What Causes Gaps in the Clouds?

A satellite captured an image of low stratus clouds framing iceberg A-56 as it drifted in the South Atlantic Ocean on June 1, 2016. NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response. Caption by Kathryn Hansen

A satellite captured an image of low stratus clouds framing iceberg A-56 as it drifted in the South Atlantic Ocean on June 1, 2016. NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response. Caption by Kathryn Hansen

The growth of cloud layer is influenced, in part, by the direction of air flow.  Cloud growth is favored by ascending air.  Clouds are formed when warm, moist air rises into higher altitudes.  Known as the lapse rate, rising air drops in temperature as the altitude of the air mass increases.  Cooler air can hold less water than warmer air. Therefore, the cooler air of the higher altitudes condenses and expands the water in the rising air, forming clouds.

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Adiabatic Warming

The inverse situation, falling air, causes a gap in cloud formation.  As air descends, it increases in temperature, a process known as adiabatic warming or heating.  This phenomenon is seen with katabatic winds, such as the Santa Anas, which descend from higher mountainous elevations, bringing warmer, drier air.

That same effect of descending air being warmer and drier is what created gaps in cloud coverage.  In the satellite image above (captured by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite), the phenomenon is visible in the low stratus clouds.  Steve Palm, a research meteorologist at NASA’s Goddard Space Flight Center, points towards the iceberg visible in the gap as a likely cause due to the effect that the feature has on the air flow around it.  Air will rise and fall around large geographic features such as islands and icebergs.   The iceberg, named A-56 is 26 kilometers long and 13 kilometers wide (16 by 8 miles) and is about five times the size of Manhattan.

“When an obstacle is big enough, it can divert the low level flow around it,” Palm said. “This causes divergence of the flow at low levels and a corresponding convergence and sinking motion above and downstream of the obstacle. The sinking motion warms and dries out the air causing a hole in the clouds. It is a common phenomenon often caused by islands.”

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