From Trader's Narrative:

On Wall Street copper is given the title of “Dr. Copper” because of its ability to predict future economic activity. When the price of copper falls relative to its prior annual price, we usually head into a recession. But we can’t look at copper priced in dollars - they are useless after all, aren’t they? - instead let’s look at copper in terms of ‘real money’. Gold.

It turns out that copper priced relative to gold is actually very correlated to the equity markets. And not only correlated, it tends to predict a few months ahead of time where the stock market is headed...

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More on copper and gold:

Barclays: It's time to short gold

Top strategist Saut: "Dr. Copper" is signaling a correction

Veteran copper trader: "We're going to see a catastrophe"