Japanese expression indicating mild surprise, corresponding to "really?" or "oh yeah?" or "uh-huh" in English. It is used on the receipt of new information by the listener.
This expression is different from so desu ne, which is used when the listener is simply agreeing with the speaker or going along with the conversation.
Kyoo wa issho ni moo benkyoo shimashoo ka.
-- Chotto, watashi no ane ga Nihon kara koko e kitte imasu.
Soo desu ka. Ii desu ne.
-- Ee, so desu ne. San-nen-kan desu kedo...
Soo desu ka.
Shall we study together again today?
-- Sorry, but my sister is coming from Japan today.
Oh really? That's good, eh?
Yep. It's been three years, so...
A mistake beginning students of Japanese frequently make is to say so desu ne when you mean so desu ka. If the other person is telling you something s/he expects you not to know yet, this will confuse them, as so desu ne has more of a "so it is" meaning, whereas so desu ka means you have heard something new. The actual degree of surprise is expressed by the tone of voice, and can range from a ho-hum "izzat so" to a heartfelt "OH MY GOODNESS CAN THIS POSSIBLY BE TRUE??". The same holds for the reverse:
O-tenki wa ii desu ne.
-- Soo desu ka.
Nice weather today, eh?
-- Oh really? (this is very surprising, I was pretty sure the weather today really sucked. Are you sure it's good weather?)
As a sidenote, I guess that could be acceptable if the 2nd person had been locked up for the entire day and literally had no idea what outside conditions were like. Better yet, just forget about it.