杏吧原创

While walking my dog on the beach in Waiheke Island, New Zealand, I spotted an interesting cloud formation consisting of small stripes (see photo). As I watched, more followed. The stripes gradually got longer and longer, and then slowly disappeared. How did these strange formations occur?

鈥淎irline pilots normally take a detour around billow clouds because they betray the presence of potentially dangerous turbulance鈥�

鈥� Stripes of clouds in repetitive patterns are ripples in the surface of a layer of humid air beneath a layer moving in another direction, much like ripples on calm water beneath a breeze. As air passes through such a region, it warms on the way down and cools as it rises. Usually we cannot see this happening, but if the humidity is such that the cooling air condenses the moisture, we see the cloud as a streak at the crest of the ripple. Air passing through the ripple crest doesn鈥檛 stop, it moves down and warms up, evaporating the cloud and then re-forming it in the next crest.

So those apparently stable streaks of cloud are an illusion. They are simply cool zones through which air ripples. Even as you watch, the droplets in any streak vanish, continuously and invisibly being replaced by incoming water droplets. Their vapour flows on to the next peak where it re-condenses and passes through the next streak in turn. Just as ripples on a pond can change, so the length and interference patterns of cloud ripples may change, sometimes bewilderingly.

Jon Richfield, Somerset West, South Africa

鈥� The photograph shows billow clouds created by wind shear. It is a manifestation of a kind of turbulence known as Kelvin-Helmholtz instability, which is created when different layers of air move at different speeds. Typically the upper layer moves faster than the lower layer. This creates eddy currents or an oscillation at the boundary between the layers, creating a ripple effect.

Airline pilots normally take a detour around billow clouds because they betray the presence of potentially dangerous turbulence.

Mike Follows Willenhall, West Midlands, UK