whether to keep waiting, the live forecast is the fastest answer, and the tonight verdict is the more useful one because it accounts for where you are standing. If you are planning rather than watching, the geomagnetic latitude calculator and the dark-hours calendar are the two that change decisions: one tells you how often the aurora will reach your latitude at all, the other tells you which months your sky is dark enough to show it.

The pairing of those last two is where most trip planning goes wrong. Going further north improves your geomagnetic latitude, which sounds like an unambiguous win, and it is not — because the further north you go, the longer the summer keeps your sky too bright, and the more of the year is closed to you entirely. Above the Arctic Circle there are months when the geomagnetic forecast is completely irrelevant, because there is no darkness for anything to be visible against. Tromsø has a better latitude than Reykjavik and a shorter season. Neither is simply better; they are a trade-off, and the two calculators together show you the shape of it.

What these tools will not do

None of them can see your sky. Cloud cover is the single most common reason an aurora night fails, it has an absolute veto over everything else on this page, and no geomagnetic model contains it. A forecast of Kp 7 under solid overcast is a night indoors. Check an ordinary weather forecast alongside anything here, and treat a clear sky as the first condition rather than the last.

None of them can promise a sighting, and we would rather say that plainly than imply otherwise with confident-looking output. Aurora forecasting past about an hour ahead is probabilistic in a way that resists precision, because the variable that decides whether energy actually enters the magnetosphere — the orientation of the magnetic field carried in the solar wind — is only measured when that wind passes the spacecraft sitting upstream of Earth, roughly half an hour to an hour before it arrives. Everything beyond that horizon is an informed probability.

The coordinate conversions use the tilted-dipole approximation rather than the full field model, which is accurate enough for planning and can differ from corrected geomagnetic coordinates by a degree or two in some parts of the world. The solar geometry uses a standard low-precision series, accurate to a few minutes of twilight time. Both are stated on the pages themselves, because a tool that hides its approximations is harder to trust than one that names them.

If you find an error in any of them, tell us and we will fix it and say that we did. Several of these calculations exist in the app precisely because we could not find a version on the web we trusted enough to link to instead.

Using them offline

Four of the five work without a network connection once the page has loaded, because the arithmetic is local. Only the live forecast and the tonight verdict need to reach NOAA. If you are heading somewhere with no signal, open the geomagnetic latitude calculator and the dark-hours calendar before you leave and note the two numbers that matter for your destination: the minimum Kp it needs, and how much true darkness the month gives you. Those do not change on the timescale of a trip, and knowing them means a single glance at any Kp figure you can get hold of is enough to make a decision.

The vocabulary, in one place

Every tool here uses at least one term that gets thrown around without definition in aurora forecasting. These are the ones worth knowing before you read any forecast, ours or anyone else's.

Kp index. A planetary measure of geomagnetic disturbance on a quasi-logarithmic scale from 0 to 9, derived from magnetometer stations spread around the world and reported in three-hour blocks. It describes how disturbed the field was globally over three hours. It is not a measure of aurora brightness and it is not local to you.

Estimated Kp. The near-real-time figure NOAA publishes every minute so that people have a current number to work with. The definitive Kp is calculated later from the full station network and can differ slightly. The live tools here show the estimated value, which is the right one for deciding whether to go outside now.

G-scale. NOAA's storm classification, running from G1 for a minor geomagnetic storm to G5 for an extreme one, mapped onto Kp 5 through 9. It exists mainly to communicate infrastructure risk to power and satellite operators, but it is a convenient shorthand for how unusual a night is.

Auroral oval. The ring-shaped region around each geomagnetic pole where aurora occurs. It is offset toward the night side and it stays roughly fixed relative to the sun while the Earth rotates underneath it. During a storm it expands equatorward, which is what a rising Kp physically means for you.

Geomagnetic latitude. Latitude measured from the geomagnetic pole rather than the geographic one. Because the two poles do not coincide, this can differ from your map latitude by several degrees, and it is the coordinate that determines your aurora odds. Corrected geomagnetic latitude is the more rigorous version, derived from the full field model rather than a dipole approximation.

Magnetic midnight. The moment your location passes closest to the midnight side of the auroral oval, which is usually the most active part of the night. It does not coincide with clock midnight, and the offset depends on where you sit within your time zone and on your longitude relative to the geomagnetic pole.

Substorm. A sudden release of energy stored in the magnetotail, lasting tens of minutes, that produces the fast, bright, structured displays people remember. Because Kp averages over three hours, a substorm can deliver an excellent night inside a block that reports an unremarkable number.

Bz. The north-south component of the interplanetary magnetic field carried by the solar wind. When it points south it can reconnect with Earth's field and let energy in, which makes it the single most informative variable in short-term forecasting and the one most consumer apps never show.

L1. The point roughly 1.5 million kilometres sunward of Earth where spacecraft measure the solar wind before it arrives. It buys forecasters something between about thirty and sixty minutes of warning, and that lead time is the hard limit on how far ahead anyone can forecast with confidence.

OVATION. The statistical model NOAA uses to produce the familiar coloured oval map, estimating auroral emission from solar-wind input. It is a model rather than an observation, and it shows emission overhead at each point — not the probability that you personally will see something from your garden.

Substorm onset. The moment a substorm begins, when a quiet arc suddenly brightens and breaks into moving structure. It is the difference between seeing the aurora and watching it, and it is the reason experienced observers stay out after a display appears to have settled down.

Astronomical twilight. The period when the sun is between 12 and 18 degrees below the horizon. Below 18 degrees the sky is as dark as it will get, which is the threshold a faint aurora needs. It is the constraint that closes the season in summer at high latitudes, and the one the dark-hours calendar is built on.