I find it interesting that "somos" is used instead of "estamos". Surely being a tourist is only a temporary state?
Paulenrique already explained how things work very well, but I feel the need to point out that this quirk in Portuguese can be used for literary effect. For example, there's a very famous story about a famous Brazilian writer (Eduardo Portella) who was once an education minister; he was a left-wing person working for a right-wing dictatorship and, sure of the temporary nature of his position, started his first press conference with “Eu não sou ministro; eu estou ministro”. This quote became famous for its elegance (and, surely enough, he was out of the job a year later).
wow..it's pretty interesting to teach your own language to others because we face things we've never thought about! Yes,your state of being a tourist is temporary, but that's not something that you "get" (Estou com sede / cansado / esgotado). It's something that is on you, so to speak... well, got it? (I hope so :-))
If you take a "role" (real or imaginary) for some time, use "ser". If you are at a hospital, "você É um paciente" (even for short time), if you take a role on a play, "você pode SER o 'mocinho'".
Ah ok, let me see if I have got this right. If we were to play a game of football then "nós somos jogadores"?
That's basically it ;) Also, one can say "nos finais de semanas sou jogador de futebol" (short period)
Thanks Paulo, I'm going to have to write this one down somewhere. It's a valuable lesson!