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University of Minnesota

Weather underground

March 24, 2009

Turbulent clouds.

Turbulent "cloud streets" stretching hundreds of miles near Iceland. Cosmic rays contain clues to weather in Arctic regions, U physicists found.
Photo: NASA

Predicting weather 10-15 miles up from half a mile underground

By Deane Morrison

One just doesn't expect cosmic rays streaming in from outer space to predict weather disturbances 10 to 15 miles above our heads. But they did, even though the gizmo detecting those rays lies under half a mile of rock.

This odd result emerged from work by a large international team of researchers, including several University of Minnesota physicists, studying cosmic rays hitting an underground detector in the University-operated Soudan Underground Laboratory, located half a mile deep in an old iron mine in northern Minnesota.

Hit parade

The frequency of cosmic ray "hits" correlated closely with a rare and sudden warming of the stratosphere called, appropriately, a sudden stratospheric warming (SSW). An SSW can affect both the severity of winters in northern regions and levels of ozone over the poles. Being able to detect and study these events will help weather forecasters and climate modelers improve their predictions.

"The advantage of this technique is, we have [cosmic ray] records from various experiments in the world that go back several decades," says team member Alec Habig, a professor of physics at the University of Minnesota-Duluth. "Now we can go back in the historical records and see [how cosmic ray data correlates with stratospheric weather events over long periods of time]. People trying to figure out how weather in the upper atmosphere works will have another tool to build models of it."

In the Arctic and Antarctic, winds whip around the poles in a circular pattern called a polar vortex. But if the heart of the vortex can be likened to the eye of a hurricane, sometimes, says Habig, that eye wobbles off center and causes various weather events.

"It's as if the vortex is taking an excursion," he says. From wobbling of the vortex to SSWs to weather on the ground, everything is connected "but it's not a simple correlation."

“It's fun sitting half a mile underground doing particle physics. It's even better to know that from down there, we can also monitor a part of the atmosphere that is otherwise quite tricky to measure."

How can weather anywhere have anything to do with cosmic rays?

These extremely fast, energetic particles are generated far from the solar system, and some reach the end of their journey by colliding with air molecules in Earth's atmosphere. Among the debris from such collisions are unstable particles called mesons. If mesons can keep from hitting other air molecules long enough, they will decay to form even tinier particles called muons.

Muons also travel fast, and many keep going right down to the Earth and deep into solid ground. About 40,000 a day hit the Soudan detector.

For many years, the rate of muon hits has been observed to rise slightly when the upper atmosphere has warmed. That's because when air warms, it expands, spreading out the air molecules. This improves mesons' chances of avoiding collisions long enough to decay into muons. Therefore, more muons are produced and more penetrate to the detector.

The rain of muons usually counts as background noise in the detector, which was set up to catch even teensier particles called neutrinos. But in February 2005, the cacophony generated a dividend that surprised even the seasoned researchers when a small but clear rise in the rate of the muon "hits" coincided with an SSW.

SSWs occur on average about every other year, but they are dramatic: In just a few days the temperature can shoot up as much as 75 Fahrenheit degrees and stay there for a couple of weeks. And they have been devilishly unpredictable. But maybe now that will change.

“It's fun sitting half a mile underground doing particle physics," says team member Giles Barr of the University of Oxford. "It's even better to know that from down there, we can also monitor a part of the atmosphere that is otherwise quite tricky to measure."

The study was led by scientists from the UK's National Centre for Atmospheric Science and the Science and Technology Facilities Council. The detector in the Soudan lab is part of the MINOS experiment, a project of Fermi National Accelerator Laboratory in Batavia, Ill.

Related Links

Physics at UMD

School of Physics and Astronomy