are different aspects of past continuous usage. In the example you
quote, 'It has been snowing', where there is no adverbial phrase,
the results or effects of the activity are still evident. It may
have stopped snowing for the time being, but the snow is almost
certainly still on the ground for all to see. Look at the following
examples of this usage. The final three examples are taken from
the Goldilocks and the Three Bears children's story when the bears
return home to find that their house has been disturbed:
Use of the present perfect continuous in the Goldilocks story increases
the suspense and makes us think that at any moment we shall discover
where Goldilocks is.
'Gosh! You're out of breath. Have you been running?'
'You've been eating garlic, too. I can smell it on your breath.'
'Somebody's been eating my porridge.' (There's very little left.)
'Somebody's been sitting in my chair.' (The chair has been damaged.)
'Someone's been sleeping in my bed. And she's still there!'