Deciphering ski moguls

I never stopped being amazed at how ski moguls form on virtually all ski runs that aren’t groomed. Yet, these amazing geometric formations are not willfully planned or constructed; they just organize spontaneously as a consequence of skiers turning and moving snow. 

It begins as a series of S-shaped turns that cross over small piles of snow left by other skiers. After a while, these mounds of snow reach a critical height that is felt and seen by skiers and force them to turn on the downhill side of the developing bump. 

The obvious paradox is that skiers descending a slope with large bumps practically can’t turn where they would like to. Trying to turn on the uphill or concave flank of a mogul prevents the skis from easily pivoting while turning on the convex side, or top of the mogul, enables them to turn easily as they scrape the snow with their tails from the downhill side of the moguls.

By doing so, skiers push snow down and pile it up on the uphill side of the following mogul. As a consequence, each mogul loses material on its downhill side that gets transferred to its uphill side. The net effect is an optical illusion in which moguls seem to migrate uphill, but the overall mass of snow keeps on moving downhill as it inexorably gets eroded through gravity. 

According to certain folks, bump skiers could be compared to fluids moving over sediments, ripples that form when a river flows over sand, the creation of sand dunes, or even washboard patterns produced by vehicles on dirt roads. The mogul spacing is a function of a skier’s turning radius and speed as well as their gravitational acceleration and the steepness of the slope. 

This is explains why moguls are usually longer at the end of a series of bumps, as skiers tend to move faster as they approach the groomed terrain at the bottom of a ski run, suddenly becoming embolden and needing less control. 

Just like other phenomena of self-organization, the high visibility, ubiquity and regularity of ski moguls, make them a work of art, the result of seemingly random ski turns. 

As for the regular, geometric pattern of ski moguls, once a mogul gets in line, it never gets out of pattern, because skiers have no other choice but ski their natural pattern, which explains why over a short period of time, the few misaligned bumps fall quickly in proper order, just like a marching band that is first totally disorganized, then lines up into rows, before all members find themselves finally marching in perfect harmony. 

Myth: Snow going up the hill (an optical illusion, in reality snow always goes down the hill!)