The average skier you see cruising down the mountain is probably going anywhere between 10 to 20 miles per hour.
Downhill racers clock out at 40-60 miles per hour, and Olympians tend to ski between 75-95 miles per hour, depending on the conditions, their equipment, and their body composition.
Speed skiers, who dress in aerodynamic apparel and ski straight down the mountain sans turning, go as fast as 120-150 miles per hour. The current world record for the fasted speed skier for men is held by Simone Origone, an Italian skier who reached 156.2 miles per hour. The fastest female speed skier in the world is Sanna Tidstrand, a Swedish skier who hit 150.6 miles per hour.
As for cross country skiers, professional racers average 15 miles per hour for continuous distances up 35 miles. Most top ski racers hit about 20-25 miles per hour on the flats, and 35-40 miles per hour on the downhills. Recreational cross country skiers tend to clock out at around 7-10 miles per hour.
Want to track your own speed on the slopes? Check out these iPhone skiing apps, many of which track your speed, miles skied, and average vertical feet.