BusyCal and other calendar apps add weather right into your calendar view, but Apple’s built-in Calendar.app on iOS and macOS doesn’t offer that feature.
Turns out, Weather Underground serves .ics files that can be imported to Calendar, as Chris Short explains:
Recently, I discovered Weather Underground provides ICS files for use in any calendar application you can think of (Google Calendar, Microsoft Outlook, and Apple’s Calendar.app). These ICS URLs do not appear to be documented by Weather Underground specifically but here are some examples to get you started.
I set it up, adding the calendar subscription to my iCloud account. Now, I can make outdoor plans without needing to jump to my weather app of choice first. Pretty neat.