![]() The air mass itself also expands and cools as it rises due to decreasing atmospheric pressure, a process known as adiabatic cooling. Surface air is warmed by the sun-heated ground surface and rises if sufficient atmospheric moisture is present, water droplets will condense as the air mass encounters cooler air at higher altitudes. Perhaps the most impressive of cloud formations, cumulonimbus (from the Latin for “pile” and “rain cloud”) clouds form due to vigorous convection (rising and overturning) of warm, moist and unstable air. More methane, on the other hand, puts more water vapor into the atmosphere, because sunlight breaks methane up into water molecules at high altitudes. In the mesosphere, carbon dioxide radiates heat into space, causing cooling. These changes may be happening because of increased levels of heat-trapping greenhouse gases such as carbon dioxide and methane in the atmosphere. Studies suggest that night-shining clouds are becoming brighter and more common, which is linked to the mesosphere getting colder and more humid. This happens most often in countries at high northern and southern latitudes (above 50 degrees) in the summer, when the mesosphere is coldest. They only form when the temperature drops below –130 degrees Celsius (-200 degrees Fahrenheit), whereupon the scant amount of water high in the atmosphere freezes into ice clouds. The clouds' high position in the atmosphere allows them to reflect sunlight long after the sun has dropped below the horizon. These rare clouds are technically called "noctilucent" or "polar mesospheric" clouds, and form at high altitudes, 80 to 85 kilometers (50 to 53 miles) high, where the mesosphere is located. ![]() These night-shining clouds were spotted over Billund, Denmark on July 15, 2010.
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