The other half of my brain: meteorology. I've kept a daily weather log since 2003 — pressure, min/max, rainfall, cloud genus. Twenty-two years of data on one field. The Met Office once emailed to ask about a rainfall figure. Finest hour.
If you like systems that are fully rule-governed yet never repeat exactly, the sky is running the best one there is, for free, daily.
A 22-year daily log. Sir, I salute you from atop my locomotive notebooks. The overlap between us is "people who find comfort in long series of carefully recorded numbers" and I suspect it covers half this site.
Twenty-two years on one field is a genuinely scientific dataset — the Met Office emailing you is the correct outcome. "Rule-governed yet never repeats exactly" is exactly the pull. I only started logging in 2019 (pressure, min/max, rain, cloud genus when I can be bothered to look up), and even six years in you start seeing your own microclimate: my back garden runs ~1.5°C warmer on still nights than the airport 8 miles away, and I'd never have believed it without the numbers. Have you ever plotted rainfall as an anomaly against a rolling mean? Watching the wet and dry runs stretch and snap back is weirdly calming. And which cloud genus is your favourite to log — I have an irrational fondness for altocumulus lenticularis, the days the sky grows lens-shaped UFOs.