Circular cloud streets in Mozambique Channel

Circular cloud streets in Mozambique Channel
  • Credit:

    Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC

This true-color Moderate Resolution Imaging Spectroradiometer (MODIS) image from August 16, 2002, shows various cloud streets in the Mozambique Channel between Madagascar (east) and southeast Africa. Cloud streets typically form along the path of low-level winds when they blow over open water, which is typically warmer than the wind itself.

Warm air from near the ocean surface rises (convects) and is then swept along by the wind. The air is thus subject to two competing influences: it’s rising up, but it is also being pushed along by the wind. These two influences cause the rising air to roll and spin, producing a row of horizontal vortices (imagine a tornado on its side) all lined up in the direction of the wind.

The vortices do not all spin the same way; in fact, adjacent vortices spin in opposite directions. To picture this, point the index finger of each of your hands toward the computer. Now trace a clockwise circle in the air with your right hand, and a counter-clockwise circle with your left hand at the same time. Where your fingers come closest together, both of them are moving upward. If your fingers were vortices, the air in each vortex would be rising right next to each other, which is where the clouds form. Where your fingers are farthest apart is where air would be spinning down toward the Earth, and no clouds form there.

This alternating pattern produces the lines of clouds (streets) like those seen in this image. In this case, some of the streets appear to be making a series of concentric circles, which reveals the wind direction.

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Metadata

  • Data Date:

    August 16, 2002
  • Visualization Date:

    September 15, 2002
  • Sensor(s):

    Terra - MODIS

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