Bortle Scale
Definition
A 1-to-9 rating of night-sky darkness, from pristine wilderness (1) to inner-city sky (9).
In plain language
The Bortle scale gives you a shared language for "how dark is this spot?" Aurora hunting works best at Bortle 4 or darker, especially toward your poleward horizon.
The science
The scale classifies sites by naked-eye limiting magnitude and visibility of features like the Milky Way and zodiacal light. Dark-site directories and light-pollution maps commonly translate measurements into Bortle classes.
Why it matters for aurora viewing
Checking a site's Bortle class before a storm night prevents the most common beginner failure: perfect Kp wasted under a bright suburban sky.
See this in action on tonight's data.