Poetry : Wings of Poesy

Read the following extract carefully and answer the questions given below: [10]
I sift the snow on the mountains below,
And their great pines groan aghast;
And all the night it is my pillow white,
While I sleep in the arms of the blast.
Sublime on the towers of my skiey bowers,
Lightning my pilot sits,
In a cavern under is fettered the thunder,
It struggles and howls at fits.
1. What natural phenomenon is expressed in the lines:

I sift the snow on the mountains below,
And their great pines groan aghast;
2. Where are the lightning and thunder located? What does the lightning do for the cloud?
3. To what is thunder compared? How is it kept imprisoned? How was it referred earlier in the poem? How does it react now?
4. Write a summary of the above extract in your own words.