Noctilucent clouds. Photographer: Andrei Nekrassov (Bloomberg)
byBrian K Sullivan (Bloomberg) In a few weeks, you may get to see evidence the atmosphere is changing — if you’re lucky.This when noctilucent clouds, the world’s highest, peak in number and show up in the night sky just after sunset as electric-blue swirls in the mesosphere, the coldest place on the planet.
Usually visible only in the polar regions, the clouds now sometimes appear as far south as 40 degrees latitude in the Northern Hemisphere, according to Cora Randall, a professor at the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder. That’s because the mesosphere, which nears the edge of space, is changing, possibly “due to a change in climate,” he said. “We believe that these clouds are a really sensitive indicator.”
The clouds first appeared in 1885 after the Krakatoa volcano erupted in Indonesia, according to the National Aeronautics and Space Administration. The initial theory was that they were related to volcanic dust, but they kept showing up.
Now there are more of them, and they aren’t confined to the ends of the globe anymore. Viewers in New York, Rome and Sapporo, Japan, may see them as they drift more than 50 miles (83 kilometers) above the earth.
NASA’s Aeronomy of the Ice in the Mesophere, or AIM, satellite mission has been looking at them from space since 2007. Changes in the clouds reflect “how we affect the atmosphere down here,” Elsayed Talaat, AIM’s program scientist, said by telephone from Washington. “If you increase the methane down below, you are going to increase the water vapor up above.”
Carbon dioxide also may play a role, Randall said. The gas, which warms the lower atmosphere, “can actually cause the upper atmosphere to cool,” she said.
Talaat said it’s too early to identify a single culprit. To make such a link would require removing other possible causes such as the solar cycle. Changing irradiance through the 11-year period affects the upper atmosphere, so Talaat said he would want to see what occurs during an entire cycle.
The AIM satellite itself, or at least the means for getting it into space, also may contribute.
“Rockets can put a lot of water vapor in the atmosphere; they inject it very high in altitude,” Randall said. “It is actually above where the clouds are.”
The noctilucent clouds show up in the late Northern Hemisphere spring. As the sun starts to illuminate the polar region, atmospheric waves push up into the higher altitudes “like waves on a beach,” Talaat said. Scientists say this push helps the mesosphere reach its coldest points in the Northern Hemisphere summer.
This year, the clouds appeared May 19 and will peak just after the solstice on June 21.
Last year, there was a double peak, which “was a little weird,” Talaat said. “We don’t really know why.”
So look up in the sky in the next few weeks. Those electric-blue swirls you see may be a piece of a puzzle that has enthralled scientists for more than a century.
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