Legolas sees many miles away the armies of Isengard

There were no clouds overhead yet, but a heaviness was in the air; it was hot for the season of the year. The rising sun was hazy, and behind it, following it slowly up the sky, there was a growing darkness, as of a great storm moving out of the East. And away in the North-west there seemed to be another darkness brooding about the feet of the Misty Mountains, a shadow that crept down slowly from the Wizard’s Vale.

Gandalf dropped back to where Legolas rode beside Éomer. ‘You have the keen eyes of your fair kindred, Legolas,’ he said; ‘and they can tell a sparrow from a finch a league off. Tell me, can you see anything away yonder towards Isengard?’

‘Many miles lie between,’ said Legolas, gazing thither and shading his eyes with his long hand. ‘I can see a darkness. There are shapes moving in it, great shapes far away upon the bank of the river; but what they are I cannot tell. It is not mist or cloud that defeats my eyes: there is a veiling shadow that some power lays upon the land, and it marches slowly down stream. It is as if the twilight under endless trees were flowing downwards from the hills.’

‘And behind us comes a very storm of Mordor,’ said Gandalf. ‘It will be a black night.’

The Two Towers, Book Three, Chapter 7: Helm's Deep.