Present perfect continuous

There 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:

• '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!'

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. 

When we use the present perfect continuous with a for/since adverbial phrase, we are talking about actions which started in the past and are still ongoing, as in your example, Jana. Further examples would be:

• 'You've been reading that book since Christmas and you still haven't finished it yet!'
• 'How long have you been waiting?' 'I've been standing here for half an hour. These buses never come.'
• 'John's been looking for a job for over a year now, but he still hasn't found one.'
• 'I've been working on this since six o' clock and now it's nearly midnight. I'm tired!

Note that when we use an adverbial phrase with for, we are talking about a period of time up to the present. When we use an adverbial phrase with since, we mention the starting time of the activity. Try not to confuse the two usages. If we wanted to re-write the final example above using a for-phrase, we would have to say:

• 'I've been working on this for nearly six hours.'