This is a guest post from Bruce Parker, Visiting Professor, Center for Maritime Systems, Stevens Institute of Technology, and author of the recent book The Power of the Sea – Tsunamis, Storm Surges, Rogue Waves, and Our Quest to Predict Disasters.
The only hurricane in recorded history whose eye passed over New York City arrived there on the evening of September 3, 1821. Its storm surge produced a 13-foot rise in water level in only an hour. This surge flooded the lower end of Manhattan up to Canal Street, the waters of the Hudson and the East rivers essentially joining to cover the sidewalks of New York. Ships were carried up onto the shore in many locations and numerous wharves were damaged. The Battery was completely destroyed. A few miles north the storm surge demolished a bridge that spanned the Harlem River from (what is now) East 114th Street to the northwest end of Ward’s Island. Although there was a great deal of damage, New Yorkers were actually lucky, because the hurricane struck at low tide. Even greater flooding and destruction would have occurred if the hurricane’s storm surge had arrived at high tide (which would have added another 5 feet of water). Also, the hurricane was only a Category 2. Two days earlier while to the east of Florida it had been a Category 5, but after passing over land several times its power was greatly reduced by the time it hit New York City.
This hurricane is actually more significant for another reason, for it played an important role in the first scientific understanding of what exactly a hurricane is. The pattern of its destruction as it passed through Connecticut and Massachusetts stimulated William Redfield to grasp for the first time the rotary nature of a hurricane. Redfield was a saddle maker turned steamboat captain and a self-taught amateur scientist. About a month after the hurricane he was traveling with his son from their home in Middletown, Connecticut, to Stockbridge, Massachusetts, the home of the family of his wife, who had just died giving birth. In spite of his great sadness (or perhaps because of it), as Redfield moved through the countryside he stared intently at the destruction that had been caused by the winds of the storm. He noticed that all the “fruit trees, corn, etc were uniformly prostrated to the north-west” in Connecticut, but when he reached Massachusetts, he noticed that the trees and corn “were uniformly prostrated towards the south-east.”
Ten years later Redfield published a scientific paper in which he used this information (plus wind data he acquired from all over those two states as well from states along the Atlantic Coast) to demonstrate that “this storm was exhibited in the form of a great whirlwind.” That paper thrust him into the scientific limelight and also into what was later called the “American storm controversy.” This “controversy” was an ongoing debate that began when James Espy, a scientist at the Franklin Institute in Philadelphia, put forth a different theory on hurricanes, one that said the air does not rotate around a quiet center (the eye) but instead that the air just rushes in from all directions directly toward a low-pressure center. Espy’s theory built upon Benjamin Franklin’s suggestion a century before that a storm was a moving system of winds and that it moved in a direction that was not necessarily the same as the direction of the winds themselves. [He had explained this with an analogy of a chimney over a fire. “Immediately the air in the chimney, being rarified by the fire, rises; the air next to the chimney flows in to supply its place, moving towards the chimney; and, in consequence, the rest of the air successively, quite back to the door [the source of cold air from the outside].” Franklin envisioned warm air rising over the Gulf of Mexico and cooler air from the northeast flowing in to replace it, this airflow beginning first near the Gulf, then extending a little farther away (for example, to Philadelphia), and then still farther away (for example, to Boston).] Now Espy added to Franklin’s analogy of heated air moving up a chimney the important idea that the updraft in the center of a hurricane is stronger if the air is moist. He used the concept of latent heat, which is the heat required to turn a liquid into gas (or the heat released when a gas condenses into liquid). Espy realized that water vapor is lighter than dry air and that, as moist air rises and expands, the latent heat released as the water vapor condenses helps the convection to continue longer. Thus, warm moist air contributes more to convection in a hurricane than does dry air, which is one reason hurricanes form over warm oceans. This was an important contribution to meteorology, but unfortunately Espy got the wind direction wrong because he did not include the effect of the Earth’s rotation.
The debate became national, with scientists in New York supporting Redfield and scientists in Philadelphia supporting Espy. Then it became an international debate when scientists in England supported Redfield, but scientists in France supported Espy. The issue was finally cleared up in 1856 in a paper by William Ferrel, who recognized that Redfield and Espy each had part of the answer (as so often happens). It is the Earth’s rotation that is responsible for hurricanes having circular wind patterns, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. A hurricane has a low-pressure center area and it is surrounded by higher pressure air masses. At the low-pressure center the air is rising because of heat and moisture from the ocean, and air moves toward the center of the storm to replace it. The air currents coming from the north, south, east, and west are all pushed toward the right (in the Northern Hemisphere) by the Coriolis effect (due to the rotation of the Earth). These multiple pushes, however, drive the rotation around the low-pressure center in a counterclockwise direction, almost like small gears around one large gear in the middle, the large gear rotating in a direction opposite to the rotation of the small gears.
For historical references see: “The Power of the Sea – Tsunamis, Storm Surges, Rogue Waves, and Our Quest to Predict Disasters.” The book includes many other stories of how scientists learned to predict storms and storm surges, and of their destructive and deadly impact throughout history.
[Note: Proof that the 1821 hurricane was the only one to ever hit New York City directly was provided by a paleotempestology study in 2001. The study found three significant overwash deposits in the New York City area, each caused by a very large storm surge. The middle sediment layer was from the storm surge produced by the 1821 hurricane. The study shows that another hurricane crossed the New York City location, but long before there was a New York City, sometime between 1278 and 1438. A third sediment layer, the uppermost and smallest layer, was from the storm surge caused by a northeaster in 1962.]