About
Borealis Radar · PublishedWhat is Borealis Radar?
Borealis Radar is a small, free companion for anyone who dreams of seeing the northern lights. It quietly watches the sky for you and answers one simple question: what are my chances of seeing the aurora tonight, right where I am?
There’s no login, no ads, and nothing to buy. Open it, glance at your score, and get back to watching the sky.
Why I built it
Aurora forecasts tend to be either overwhelming or vague. I wanted something kinder: a clear, honest answer for your exact spot, in the time it takes to look out the window.
So when the sky is about to put on a show, you’ll know it’s worth stepping outside — and when it isn’t, you can rest easy and try another night.
How it works
Behind the scenes, Borealis Radar keeps an eye on the things that decide whether the lights can be seen: how lively the sun has been, how dark your sky is, how bright the moon is shining, and whether clouds are standing in the way. It gently blends all of that into a single score and a plain-language verdict — no charts to decode, no jargon to learn.
A personal project
Borealis Radar is a labor of love, built and cared for by one person. If you’d like to say hello, share an idea, or help with translations, the Feedback button in the app comes straight to me.
Aurora questions, answered
What is the aurora borealis?
The aurora borealis, or northern lights, is the glow produced when charged particles from the sun are funnelled along Earth’s magnetic field and collide with gases in the upper atmosphere, roughly 90 to 400 km up. Oxygen gives the familiar greens and deep reds; nitrogen adds blues and purples. It happens far above the weather, which is why cloud hides it completely.
What Kp do I need to see the aurora where I live?
The auroral oval sits near 66° latitude when Kp is 0 and pushes about 2° further from the pole for each Kp level, so the minimum Kp your location needs is roughly (66 − your latitude) ÷ 2. Tromsø needs almost nothing; Reykjavík and Fairbanks need about Kp 1; Oslo about Kp 3; Edinburgh about Kp 5; Edmonton about Kp 6; Calgary about Kp 7–8; and Seattle needs a storm beyond Kp 9, which is why sightings there are a once-a-decade event. Borealis Radar applies this to your pinned location, so Kp only lifts your score when the oval can actually reach you.
What is the Kp index?
Kp is a planetary index of geomagnetic disturbance from 0 to 9, published every three hours. It describes how far the aurora has spread away from the poles, not how bright it is — a high Kp is only good news if it is also dark and clear where you are standing.
Why does Bz matter so much?
Bz is the north–south direction of the magnetic field carried by the solar wind, and it is the strongest short-term predictor there is. When Bz turns and stays southward — a negative value — it links up with Earth’s own field and lets energy pour in, often lighting the sky within the hour. A strongly northward Bz can leave a promising forecast producing nothing at all.
When is the best time to look?
Activity peaks around magnetic midnight, so roughly 10pm to 2am local time is the richest window, though a strong storm can show at any dark hour. In the northern hemisphere the season runs from late September to late March, simply because you need long, dark nights. The weeks around the March and September equinoxes tend to be the most geomagnetically active of the year.
Why is my score low when Kp is high?
Because Kp on its own does not get you a sighting. A score can stay low through daylight, thick cloud, a bright moon washing out faint arcs, wildfire smoke, or simply your latitude being too far south for that Kp to reach. Borealis Radar weighs all of these together instead of reporting Kp alone, which is why its answer sometimes disagrees with a headline space-weather number.
Contact
Questions, corrections and translation help are all welcome. The Feedback button inside the app comes straight to me, or say hello on Instagram: @max.a.k.a.yuri.