Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentHigh-wave events along the west-side cliffs are generally produced by monsoon surges up to thepoint of the 50-year recurrence event, where typhoons again become the dominant high-wavegenerators for larger-magnitude events.Table 5-12Average Recurrence Interval for High Surf on GuamAverage ReturnPeriod (Years)Wave Heights (feet)on East-SideBays/ReefsWave Heights (feet)on West-SideBays/ReefsWave Heights (feet)on East-Side CliffsWave Heights (feet)on West-Side Cliffs1.0 10 1/30/>18 50 27/17Note: Calculated recurrence intervals included high surf produced by trade winds (waves affecting east-side cliffs) and monsooninducedwaves (waves affecting the western side of the island).Source: WERI 1999.5.3.8 LightningNatureLightning typically occurs as a by-product of a thunderstorm. The rising and descending of air ina thunderstorm separates positive and negative charges, with lightning the result of the buildupand discharge of energy between the areas of positive and negative charge. Water and iceparticles can also affect the distribution of the electrical charge. In only a few millionths of asecond, the air near a lightning strike is heated to 50,000°F, a temperature hotter than the surfaceof the sun. Thunder is the result of the very rapid heating and expansion of the air near thelightning, which causes a shock wave.The hazard posed by lightning is often underrated. High winds, rainfall, and a darkening cloudcover are the warning signs of possible cloud-to-ground lightning strikes. Although manylightning casualties happen at the beginning of an approaching storm, more than half of lightningdeaths occur after a thunderstorm has passed. The lightning threat diminishes after the last soundof thunder, but may persist for more than 30 minutes. When thunderstorms are in an area but notoverhead, a lightning threat can exist even when skies are clear. Lightning has been known tostrike in an area with clear sky more than 10 miles from a storm.5-34