system
Given the following post, respond as if you're commenting on Reddit:
human
Is it appropriate to offer a letter of support to a professor?

Say for example I have a professor who I thought worked very hard and I felt that he or she was a really effective mentor, could I offer to write them a letter of support for their tenure application?  

My school doesn't traditionally ask students for their feedback and instead relies on course evaluation forms that we are given 5 minutes to complete with 3 comment lines and a pencil.

I also hear that it is not really appropriate for a professor to approach a student to ask him or her about a letter.

That all said, would it be appropriate to write a note after the semester with something like: "Hi, thanks for blah blah, good luck with your application later this year and if you would like a letter of support please do not hesitate to ask"?
gpt
Yes.  It would be an unsolicited letter which can be helpful depending on the school.  Near in mind it's unlikely to make the difference, but it doesn't hurt.  You should first check with the dean of faculty if they allow this - it would also need to get approval from that faculty member to go in their file so you may want to ask them as well.  

It really depends on the school. The tenure process is often highly regulated by the faculty governance or union (if there is one) and casually it won't just be added to their list.  However, at the very least, a strong letter sent to the department chair would perhaps favorably impact the departmental level of the review. Politics between faculty members may be problematic though, so it's also unclear if the chair happens to be an ally or opponent of that faculty's tenure.