Methods of estimating runout distance:
(1) based on the runout (or alpha) angle
(2) based on evidence of past runouts
runout distance along the base level in feet =
= (height of the top of the avalanche break above base level) / SIN(runout angle) * SQRT(1 – ((SIN(runout angle))^2))
where:
• The runout distance is measured from the vertical line drawn down from the point of the avalanche break and not the base of the mountain.
• SIN(runout angle) = (height of the top of avalanche break in feet) / (hypotenuse of right angle triangle for height and runout in feet)
• Hypotenuse of the right angle triangle = SQRT(((height in feet)^2) + ((runout)^2))
• Runout angles in the United States range from 19 to 25 degrees depending on location (Continental climate 19-22 degrees, intermountain 20-23 degrees, maritime 23-25 degrees).
As a basic rule of thumb, a camp should be located a horizontal distance 2.5 times the vertical height at the top of the avalanche break.
Evidence of previous runouts:
(1) abrupt transition in tree age (from young to old)
(2) knocked down trees
(3) trees with branches torn off of the uphill side but still present on the downhill side
