graduate school - Is it okay for a professor to leave the classroom only 5 min past the class start if nobody has shown up?

Last week, because of a subway delay due to bad weather I arrived 15 minutes late to class to find out my professor had left 10 minutes before because nobody else had arrived.

Was it okay for him to do that? Would it be rude to complain about this matter?

We are just 4 students between mid 20's to mid 30's, this is a grad school elective course with a single session of 3 hours every week, starting at 17:00 ending at 20:00.

I'm also a former TA and currently a lecturer of an undergrad Physics 101 course, so I'm aware of how it feels when even half of the class is missing.

Update version 2 Today I was the only one to attend (unsure about the reasons my classmates had, the weather was OK this time). And talked to the professor about the issue. He said this situation wasn't his problem. About the weather/commute issue he said this: "Of course I know that happens, I'm much older than you". Then decided there wasn't enough people, handed me the class notes saying we will skip theory again because "everyone is irresponsible" (exact words again) and left. I'm dumbfounded a second time.

Final update I decided to let it be for now. I'm not going to pick a fight alone like a crazy person (my classmates don't see a problem, neither with the professor's behaviour, nor theirs). If the situation takes a turn to the worse, I'll just withdraw for the course and hope someone else teaches it next year. Thanks for everyone's interest :) never expected so many comments!

Answer

This is something that is going to vary quite a bit from one country to another. My personal opinion about the ethics of this, for a three-hour class that meets once a week, on a day of a torrential downpour (and I have lived in places that featured this kind of bad weather, so I get the picture): the professor should have waited a bit longer. Also, it would have been considerate for him to go to an office or library nearby, to get some work done, leaving a note in the classroom.

But I will focus my answer on a positive action you can do, now, about what happened.

You had prepared some questions. Good. Now bring those questions to the professor in office hours.

If the professor looks at his watch in a nasty way every three and a half minutes, ignore it and pretend he is being polite. And be polite.

Do not make any snide or subtle remark connecting your visit to office hours with what happened the day you were late. You may briefly apologize for being late, if you can trust yourself to do it completely deadpan.

This approach has a couple of things going for it:

1. 
   

   You get your questions answered

   
   


2. 
   

   The professor may put two and two together and realize that it is less convenient to answer questions in office hours than in class.

   
   
   

But don't overdo it. Don't spend more than half an hour in his office discussing your questions with him.

It might be inconvenient for you to make a special trip to campus just for 20 or 30 minutes in office hours, but it will worth it, as this can be more effective than grousing.

Note: This answer was written in response to the un-updated version of the question. If OP would like me to write an answer to the question in its new form, a separate question should be posted.