March 19, 2006
Three types of ripples in water ice were observed near Mt. Monadnock, New Hampshire on January 16, 2006. Ripples may be defined as a banded pattern of wavelike disturbances. They usually occur in fluids, but as seen here, they can occur in solids as well.
Leftmost: ripples formed by wind action on a shallow puddle, subsequently frozen solid. Formation of ripples by wind passing over a water surface is a classic example of an emergent phenomenon: small disturbances in the surface self-stabilize into a banded pattern when a current of one fluid (wind) passes over the surface of another (water). What's interesting here is that the ripples persisted during the freezing process. Normally, wind ripples on water collapse and re-form with each gust of wind.
Center: The pattern of horizontal bands seen here in a small iceflow have nothing to do with the shape of the underlying surfaces, nor were they formed by wind. Instead, the necessary current is supplied by meltwater passing over the surface of the ice. The ice itself is not truly fluid, but it may melt away or thicken, depending in part on the amount of water passing over it. As the volume of water moving over any spot depends on the shape of the ice near that spot (just as local wind forces at a water surface depend on the shape of the surface), ripple patterns can emerge.
Right: When liquid water passes over a rocky stream bed, the rocks divert the flow, forming shock waves which exhibit interference, sometimes forming banded patterns. As the stream freezes over (beginning at the shore, out of frame to the left), the larger disturbances are preserved in the shape of the ice.
In all three cases, the ripples have wavelengths on the order of just a few centimeters (an inch or so).Related Links: