Wave clouds across the western United States
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Wave clouds across the western United States

June 4, 2011
On May 27, 2011 the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite passed over the western United States and captured this stunning true-color image of wave clouds stretching across the region.

Near the left border of the image, a cap of solid white clouds can be seen covering the Sierra Nevada Mountains. Just to the east of this cap cloud, a pattern of parallel rows of clouds interspersed with cloud-free rows begins. This particular formation is called a wave cloud, because the appearance is reminiscent of white caps on an ocean. Similar to the formation of white caps at the peaks of ocean waves, lines of white clouds form at the peaks of rolling air, while the troughs of the waves are cloud-free.

When westerly airflow increases in speed over the crest of a mountain, spectacular clouds can often be seen over the leeward valleys. These clouds are called mountain lee waves, and are a result of the wave pattern created as fast-moving wind passes a crest then plunges downslope, rolls upward, and drops downward again in a series of crests and troughs. The wavelength of this flow - and thus the space between the clouds on the crests - depends on the airflow characteristics, primarily the variation of air temperature with height (lapse rate) and the increase of wind speed with height (wind shear).

The Sierra Nevada Mountains have long been well-known for generation of strong wind phenomenon, such as large amplitude lee waves, rotors, strong updrafts and clear-air turbulence. The gentle upwind slope, and the steep leeside slope make the Sierra Nevada an excellent mountain wave generator. Also, the proximity of the Pacific Ocean to the west serves as a good source for upper-level moisture that gives rise to the clouds atop the wave crests.

Most mountain lee waves in this region are generated from October through May, when conditions are most favorable. The drier air of summer, along with weaker westerly winds, makes wave formation difficult. In season, the number of wave clouds formed can vary from a single small cloud hugging an eastern slope to long expanses of wave clouds stretching far to the east.

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