I have the standard Weather calendar added to Google Calendar, and I can see the weather for today on goocal in my desktop browser just fine.
I noticed that the weather was no longer showing up on my calendar. It was showing the weather for last week - Mon-Tue-Wed, but there was no entries for this week. So I deleted the weather calendar completely. Verified it was no longer showing up on my Android. Then I added it back again. Now I can (still) see it on my desktop in the browser, but in Android the calendar no longer shows up at all. The Weather calendar is quite definitely selected as "Synce, Visible" in the calendar settings. I also tried another calendar app that uses the Google Calendar database (Business Calendar); again, the Weather calendar is selected to be shown, but it does not show up.
Before you ask why I don't just use the weather widget, to be honest I don't particularly care about the weather, I just like having something on my calendar every day - it makes it more obvious in "Agenda" view that I have nothing going on today
UPDATE: I just tried the "Day of the Year" calendar and that shows up just fine. My speculation is that the weather calendar requires your current location, and that's the source of the problem. I'd rather see the weather, which is at least marginally useful, than the day of the year though.