The past perfect continuous is used to express that an actionA general term expressing that somebody or something does something started before a point of timeA particular moment in time. E.g.: She came home at six. ("six") I was born in 1980. ("1980") in the past and that (a) it was still in progressAn action is in progress when it is happening. E.g.: When I arrived he was surfing. ("he was surfing") or (b) it had just stopped. This point can be defined with a time expressionA word or group of words used in a particular situation. E.g.: Good morning, at six, on the sofa or a clauseA group of related words containing a subject and a verb.. This clause is in the past simple. 

By 2001 George had been living in Scotland for 60 years. (action in progress; a time expression is used)
When I got to her house she had been waiting for hours. (the action had just stopped; a clause is used)
It had been snowing all night.

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It had been snowing all night.

Form: past perfect continuous
Past perfect continuous Past perfect continuous for past habits