Isabel’s winds really weren’t that impressive. By the time she got here, she surely wasn’t a hurricane, which is defined as a tropical cyclone with sustained winds (not mere gusts) of 74 mph or higher. Her maximum sustained winds at onshore stations in tidewater Virginia averaged in the mid‐50s (with the one exception of Norfolk Naval Station, where they registered 69 mph). At Reagan National Airport, they topped out at 45. In Richmond, where the percentage of residents without power was almost twice that in Northern Virginia, sustained winds never even made the minimum criterion for a tropical storm — just 39 mph.
No, in plunging us into 18th‐century‐style dusk‐to‐dawn darkness and a morass of melting comestibles, Isabel had an agent — the forest of urban and suburban trees we worship so much that we keep them around long after they should have been put to sleep. They’re old, they’re fragile, and many of them are so big that, when blown over by tropical‐force storm winds, they’re likely to find a power line that was once thought to be safely distant.
It’s not as though we haven’t had prodromal indications of what could happen if a storm like Isabel came along. Heavy (but normal) thunderstorms and moderate ice storms — with less than a half‐inch of accumulation — have been turning off the lights for tens (and sometimes hundreds) of thousands in the mid‐Atlantic over the past decade or two. Why? Because these storms are tearing up — and, in some cases, uprooting — trees at an alarming rate.
A trip down to the local county historical society graphically illustrates the changes in regional vegetation over the past century. In 1900, photos of rural Virginia show most trees about one‐third the size of today’s. This was the period when the land was recovering from intensive logging, and nascent arborist movements began to exert political influence. That influence has grown today to dominate domestic silviculture and, above all, suburban culture. We are preternaturally crazy about our trees. Yet left untended and allowed to remain standing far longer than they probably should have been, many of the same trees from those old photos have become tattered wrecks, with diseased branches vulnerable to every passing ice storm and decaying hurricane.
The postwar explosion of suburban Washington created another cohort of what are now aging trees. A 50‐year‐old free‐standing black locust, a popular suburban variety because of its rapid growth, is likely to be hollowed by disease, making it an easy target for a mere thunderstorm or tropical storm.
The bigger they are, the easier they fall. The force that a tree intercepts is a function of the square of its area. Double the diameter of a tree’s crown and it intercepts four times the wind force. Allow the diameter to quadruple out of misplaced love as in, say, Takoma Park (where it’s illegal to chop down a tree without an almost‐impossible‐to‐get permit) and the tree intercepts 16 times more force from the same wind. And tall trees also stick up more in the wind, like skyscrapers.
These immutable physics have inescapable consequences. We have such a thorough history of hurricanes and their surrounding weather that we can do a pretty good analysis of whether it was Isabel herself or the assist she got from the vulnerability of our vegetation that caused our extended powerlessness.
The modern wind speed champion for our area is 1954’s Hurricane Hazel, which hit the North Carolina‐South Carolina border as a Category 4 (out of 5) storm. Thanks to a kick from the jet stream, Hazel took a mere four hours to cross the Virginia border. Sustained winds topped out at 78 mph at both National Airport and Norfolk, and 68 mph at Richmond.
Just as the force sustained by a tree is the square of its area, the force that the wind exerts is the square of the speed. In general terms, Hazel was three times more powerful than Isabel. There were of course fewer households then, but if we adjust for the population increase since the 1960 Census, Hazel’s downed trees and branches deprived only one‐quarter as many people of power as did Isabel’s. Forty‐eight hours after Hazel, only 30,000 were without electricity in the Washington area , or about 20 percent of those initially affected. By contrast, 48 hours after Isabel struck, 600,000, or 60 percent, were still out. Obviously, what Hazel put down was a lot easier to clean up. And Hazel was mopped up after without the aid of pneumatic cherry pickers or cell phone communications.
Perhaps, you might argue, comparison to Hazel is unfair because she hit during a mini‐drought, while Isabel showed up after one of the area’s soggiest summers in history. After all, as every gardener knows, it’s much easier to pull (think “uproot”) big weeds (i.e. trees) out of saturated muck than out of dry ground.
Fortunately, we have another test storm: Hurricane Donna struck southeastern Virginia on Sept. 12, 1960, with a huge rain shield soaking already‐saturated ground. Her maximum sustained wind at Norfolk was 73 mph (one mile shy of hurricane force), or about 2.5 times more powerful than Isabel’s. An impressive 138 mph gust hit the Chesapeake Light tower at the mouth of the Bay.
Two days later, according to the Norfolk Virginian Pilot, “Electricity failure still plagued 3,000 to 4,000 homes in Norfolk and South Norfolk [which] were the worst affected.” Census figures show a metropolitan area population of 803,000 in 1960. Do the math. That’s less than one percent of the population. Two things grew like Topsy in Norfolk, Richmond and Washington in the succeeding four decades: suburban sprawl and sprawling trees.
Forest ecologist D.R. Foster has studied the effect of hurricane winds on aging trees. By age 25, nearly 100 percent of all pines are knocked down, he found. (Fast‐growing pines are another favorite of local developers and homeowners.) At age 60, about 60 percent of the hardwoods go. But the 1988 study pertained to the “natural” forest of New England, where a tree’s size is limited by the close proximity of its neighbors. In suburbia, our more widely spaced trees grow to unnaturally gargantuan proportions so that we can enjoy maximum shade. The math becomes obvious. A tree double its wild size will be felled by a wind of half the speed required to take one down in the woods. Consequently, the forests of Northern Virginia and suburban Maryland (and the even bigger trees in Richmond and the District) are pushovers, even for a weakling like Isabel.
That’s the bad news about our overgrown suburban woods. Even worse news comes from the great “Harry Cane” of 1667. Here’s an excerpt from the pamphlet “Strange News from Virginia, being a true relation of the great tempest in Virginia,” published in London in 1667:
