To dispatchers in Lower Manhattan, that is the most obvious sign that all is still not well with PATH, a vital link for New Jersey commuters that still isn't back to full operation nearly three months after superstorm Sandy.

WSJ: PATH's Road Back Is Rocky

The rise and fall of signal strength in each "link" - the path from one mast to another - provides an average measure of how much rain there is between them.

BBC: Rain tracked with mobile network

Then, they reach a parking area and take a quarter-mile-long path toward the mountains that leads to a couple of acres of ground enclosed by an eight-foot chain-link fence.

NPR: Revisiting the Birth of the Bomb