T he principal difference in the categorisation of a Hurricane over a Tropical Storm is the sustained wind speed. A storm has sustained wind speeds that are between 39 and 73 miles per hour (mph). Hurricanes have wind speeds that may range from 74 miles per hour to more than 160 mph. Both systems can have wind gusts that are higher than the sustained winds. In general terms, winds are not the hurricane-generated hazard that cause the highest numbers of fatalities. However, the winds do cause fatalities and extensive damage. It is useful to think of the effects of hurricane winds in two distinct but related categories, direct and indirect effects. Let us consider the effects of hurricane winds. DIRECT EFFECTS OF WIND DAMAGE Hurricane winds can do extensive damage by the sheer physical force of the winds. This happens when trees are unrooted, electrical poles toppled and perimeter fences flattened. The direct effects of wind damage can amount to millions of dollars in each event. In 2017 colleagues and friends in the British Virgin Islands (BVI) described scenes like the following: Yachts removed from marinas and deposited in main roads Cars lifted unto porches Roofs deposited in neighbouring front yards Washers and dryers removed from verandas and put into the streets Windows shattered by the force of the wind alone Cutlery from one apartment being blown into another In 2004 in Grenada there was a famous story about a house that had “crossed the road” in a coastal village due to Hurricane Ivan. Hurricane winds can also damage buildings directly because of the force of pressure differentials. The pressure exerted by the wind on one side of a building or on the roof might be greater that on another side of the building. In a major hurricane that difference can be so great that in a manner of speaking the building virtually explodes. Buildings that have roof coverings made of materials other than concrete or have a very low pitch are particularly susceptible to this form of damage. The changes in force and pressure can be particularly marked if the eye of the storm passes over or near a particular location. In Hurricane Hugo in Montserrat in 1989, I can recall that my ears “popped” near the time of the passage of the eye of the storm because of pressure differentials. INDIRECT EFFECTS OF WIND DAMAGE The indirect effects of hurricanes can be thought of as a both a companion to and largely a consequence of the direct effects. The force of the wind can topple huge trees. When those trees fall if they fall on a house or vehicle that can do great damage. Sadly, there have been cases where people have been killed when trees fall on houses. Continues on next page 13
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