I've heard that APRS is only allowed for non-commercial use. I was thinking of a building a weather station that uploads data via APRS (so far, non-commercial). If I then include data from that weather station on a website with advertisements, is it commercial use (and thus not allowed)? If that is not commercial use, what would be an example of commercial use?
There is no rule specific to APRS; the relevant regulations do not care about what mode, protocol, etc. you are using. From §97.113:
§97.113 Prohibited transmissions.
(a) No amateur station shall transmit:
Communications specifically prohibited elsewhere in this part;
Communications for hire or for material compensation, direct or indirect, paid or promised, except as otherwise provided in these rules;
Communications in which the station licensee or control operator has a pecuniary interest, including communications on behalf of an employer, with the following exceptions:
I am not a lawyer. This is not legal advice. This is speculation based on a layman's understanding. If you actually need a reliable answer, consult a lawyer.
These rules prohibit what you are proposing, because you are making transmissions which in which you have a pecuniary interest: your site, from which you profit via advertisements, would be of less value if you did not have the weather station transmitting.
On the other hand, if you are only doing one of the two things:
If you have a weather station transmitting and no one's paying you to do it, there is no pecuniary interest, so the transmission is not prohibited.
If you are receiving APRS and displaying the data on a web site, you are not transmitting, so §97.113 does not apply. In fact, the well-known APRS data site aprs.fi contains ads.
This is not only legal but highly prevalent throughout the amateur community today, especially with APRS.
What you are referring to is essentially hosting an online logbook. You will log some traffic previously transmitted in some format over amateur radio and display it publicly online.
In order to help pay for website hosting (presumably) you wish to show ads on your website.
The ads on your website give you pecuniary interest in your WEBSITE but not the original APRS transmissions! Any amateur could already intercept your weather data with their own amateur radio gear and decide to view, collect and republish your weather data! Therefore the transmissions themselves are open and freely available to any amateur. If you want to get even more technical even an unlicensed individual could listen to your APRS packets and collect your weather data as no transmission is required to gather APRS packets (as opposed to other digital modes which do require both stations to transmit).
Real World Examples
For a very similar example to what you are proposing, take a look at "aprs.fi" which aggregates ("logs") APRS location data online - APRS.fi has google ads everywhere!
Another APRS website doing the same thing is OpenAPRS.net - ALSO has web ads and explicit offers to purchase advertising.
Looking outside of digital mode logging, you could look at QRZ.com (yup lots of ads) which hosts some QSO logging software.
How about ARRL.org - yup lots of ads even on the LOTW login page the banner has a yaesu sponsor notice.
You could go on for quite sometime enumerating the numerous other amateur radio websites which have ads on them, but if the ads are only on the web (never transmitted on the air) they only provide pecuniary interest for your online activities - not your on air activities.
Weather Station reports are an important and intentionally designed component of APRS. Use it to the maximum of your capabilities!
UPDATE: Answering the original question better - When would APRS use be commercial?
APRS (or any transmission) is commercial when either the content is for commercial purposes or you are being paid to make the transmission.
For the first example regarding content, this could be making transmissions for a business or coordinating commercial activities. For example if you owned a plumbing company and installed APRS tracking on all your plumber's vehicles in order to facilitate better dispatching - that would be commercial use as the transmission is intended to facilitate business operations.
Likewise if the content of the transmission is of a more obvious commercial nature that would of course be commercial. For example you could not transmit an APRS message that read "Sale on Products at my store! Come visit my store!". That would of course be transmissions with commercial intent and pecuniary interest.
For the second example, if you are being paid for the transmission. This really stems from old times when it cost money to send a message (ie: "telegram") to friends/family, but still applies today. I can't pay you to send a message across country for me. You cannot receive remuneration for making the transmission.
I was browsing and saw this and I couldn't help but put my "2 cents" in. I think it would truly take a lawyer to properly and completely answer this question.
I note the following:
1) It may or may not be illegal. It seems to me that it depends on WHY the person did this. To get money? Simply as a method of providing a service/information? To me it seems that the transmission itself is NOT illegal, it is ONLY the other circumstances associated with it that are in question.
2) It would, IMHO, be very difficult to prosecute this situation. A lot of the crux of the situation has to do with intent. Intent can be hard to prove. The government may be reluctant to prosecute, whether it is or is not illegal. They may not be able to PROVE the intent, and thus whether it is actually illegal or not.
3) You can't make it illegal for one person to do a thing and illegal for another to do essentially the same thing under approximately the same circumstances (with some obvious exceptions). In this case it seems perfectly legal for one person to transmit and that transmission be used on the internet, even though the person controlling the internet gets money. The person transmitting did nothing illegal (got no money) and the person controlling the web site did nothing illegal (didn't transmit anything). Then how can the same type of transmission or its use on a website suddenly become illegal, when it is used in exactly the same way and for the exact same purpose just because it is on another website (regardless of who is doing it)?
4) That is not to say that the person doing this is completely in the clear, legally. He DID get money, but was it really his INTENT to do anything but provide information, just like others? Or was he trying to generate money by including it on his website? Does the money relate to the transmission or not? Or does the money relate to the website only? Does the money somehow not count in this situation?
It is an interesting and thought provoking situation in any case. I am no lawyer but I would think that this is a reasonable, even useful and helpful, use of the transmissions. The money seem unrelated to the transmitting although I admit if it suddenly increased the revenue of an existing it would not look so innocent. But I still believe it should be an allowed activity, since others can do it without being illegal, even if it needs a change to the rules to accomplish this.
As a side note, I believe that being sure that the rules are followed and legal issues avoided with some good pre-planning. Let different people take charge (at least legally, if not actually) of the website and the transmitter. Thus the money is separated from the transmissions, but the goal is still accomplished.