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Lightning may precede heavy bands of snow.
A flash of lightning during a snowstorm may be nature's way of signalling that the worst is yet to come.

The combination of snow, lightning and muted thunder occurs occasionally in storms across the temperate regions of North America, Europe and Asia, often near lakes or cyclones. Eyewitness accounts of these rare events, known as "thundersnows", date back thousands of years to ancient China, and suggest that lightning tends to strike in parts of the storm where the most snow is falling.

To verify such accounts, Patrick Market and Amy Becker at the University of Missouri, Columbia, studied 1000 lightning strikes in 24 thundersnows. They had previously shown that bigger snowstorms tend to produce more lightning, reinforcing the expectation that lightning would coincide with the heaviest bands of snow.

In fact they found that lightning tended to strike about 15 kilometres downwind of the heaviest bands of snow, so the heavy snow did not arrive until later (Geophysical Research Letters, DOI: 10.1029/2008GL036317).

The finding may give meteorologists a way to predict snowstorm behaviour in near real time. Joe Schaefer of the National Oceanic and Atmospheric Administration in Norman, Oklahoma, says this is difficult with existing techniques, as "unlike rain, snow doesn't show up well on our current radar systems".