I want to know if there is any difference of meaning between the following two sentences:
1- It is hot here/there.
2- It is hot in here/there.
Both expressions are used to mean that it's hot where you are, with the following nuances:
(1) here is used for the climate of a city or country:
This is Maine. It's too hot here to grow oranges!
(2) In here is used for any building or enclosure that you're in:
I'm sorry it's so hot in here - I'm baking a turkey.
(3) in here is used to contrast the interior from the exterior:
It's freezing out today, but it's nice and hot in here.
(4) It's (a little) hot in here can be a subtle (and convenient) hint when you want someone to turn up the air-conditioning without telling them to do so outright. I live in Florida, where this is very useful, but this may be foreign to most. Similiary, when visiting a friend, he might ask you (the guest) "Is it hot in here?" meaning Would you like me to turn up the ac?
NOTE that (1) and (2) can both be expressed with simply "here" - so even if you're in an enclosure, you can say "it's hot here."
"It's hot in here" means roughly "It's hot in this building" or "It's hot in this room" (or "It's hot in this tent" or "It's hot in this cave" . . . you get the idea), whereas "It's hot here" more likely means something like "It's hot in this part of the country."