to 61 km/h) to Force 8 Fresh Gale (39 to 46 mph, 62 to 74 km/h). Trees that fail during Fresh Gale and stronger force wind events often do not exhibit visible defects or conditions that would have warranted preventive risk mitigation. Force 10 is a Storm with wind velocity of 55 to 63 mph (88 to 101 km/h); in many areas these wind speeds are considered extreme. Force 11 Violent Storm (64 to 72 mph, 102 to 116 km/h) and Force 12 Hurricane (≥ 73 mph, 117 km/h) winds are considered abnormally extreme in most geographic areas. At these wind velocities, failure of defect-free trees can be widespread. See Appendix 5 for a more complete discussion of the Beaufort scale as related to tree failure and outages on overhead electric utility systems. Tree-Caused Outages It is important for tree risk assessors to be familiar with the normal weather conditions for the region in which they are assessing trees. Normal weather conditions are those that occur at a frequency where trees have likely adapted to those conditions. Tree failures are most often associated with forces that exceed normal conditions. To help assessors calibrate, weather condition guidance is built into the definitions for the ratings of likelihood of failure. Tree-Caused Outages Predicted Damage 15 25 35 45 55 65 75 Max 2-Min Wind (MPH) Force 4 Force 5 Force 6 Force 7 Force 8 Force 9 Force 10 Force 11 Figure 4. While tree-caused outages do occur at relatively low wind speeds, there is a sharp increase as winds exceed Force 7 (38 mph, 61 km/h). 30