Hurricanes are tropical cyclones. They have lowpressure centers, called eyes, and rotation is cyclonic (counterclockwise in the Northern Hemisphere) near the surface. The tropical cyclone core is warm, which causes high pressure to form in the eye near the top of the storm. This high pressure drives diverging, anticyclonic winds out of the tropical cyclone.
Tropical cyclones are born over tropical oceans with temperature ≥26.5°C over 50 m or more depth. Evaporation from the warm ocean into the windy boundary layer increases the energy in the storm, which ultimately drives a circulation similar to a Carnot-cycle heat engine. Tropical cyclones die over cold water and over land, not due to the extra drag caused by buildings and trees, but due mostly to the lack of strong evaporation from the ocean.
Because tropical cyclones are born in the tradewind regions of the global circulation, they are initially blown westward. Many eventually reach the eastern shores of continents where the global circulation turns them poleward. At the equator there is no Coriolis force; hence there is no rotation available to be concentrated into tropical cyclones. Thus, tropical cyclones are most likely to form between 10° to 30° latitude during autumn.
Updrafts are strongest in the eye wall of thunderstorms encircling the clear eye. Rotation is initially gathered from the absolute angular momentum associated with the Earth’s rotation. As the storm develops and gains speed, centrifugal force dominates over Coriolis force within about 100 km of the eye, causing winds that are nearly cyclostrophic in the bottom third of the troposphere. Simple analytical models can be built to mimic the velocities, temperatures, and pressures across a tropical cyclone.
While near the shore, tropical cyclones can cause damage due to storm-surge flooding, wind-wave battering, beach erosion, wind damage, heavy rain, tornadoes, and lightning. The surge is caused by Ekman transport of water toward shore, and by the reduced atmospheric pressure head within the eye.
In 1989, category 5 hurricane Hugo moved directly over the US. Virgin Islands in the Caribbean Sea. When the hurricane reached the island of St. Croix, it temporarily stopped its westward translation, allowing the intense eye wall to blast the island with violent winds for hours. The following is an eyewitness account.
“It had been many years since St. Croix was in the path of a major storm. Hurricane Hugo reached into the Lesser Antilles with a deliberate vengeance. St. Croix was somewhat prepared. Many hundreds of people had moved into schools and churches to take refuge. But no one was ready for what happened next. By 1800 hours winds were a steady 50 kts [25 m s–1] with gusts up to 70 kts [35 m s–1] from the northwest. I was on the top floor of the wooden Rectory at the St. Patrick’s Church in Frederiksted with my husband and 8 month old son.“
“By 2000 hours it was apparent that our comfortable room with a view was not going to provide a safe haven. The electricity had been out for some time and a very big gust from the north blew the air conditioner out of the window, landing at the foot of our bed. We evacuated with only one diaper change and bottle of baby juice, leaving behind the playpen, high chair, and bundles of accessories brought from home. We followed Fr. Mike down the wooden staircase. Drafts were everywhere and glass doors exploded just as we passed on our way to Fr. O’Connor’s main living quarters on the first floor, where the walls were made of thick coral blocks.”
“We settled in again in spite of the persistent crashing and banging against the heavy wooden shutters. We had to shout to hear each other across the room and our ears were popping. In the bathroom, the plumbing sounded like a raging sea. The water in the toilet bowl sloshed around and vibrated. Mercifully the baby slept.”
“Soon the thick concrete walls and floors were vibrating, accompanied by a hum that turned into the ‘freight train howl’. The banging intensified and persisted for the next 4 hours. By 0100 hours we were tense and sweaty and wondering if it would ever end and if there was anything left outside. Fr. O’Connor was praying and feared that many people must be dead. He got up to open a closet door and a wall of water flowed into the bedroom. At that point we moved to the dining room with a group of 8 other people trapped in the rectory and waited.“
“There was concern that the rest of the roof would go and it was decided we would make a run for the schoolhouse made of 2-foot [0.61 m] thick concrete walls. I held the baby in my arms and with flimsy flip-flops [sandals], just about skated across the cement courtyard dodging flying branches and sheets of galvanized aluminum. The window was opened for us as a big, old mahogany tree blocked the door.”
“Shortly after, the eye was over us. The thick wooden shutters were flung open and about 100 people outside climbed in the window. The housing project nearby had been stripped of its north and east walls. The eye remained over us for 2 hours then the wind started up with the same intensity coming from the southwest. Only now the room was packed. Strangers were sharing the same mattresses. People slept in desks and chairs made for elementary children, and it was hot. Toddlers and infants wailed.”
“There was no generator, only an occasional flashlight could be seen. Fears of [storm] surges were on everyone’s’ minds. We were only 200 meters from the west shoreline. Hugo had slowed down its eastward track to 4 mph [1.8 m s–1] and the eye passed straight through the middle of the 23-mile [37 km] long island of St. Croix. It seems like the storm was in a fixed permanent position.”
“When dawn broke, the winds still howled. By 0800 it was safe to open the windows and the landscape made me burst into tears. There was not a leaf left on a tree, there was not a tree left standing, just tangled branches lying sideways everywhere and not one blade of green grass. The wind had burned the ground and turned everything brown. The gray skies, light rain, and brown landscape persisted for several weeks.”
“There were only 2 deaths reported but within weeks several dozen people died from heart attacks, strokes, electrocutions and other accidents associated with reconstruction. The majority of the island residents functioned without power for 3 to 6 months, using generators or candle power and gas stoves.”
– Susan Krueger Allick Beach, 1999