In Chapter 17, Gandalf arrives with news that Bolg, a new goblin leader, is leading the goblins and wargs into battle with the dwarves, men, and elves. As Bolg advances, Tolkien describes the atmosphere using imagery and alliteration:
Still more suddenly a darkness came on with dreadful swiftness! A black cloud hurried over the sky. Winter thunder on a wild wind rolled roaring up and rumbled in the Mountain, and lightning lit its peak.
The thunderstorm's darkness is soon amplified by a cloud of bats that flies in to fight on behalf of Bolg. Tolkien does not simply tell the reader that there is a storm, but moreover emphasizes the sensory experience of standing in a storm as it begins. The sky turns dark so fast it is "dreadful," like an omen of terrible things to come. Darkness typically represents danger in this novel, whereas light represents reprieve. The sudden deprivation of light in the middle of the day suggests that there is no longer anywhere for the protagonists (especially Bilbo) to hide from the dangerous enemies they have accumulated along their way to the Lonely Mountain. Tolkien emphasizes the unusually cold wind for a thunderstorm, as well as the deep, "roaring" sound of the thunder that closes in on the mountain. The sound is not dissimilar to Smaug's "roar." The dragon may be defeated, but the sound signals that the adventurers have yet to face the greatest danger of all. Tolkien also describes a flash of lightning, which lights up the sky but also raises the hair on the back of the reader's neck. This is not the kind of light that saves anyone from the dark, but rather the kind of light that might set the whole place on fire. Again, there is the sense that Smaug's presence still lingers over this place.
Tolkien uses alliteration to make all this imagery even more effective. The "darkness" is "dreadful," "sudden," and "swift." The "wild wind" brings "winter thunder" that "roll[s] roaring up and rumble[s]." The "lightning [lights]" up the top of the mountain. Some of this alliteration is onomatopoetic and transports the reader into the scene. The repeated "r" sounds echo the drawling sound of thunder even as the words describe that sound. Spoken aloud, the repeated "w" sounds require the reader to recreate the sound of wind by forcing air through their mouth. Even the alliteration that is not onomatopoetic makes for a sense of breathlessness that helps build tension and signal that the the coming battle will be monumental to the plot and to Bilbo's character development.