12 by Franklyn Michael 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 storm/hurricane-generated hazard that cause the greatest 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 uprooted, electrical poles toppled and perimetre fences flattened. The direct effects of wind damage can amount to millions of dollars in each event. In 2017 some of my colleagues and friends in the British Virgin Islands (BVI) described scenes like the following: Yachts removed from marinas and deposited in main roads Cars were lifted unto porches Roofs were deposited in neighbouring front yards Washers and dryers were removed from verandas and put into the streets Windows were shattered by the force of the wind alone Cutlery from one apartment was blown into another In 2004 in Grenada there was a famous story about a house that had “crossed the road” in a coastal village during Hurricane Ivan. Hurricane winds can also damage buildings directly because of the force of pressure differentials. The pressure exerted by the wind one side of a building or on the roof might be greater than on another side or part 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 (angle) 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 There are indirect effects of hurricanes that 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 great damage may result. Sadly, there have been cases where people have been killed when trees fell on houses. One of the most dramatic features of a major hurricane is the nature and extent of flying debris. Debris driven by hurricane force winds can do extensive damage. Portions of roofs, tree branches, garbage cans, outdoor furniture and construction debris can become truly lethal missiles on the winds of a major hurricane. One of the ways in which major hurricanes create indirect damage is through the impact of “horizontal rain”. If you have not experienced it, you might find it hard to believe. The force of the winds is enough the send the rain forward parallel with the ground at building level. Most buildings especially houses are designed for normal rain that falls vertically; horizontal rain can cause extensive interior flooding. I attempted to capture the experience poetically in the poem, “Facing the Force” that we published in the 2021 edition. Continues on next page
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