What is strange with the thunderclouds?

A description of the clouds is difficult without know which section of the text your question pertains to. Below you will find three quotes I was able to find in the book.

From the text:

Overhead, a huge storm was brewing, with clouds blacker than I’d ever seen over the city. I figured maybe it was global warming or something, because the weather all across New York state had been weird since Christmas. We’d had massive snow storms, flooding, wildfires from lightning strikes. I wouldn’t have been surprised if this was a hurricane blowing in.

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And there it was again-distant thunder on a cloudless day.

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Thunder rumbled across the valley. The storm clouds had now reached the edge of the beach. As far as I could see, the sky and the sea were boiling together.