Explain the fear experienced by Helen Keller. How did Miss Sullivan pacify her?

One day, Helen and Miss Sullivan went walking in the woods.  They found a nice shade tree, and Miss Sullivan suggested that they eat their midday meal under it.  Helen decided to stay in the tree while Miss Sullivan brought the food for the meal back from the house.
Helen waited on a branch in the coolness of the shade for her teacher to return.  Though Helen could not see or hear the approaching thunderstorm, she did feel it coming:

Suddenly a change passed over the tree.  All the sun's warmth left the air.  I knew the sky was black, because all the heat, which meant light to me, had died out of the atmosphere (The Story of My Life, Chapter V). 

The scent of the approaching storm was something Helen could smell.  As Helen waited, clinging to the tree branch, "a nameless fear clutched at [her] heart."  Helen was fearful because she felt alone in the storm.  She could not see or hear, and therefore could not find her way home by herself.   Helen explained the fear and loneliness she felt while in the tree:

The immense, the unknown, enfolded me.  I remained still and expectant; a chilling terror crept over me.  I longed for my teacher's return; but above all things I wanted to get down from that tree.

The winds from the storms came and violently blew the tree branches around.  Helen continued to cling to her branch.  She was terrified:

A wild impulse to jump seized me, but terror held me fast.... I felt the intermittent jarring that came now and then, as if something heavy had fallen and the shock had traveled up till it reached the limb I sat on.  It worked my suspense up to the highest point, and just as I was thinking the tree and I should fall together, my teacher seized my hand and helped me down.  I clung to her, trembling with joy to feel the earth under my feet once more.

Miss Sullivan calmed Helen's fears by returning to the tree and helping her down.  With her hand in Miss Sullivan's, Helen experienced comfort.  Miss Sullivan let Helen cling to her.