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Lesson 9: Background: Marine Stratus It is common to find stratocumulus clouds in the vicinity of the descending branch of the Hadley Cell that lies over cold ocean regions. This is evident in the accompanying satellite image of the Western Hemisphere. The marine air near the surface is cool and humid. The upward and downward air movements generated by turbulent mixing create this marine air layer. As the air in the upper troposphere descends it warms adiabatically, and its relative humidity lowers. When a large area of the atmosphere sinks, a temperature inversion can develop due to adiabatic compression. The upper region of the layer sinks farther than the lower region and thus warms more. After subsiding, the depth of the atmosphere the top of the layer is warmer than the bottom. This is called a subsidence inversion because it results from descending, or subsiding, air. This particular subsidence inversion is also called the trade wind inversion because it occurs in the region of the tradewinds. The stratus clouds observed in the satellite image lie just below the trade wind inversion. The stratus clouds form when the air approaches saturation due to mixing of the dry subsiding air with the moist air near the ocean surface. Because of the tradewind inversion, which inhibits vertical motion, the clouds cannot develop vertically.
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