Question: Cold and dark, I'm the same as my neighbor.
Flatter than a pancake, faster than a panther.
Lighter than a feather I still prevail,
Yet a million mean trying to lift me would fail.
Expanding and shrinking throughout the day,
When heat is highest, underneath I lay.
I leave in the darkness of the night,
And don't come back if there's rain in sight.
What am I?