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Very big, very strong. Typhoon Haiyan as it approached the Philippines spanned more than 600 kilometers and had winds of just over 300 kilometers per hour.

Super Typhoon Haiyan: Full of Sound and Fury and Signifying … ?

Typhoon Haiyan was the most powerful tropical cyclone ever to hit land and perhaps the most powerful in recorded history. But what lessons can scientists draw from an awesome storm with winds of just over 300 kilometers an hour? Not as much as they might wish.

Why was Typhoon Haiyan so strong?

“I wish I knew,” says meteorologist Jeffrey Masters of Weather Underground Inc. and its website. “We don’t have very good observations” that would illuminate the inner workings of Haiyan, Masters says, because instrumented aircraft do not routinely fly into western Pacific cyclones. In satellite observations, conditions did not seem exceptionally favorable for intensification, he says, but then, meteorologists don’t really understand how and why tropical cyclones grow stronger.

Is Haiyan part of a trend toward stronger tropical cyclones?

Not obviously. Haiyan is the fifth Category 5 storm (the top category) on Earth so far in 2013, Masters says. The global average since 2000 has been 4.4 Category 5s per year, and the record was set in 1997 with 12 of them. September’s massive assessment from the Intergovernmental Panel on Climate Change (IPCC) found only “low confidence” that intense tropical cyclone activity had increased measurably since 1950.

But might tropical cyclones become more powerful under global warming?

The IPCC found that, “more likely than not,” global warming will drive an increase in intense tropical cyclone activity in the western North Pacific and North Atlantic by late in this century.

If Haiyan was so powerful, why was the tropical cyclone season in the Atlantic a near no-show?

“We did anticipate this season to be more active than normal,” says meteorologist Christopher Landsea of the National Hurricane Center (NHC) in Miami, Florida, “but it did not turn out that way. The season is one of the quietest in 60 years.” NHC forecasters had factored in conditions such as warmer waters in the tropical Atlantic and the absence of an El Niño, but, as Masters notes, “not everything crucial to a forecast can be forecasted months ahead.”