The 2018 hurricane season is in full swing, with a total of four named storms currently in the Atlantic Ocean and Hurricane Florence surging ashore at the North and South Caroline border. Fortunately for the residents of these states, the formerly Category 4 hurricane has diminished to a Category 2, but it is still tearing roofs off of buildings, overturning vehicles and even toppling less substantial buildings.
Hurricanes bring with them a huge amount of destructive force, which can be spread over a huge area. In the case of Hurricane Florence, the storm is spreading over an area of 200 to 250 miles wide. But that’s nothing in comparison to Hurricane Sandy in 2012, which topped 1,000 miles wide.
There are two basic dangers that hurricanes bring: wind and flooding. Both can be extremely destructive, as we have little which we can use to battle these forces of nature. By comparison, if we were to drop an atomic bomb, we would find that a hurricane can expend as much energy as somewhere between 10,000 and 500,000 nuclear bombs!
Hurricanes are categorized by the wind speed they produce. As higher wind velocities create more damage, this makes sense, at least to some extent. But not all the damage comes from the wind. These categories are based upon the Saffir-Simpson Scale:
|Category 1||Winds of 74-95 mph||Winds of 119-153 km/h|
|Category 2||Winds of 96-110 mph||Winds of 154-177 km/h|
|Category 3||Winds of 111-129 mph||Winds of 178-208 km/h|
|Category 4||Winds of 130-156 mph||Winds of 209-251 km/h|
|Category 5||Winds over 157 mph||Winds over 252 km/h|
As wind speed increases, the amount of damage caused by the wind increases also. A basic idea of the danger from wind damage, for each category is:
- Category 1 – can topple unanchored mobile homes, uproot weak trees and tear poorly attached shingles off of roofs.
- Category 2 – damage roofs and inflict damage on poorly constructed doors and windows. Sings and piers can receive considerable damage. Mobile homes are typically damaged.
- Category 3 – can cause structural damage to residences and utility buildings, especially those of wood frame manufacture. Buildings without a solid foundation are typically destroyed.
- Category 4 – typically cause structural damage to residences. Gas station canopies are usually totally destroyed. Trees, other than the hardiest, are typically uprooted.
- Category 5 – cause complete roof failure on many homes and industrial buildings. Small utility buildings are often blown away. Heavy, irreparable damage is caused on most wood frame structures.
In addition to the damage caused by wind, there is also considerable damage caused by flooding , as part of the danger of hurricanes. This flooding comes from two sources: storm surge and rainfall. Storm surge is ocean water, which rises due to the wind. This can reach as much as 20 feet, in cases where the storm surge coincides with the normal high tide.
Hurricanes carry an enormous load of water, dumping it, in the form of rain, over the entire area of the storm, but predominantly in the northwest quadrant. How much rain is actually dumped on any particular piece of ground not only depends on the amount of rainfall, but also on the speed at which the storm is moving. Hurricane Harvey, for example, dumped 51 inches of rain on Houston in 2017. Forecasters are saying that Hurricane Florence could dump as much as 40 inches of rain on North Carolina.
The reason why Hurricane Harvey was able to dump so much rain on Houston was that it stalled over Houston, staying in the same place for days. Had the storm continued to move inland, flooding would have been much less, with the rainfall spread out over a much larger area.
Hurricane categories can be deceptive in this, because the category deals only with the wind speed and not how much rain the hurricane is likely to cause or the flooding which will result. Hurricane Harvey’s wind speed had died to the point where it was considered a tropical storm, by the time it hit Houston, yet it still caused $125 billion in damage, making it the second most expensive hurricane to hit the US.