Like the weather, markets can run hot or cold. But as much as you may question the accuracy of your favorite weather forecaster, the track record of market prognosticators has to be worse.

For example, I could reasonably say that I expect the weather in Philadelphia today to be sunny with a high temp of 80.

But it would be lunacy for me to say what I expect Philadelphia stocks to do today. First of all, the daily changes of the stock market have less to do with local geography than they do with industry sectors, economic data and investor whim.

It’s also impossible for anyone to tell you where he or she expects the Dow Jones industrial average to finish today. Yesterday, the Dow fell 161.27 points to 8163.60. That has no bearing on what today will bring.

Even investors who put their money at risk everyday won’t make a short-term call like that. Rather, they’ll stick to a strategy that reflects what they think the future will bring: Oil will hit $100 a barrel. The U.S. economy will grow at a 2 percent annual rate. Wind energy will double as a source of energy in the next five years.

Sometimes they’re right and get dubbed geniuses. Often they miscalculate. Take T. Boone Pickens, the oil man turned renewable-energy champion. He told the Associated Press yesterday that he’s scrapping year-old plans to build the world’s largest wind farm in Texas.

The hurdle, he says, was not that oil had collapsed from $147 a barrel last summer to yesterday’s $62.93. He couldn’t get the power generated by his farm to a distribution system.

With $2 billion sunk into orders for 687 wind turbines, Pickens still believes in wind power, and I suspect many of us believe that the U.S. is now committed to investing in alternative and renewable energy. But, as Pickens learned, there are many things that can go wrong.

You can follow the ups and downs of the publicly traded companies focused on clean energy by tracking the WilderHill Clean Energy Index. It’s made up of 52 stocks, including First Solar Inc. and Itron Inc.

Before you follow Pickens’ lead and go all in on clean tech, here’s a dose of reality: The WilderHill index is down 49 percent versus the 27 percent decline in the Dow.