Cape Verde's vortex streets

Cabo Verde is a ring of volcanic islands off West Africa, in the north-east trade wind.

What the satellite saw

A long chain of staggered cloud eddies west of the Cape Verde islands
Cape Verde, 20 December 2020: a long staggered street off the tallest islands, in the north-east trade wind. NASA Earth Observatory image by Lauren Dauphin, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview ("The Stability of von Kármán's Vortices"). Public domain.
The wake behind Cape Verde as Wind Tunnel draws it, the NASA photo of the real one laid over the lower right corner
Cape Verde in Wind Tunnel with Satellite photo on: the simulation, made in your browser, with NASA's photo in the corner.

NASA's Terra satellite photographed them with MODIS on 20 December 2020 (Earth Observatory, "The Stability of von Kármán's Vortices"). A long street stretches west of the tallest islands, its eddies offset, never side by side.

That offset is the point of NASA's title: in 1912 Theodore von Kármán showed that only the staggered arrangement of two rows is stable. Rows that line up break apart.

Why Cape Verde

Height. Pico do Fogo rises to 2,829 m; Fogo, Santo Antão and São Nicolau are the tallest islands, tall enough, NASA writes, to make rain shadows.

The trade wind, steady from the north-east in winter.

Flat neighbours. The lower islands make little or no street, which shows how much height matters.

Make it in Wind Tunnel

  1. Open the link below, or press Cape Verde under Island: the wind starts from 40°, north-east.
  2. Press Cloud under Look and Satellite under Colour: white cloud on a blue-grey sea, as the photo sees it.
  3. Turn on Satellite photo under Island to lay this photo over the corner of the frame, beside the flow.
  4. Press Vorticity under Look: the two rows show in two colours, one for each spin, and you can see they never line up.

Open Cape Verde in Wind Tunnel