|Photo of toppled tree behind the White House|
Note the stump and the break about 2' off the ground
Photo from CNN.com, no photographer given
Natural tree blowdowns are well known. In 1999, a severe storm called a "derecho" traveled from North Dakota through Quebec, 1300 miles in 22 hours, damaging homes, knocking down power lines, and blowing down an estimated 25,000,000 trees. A derecho is a straight-line windstorm common in here in the Midwest, and they can pack winds blowing up to 120 miles per hour.
So, is it difficult to blow down a tree? The branches and canopies cause the drag on trees to be high, although trees change shape under wind conditions to minimize drag. Drag coefficients depend on a number of parameters, including the wind velocity itself. They are highest for fir and spruce, and lowest for Western hemlock and Lodgepole pine. Douglas fir and Scotts pine are intermediate. Typical velocities that knock over trees are about 100 feet/second, or 68 mph.
Drag on a tree is proportional to both the wind velocity and the density of the wind itself (clear air). At Mount St. Helens, the "wind" was full of rocks and ash (best described as an ash hurricane), so the density was higher perhaps by one to two orders of magnitude. The velocity required to blow down the trees there was accordingly less, perhaps by one or two orders of magnitude as well.