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

Ask Dr. Flake

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


, 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