A central conceit of George R.R. Martin’s A Song of Ice and Fire books (and the popular HBO series Game of Thrones based on them) is that the seasons of the planet where they take place are not as predictable as the Earth’s annual cycle. Somehow the phrase “winter is coming” wouldn’t seem as foreboding if you could reply, “Yes, that usually happens in December through February.”

But how could a planet have unruly seasons? Earth’s seasons are due to the tilt of its axis. During one part of Earth’s orbit, the Northern Hemisphere is tilted away from the Sun, with the resulting indirect sunlight spread thin over the surface of the hemisphere, causing winter. On the opposite side of its orbit, summer comes as this hemisphere is tilted toward direct sunlight. There isn’t much room in such clockwork for randomness.

Well, if you’ve ever wanted to debate fan theories, here’s an excellent new resource for you to draw from: a real climate model simulation of Westeros and Essos.