These guides explain the numbers on our forecast pages and what to do with them: what the Kp index means at your latitude, how to read tonight's forecast and decide whether to go out, how to see and photograph the aurora with a phone, and how the forecast itself is computed.
Each guide is written and signed by its author. Numbers come from primary sources — NOAA's Space Weather Prediction Center, peer-reviewed papers — or from our own data, and each guide says which. If something here is wrong, tell us.
For tonight's conditions, start from the forecast on the home page.