Lesson 9: Background: Marine Stratus

High-pressure systems are supposed to be cloudless. So why aren’t the world’s subtropical highs always sunny? 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 satellite image of the Western Hemisphere at right. The marine air near the surface is cool and humid. As the air in the upper troposphere descends, it warms adiabatically. When a deep layer of the atmosphere sinks, a temperature inversion can develop as a result of adiabatic compression. This is illustrated in the chart above. The upper region of the layer descends over a greater distance than the lower region and thus warms more. After subsiding, the top of the layer is therefore warmer than the bottom. This is called a subsidence inversion because it results from descending, or subsiding, air. In meteorology, subsidence denotes sinking motions. This particular subsidence inversion is also called the trade-wind inversion because it occurs in the region of the trade winds. The temperature increases sharply with altitude in the tradewind inversion. Stratus clouds form when the air approaches saturation as a result of mixing of the dry subsiding air with the moist air near the ocean surface. The marine stratocumulus clouds observed in the satellite image lie just below the trade-wind inversion where there is enough instability for the stratus to become partly convective, forming stratocumulus clouds. Because of the trade-wind inversion, which inhibits vertical motion, these clouds cannot grow taller.

Return to Lesson 9

Return to Satellite Meteorology Main Page