From Virgina Woolf's To the Lighthouse:
It was fringed with joy. The wheelbarrow, the lawnmower, the sound of poplar trees, leaves whitening before rain, rooks cawing brooms knocking, dresses rustling -
How do leaves "whiten before rain"? Is this something common to all plants?
Additionally, is a "rook" a type of raven, or what is a rook?
Per the Farmer's Almanac, the leaves of some deciduous trees do indeed curl upwards before storms, reacting to atmospheric changes.
But does seeing the undersides of leaves really mean rain is on the way? In this case, our forebears were definitely onto something. The leaves of deciduous trees, like maples and poplars, do often to turn upward before heavy rain. The leaves are actually reacting to the sudden increase in humidity that usually precedes a storm. Leaves with soft stems can become limp in response to abrupt changes in humidity, allowing the wind to flip them over.
From below they would appear to whiten before the storm or as the old adage puts it,
“When leaves show their undersides, be very sure rain betides.”
As to the latter point, this is a rook (in this case an American Rook). No special symbolism required.
