What causes trees to fail in high velocity winds?

There are several reasons trees fall or shed parts in severe weather. Decay within the trunk and branches is one defect that can cause failure due to a lack of supporting wood. Hollow trees can remain intact, but need a good solid outer wall of wood for support. Cracks in wood are also a main reason for failure. If wood is cracked through on both sides of the tree strength is severely compromised. Cracks are easy to detect and sometimes can be corrected by proper cabling or bracing techniques. Root problems are another cause of wind throw. This problem is even more complicated because roots are under the ground and not visible. Roots could have been cut for a sidewalk or sewer, removing the anchor roots for the tree. Roots also can decay and lose strength. Often times the tree seems to be healthy and shows no outward sign of health problems. Occasionally soil volume is too small or too saturated to support the tree and entire root systems will uproot. Winds in excess of 50-60 MPH will topple even healthy trees, making prevention virtually impossible. Have trees checked periodically by professionals especially after storm events. A consulting fee is a small price to pay to prevent tree failures.