The average annual wind speed measured at the Blue Hill Observatory in Milton has dropped by more than 10 percent over the past 30 years, and no one knows why.

As wind energy projects are picking up, the wind is dying down. That’s the case in the Blue Hills, at least, where the average annual wind speed – the strength of a day’s top wind, averaged out over 365 days – has gotten 10 percent weaker over the past three decades.

Other parts of the United States – particularly on the East Coast and in the Midwest – have also recorded diminished wind speeds in recent years.

“It’s not just happening here,” said Charles Orloff, executive director of the Blue Hill Meteorological Observatory. “It’s happening in other parks around the country, even those not as influenced by development.”

For much of the last century, the annual average wind speed at the observatory was 15 to 16 mph. It began to drop during the 1950s, and last year marked a record low annual average of 12.6 mph.

Decreasing wind speeds could pose a problem, and not just for kite enthusiasts.

Milton is looking into building two turbines on town-owned land near the Granite Links Golf Club. Most wind turbines operate best with winds at 8 to 16 mph.

“If this trend is going to continue, you’ll get to a point where (turbines are) not going to be as cost-effective,” said Orloff.

Why are winds getting weaker? Mike Iacono, who has volunteered at the Blue Hill observatory a day a week for 22 years, said local, regional and global factors all likely contribute to the decreased wind speed.

Specifically, he pointed to reforestation in some cases, increased development in others and altering weather patterns – some of them related to climate change – as potential culprits.

The storm tracks that brought high winds over Blue Hill are starting to shift north, for reasons still unknown, Iacono said. Global warming has helped to warm the polar air, creating less of an interaction with the warm air coming up from the south, Iacono said.

“The temperature contrast is being reduced, and that in itself slows down the wind speed,” he said.

Patriot Ledger staff reporters Brad Kane and Gal Lotan contributed to this report.

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HOW THE OBSERVATORY MEASURES WIND SPEED

The Blue Hill Observatory in Milton uses a contacting anemometer that dates back to the late 1960s.

The three-cup instrument is mounted at the top of the highest wind mast on the observatory roof about 50 feet above ground level.

The height of the anemometer has been the same since 1908.

The anemometer is calibrated so that a nearly constant 640 spins of the instrument equal one mile of wind passing the observatory.

Each time a mile mark is reached, an electrical contact sends a signal to an indoor recorder, which makes a mark on a moving chart.

The marks are counted for each hour to derive the average hourly speed – the number of miles per hour.

Source: Blue Hill Observatory