Billow Clouds Seen from Above and Below

May 05, 2016

K-H_Billow IMG_2449 (1)

Billow Clouds2P1100806
Photographer: Patti Weeks
Summary Author: Patti Weeks
The cloud formation above (at top) is a classic example of Kelvin-Helmholtz instability, named after 19th century scientists Lord Kelvin and Hermann von Helmholtz. These clouds are known as billow clouds. Wind shear is a key ingredient to their formation -- unstable atmospheric layers of varying densities are moving at different velocities. Here, the thinner top layer is moving faster than the denser layer below, causing an upward and backward curling action at the top of the cloud. This photo was taken over Greenville, North Carolina, at dusk on April 12, 2016.
The bottom photo taken from a commercial airplane over Oklahoma shows a widespread area of billow clouds taken during the late afternoon on January 31, 2016. When sufficient moisture is present billow clouds can form in distinct parallel rows. They 're a warning to pilots of potentially dangerous turbulence.

Photo Details: Top - Camera Model: iPhone 5; Focal Length: 4.12mm (35mm equivalent: 40mm); Digital Zoom: 1.200x; Aperture: ƒ/2.4; Exposure Time: 0.017 s (1/60); ISO equiv: 64. Bottom - Camera Maker: Panasonic; Camera Model: DMC-ZS7; Focal Length: 4.1mm (35mm equivalent: 25mm); Aperture: ƒ/5.6; Exposure Time: 0.0020 s (1/500); ISO equiv: 80.