Many dogs overreact to sudden loud noises like a clap of thunder.
Not all canine responses to startling noises are the same. Some dogs express hyper-vigilance when they hear loud noises, while others hide in fear. Outright panic is also not an unusual reaction for some dogs.
It's important to understand the difference between the normal fear a dog expresses and fear that has become pathologic. In the latter situation a dog begins to generalize his fear of, for example, a clap of thunder, to every sudden or loud noise in his environment.
If on a bright, sunny day your dog suddenly displays his fear-of-thunder response to the rattling of pots and pans in the kitchen or the noise of a garbage truck down the block, he's generalizing his fear of the noise of a thunderstorm to every loud or unusual sound he hears.
Noise phobia, of which storm phobias constitute one class, is defined as a sudden and profound, nongraded, extreme response to noise, manifested as intense, active avoidance; escape; or anxiety behaviors associated with the activities of the sympathetic branch of the autonomic nervous system. Behaviors can include catatonia or mania concomitant with decreased sensitivity or responsiveness to pain or social stimuli. Once fully developed, repeated exposure results in an invariant pattern of response.
Some dogs react to noise phobias by freezing and withdrawing, while others respond by crashing through windows or chewing through restraints or enclosures. While the former behavior may seem less extreme, the fact is both reactions indicate profound suffering and damage to nerve cells.