when to use عند and when to use في in arabic

[oceanic]

Hi

When do you use عند and when do you use في in arabic?

I was under the impression in-da meant at/near, whilst fi meant in/within. But i recently came across two very similar otherwise sentences where one used in-da and the other used fi, and i can't figure out why the difference given the sentences similarity.

The sentences were:-

The weather is sunny today in the mountains (used fi)
&
The weather is sunny today at the beach (used in-da)

In english both make sense, but i was wondering why the different usage of the two words, the sentences are otherwise very similar.

thanks

salam