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What's the difference between grade (percentage) and slope (in degees)?

Ask Dr. Flake
posted: 10/18/2005

Grade and slope are both ways of expressing steepness, really. It's just that highway depart-ments and skiers, generally speaking, have different ways of expressing themselves. Highway engineers use grade, a ratio (which later gets assigned a percentage) that indicates how long you must travel up or down a road for a vertical rise or drop of one foot. A 100 percent grade, therefore, would be 45 degrees-and really dangerous, given the lousy brakes on Flake's '78 Subaru Brat. If one were to drive down Utah's Big Cottonwood Canyon, one would see signs reading 5.5% GRADE, meaning that stretch of highway between Brighton and Salt Lake City is 5.5 percent of vertical. Skiers and avalanche experts, on the other hand, go by simpler geometry-class measurements-expressing steepness as an angle of degree, with 90 degrees being sheer vertical and zero degrees being flat. The takeaway? Skiing is most enjoyable on slopes between 30 and 55 degrees; driving is not.

October 2005

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