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HIGH-ENERGY particles from deep space that constantly slam into our upper
atmosphere may have a hidden use: predicting the weather. When they strike the
atmosphere, these “cosmic rays” produce showers of other particles that rain
down on the Earth. Measuring how many there are could tell us how air
temperature changes with altitude—crucial information for forecasters.

Meteorologists currently measure temperature profiles by releasing balloons
carrying sensors. “Airports send them off every one to six hours, and each one
costs about £150,” says David Stephenson of the University of Reading. The
sensors aren’t retrieved, so that adds up to £220,000 a year. They’re also
tricky to use in strong winds.

Joseph Mok and Clement Cheng of the Radiation Health Unit in Hong Kong think
they may have a cheaper alternative. They knew that the number of particles
generated by cosmic rays that reach the Earth’s surface varies with the
temperature of the atmosphere. Muons, for example, are formed when pions and
kaons hit the upper atmosphere, where the pressure is about a tenth what it is
at sea level. The altitude at which this pressure occurs increases as the air
gets warmer and expands. On a hot day, the muons are formed relatively high up
and take longer to reach the ground, so more are lost through decay.

Researchers had already noticed the number of muons changes with the
seasons—more reach the ground in winter than in summer. So Mok wondered if
it would be possible to track short-term changes too.

Mok and Cheng set up two detectors at a weather station to count how many
particles reached sea level over a period of 5 hours during the day and 5 hours
at night for 10 months. They found the muon count went up a few per cent during
the night, as expected, since the nights were cooler. But this trend reversed
when the weather was changing. If a rainy low-pressure front was advancing, for
example, the muon count would go up during the day.

That could make a handy way to track and possibly predict changeability in
the weather, says Mok, though he adds they need to account for things like solar
flares, which affect the Earth’s magnetic field and so can deflect cosmic
rays.

Sean Clarke from Britain’s Meteorological Office says the information from
muons might not be detailed enough. Monitoring muons could be cheaper, but
weather balloons give a very good view of the temperature profile of the
atmosphere.

“They present a very intriguing idea,” says Rodney Viereck of the National
Oceanic and Atmospheric Administration in Boulder, Colorado, although he thinks
the research falls short of proving the connection. “There are others who
conclude that the change in the cosmic ray flux is what changes the weather and
not the other way around. It’s very controversial.”