On October 7, 2013, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite passed over Peru and captured a stunning true-color image of beautiful cloud formations, over both the Pacific Ocean and the Andes Mountains.
A broad bank of marine stratocumulus clouds covers the Pacific Ocean off the western coast of Peru. Such extensive and persistent stratus cloud banks are a prominent feature off the Peruvian coastline, and occur most frequently in October. These clouds form in the stable air which overlies ocean water which has cool surface temperatures, usually caused from upwelling of cold water from deeper waters.
Inland, over the green vegetation found in the Andes Mountains, sheets of popcorn clouds speckle the sky. Such clouds are formed when plants release water vapor, which rises and condenses into clouds as it cools at higher altitudes.
While these formations are beautiful, the most striking formation seen in this image is a long ribbon of cloud that stretches from near Chincha Alta, Peru southward to end near the Bolivian border. At the northwest end of the formation the cloud appears tightly rolled and sharply demarcated, and casts a dark shadow on the tan, arid land below. The ribbon progressively expands towards the southeast until, west of Lake Titicaca, only windblown puffs remain.
This unusual cloud formation has the characteristics of a roll cloud, which is a horizontal, tube shaped type of cloud usually formed by outflows of cold air from sea breezes, or sometimes from cold air pushed ahead of a cold front. In this case, it is likely that the chilled, moist air over the cold sea surface has been blown inland by a westerly breeze. The steep rise of the mountains has lifted the air in a solitary, stable wave and, when the air cools, a cloud forms on the crest of the wave. Stable, self-reinforcing, solitary atmospheric waves upon which roll clouds form are relatively uncommon, so while not extremely rare, such strikingly beautiful clouds are unusual.