Recent modeling studies indicate that while the average number of Atlantic hurricanes per year will probably not increase by the end of the century, the intensity and amount of rainfall produced per storm most likely will rise. Global warming is thought be one contributor to these changes. As climate warms, ocean temperatures warm, causing increased evaporation and cloud formation.

A 2008 study by NOAA’s Geophysical Fluid Dynamics Laboratory, last revised in August, 2011, indicates that global warming will very likely bring about these outcomes: (1) a 2% to 11% increase in hurricane intensity; (2) a doubling in the frequency of very intense – categories 4 and 5 – hurricanes; (3) higher rainfall rates than present day hurricanes, with a projected increase of 20% within 100 km (60 mi) of the storm center; (4)  no increase in the number of storms annually; (5) changes will be gradual, and probably not detectable for several decades.

Hurricane Irene and Tropical Storm Lee, the two 2011 tropical storms that made landfall in mainland United States up to September 13, were not as intense as originally forecast, but were heavy rainmakers and caused considerable property damage and a number of deaths.

Irene, which started as a tropical wave off the west coast of Africa, grew to a category 3 hurricane in the Caribbean, but had dropped to a category 1 when it made landfall on August 27 in North Carolina with a wind speed of 85 mph (140kph). After going back out to sea, Irene made its second landfall in New Jersey, and had been downgraded to a tropical storm when it made its third landfall in Brooklyn, NY.  Heavy rain associated with the storm caused widespread flooding in New Jersey and Vermont. 55 people were confirmed dead as a result of the storm. Property loss was estimated at $10 billion.

Lee started as tropical depression in the Gulf of Mexico and was upgraded to a tropical storm on September 2. It came ashore in Louisiana on September 3 with sustained winds of 45 mph (80kph), but was a slow moving and very wet storm, depositing 11 inches (28mm) of rain on New Orleans and Mobile in the first 24 hours. It tracked north, delivering 13 inches (33mm) to parts of Pennsylvania, causing the Susquehanna River to  crest at just over 42 feet (13m), the highest ever recorded. Wilkes-Barre, PA and Binghamton, NY sustained substantial flood losses.

Earlier, Tropical Storm Arlene, the first of the season, produced heavy rain in several Mexican states, triggering mudslides that killed 22.

While this was happening in North America, Typhoon Talas struck Japan. It, too, was a low intensity, slow moving storm that produced very heavy rain. Wind speed didn’t exceed 65 mph (100kph), but parts of Japan received 79 inches (2,000mm) of rain between September 3 and September 8. 59 people died and 50 were missing as a result of flooding and mountain mudslides.

Storm surge was not a factor in either Irene or Lee, but In stronger hurricanes, more people die from the storm surge than from the high winds. A storm surge is created by the wind’s piling the ocean’s surface higher than ordinary sea level. Low pressure at the center of the weather system has a secondary effect in the buildup of the sea and the energy of the surge. A category 4 hurricane tends to build an 18-ft (5.5m) surge, but during Katrina in 2008, 20-to-30 ft (6.1 to 9.1m) waves were reported along parts of the U.S. Gulf Coast.

Hurricanes and all tropical cyclones start as a cluster of thunderstorms moving over warm ocean water registering 80F (26C) and greater. Thunderstorms form in areas of wind convergence. Off the west coast of Africa, the northern and southern equatorial winds collide and force warm moist air to rise and condense to form storm cluster formations called tropical disturbances. As a tropical disturbance grows and organizes, more water vapor condenses in rising air, causing the surface air pressure to drop.

As more warm moist air rises and condenses, the storm system increases in size, the surface pressure drops further, and the storm becomes a tropical depression. The earth’s rotation can impart a spin to the storm clouds at this point, causing even more warm moist air inside the spiral to rise and condense, enlarging the storm area, and increasing the storm’s wind speed. The formation becomes a tropical storm when wind speed reaches 39 mph to 73 mph (62-117 KPH). The storm becomes a category 1 hurricane when the wind strengthens to 74 mph to 95 mph. Here are the hurricane categories:

Category         Wind MPH       KPH                  Surge Ft           Meters

1                      74 to 95           118-152           5                      1.5

2                      96 to 110         153-176           8                      2.4

3                      111 to 130       177-208           12                    3.7

4                      131-155           209-248           18                    5.5

5                      155+                248+                18+                   5.5+

Tropical Cyclones are called hurricanes in the Atlantic, typhoons in the Western Pacific, and cyclones in India and Australia. Even though the North American Eastern and Gulf Coasts have experienced many highly destructive hurricanes, tropical cyclones with even more devastating consequences have occurred in the Bay of Bengal, where much of Bangladesh and parts of India are low-lying wetlands and wide open to storm surge damage. The northern end of the Bay of Bengal is funnel shaped, and storm surges become tidal bores that sweep many miles inland. The Bhola cyclone in 1970 had sustained winds of 140 MPH (224 KPH) and a storm surge of 35 feet (10.7m). 500,000 died. In April, 1991, a similar storm in the same area killed 150,000. The biggest storm surges recorded occurred in India in 1839 when a 40-ft (12.2m) surge killed 300,000; and in Bathurst Bay in Queensland, Australia in 1899 when a 42-ft (12.8m) surge killed 400. It was reported at the time that dolphins and fish were found atop cliffs surrounding Bathurst Bay.

One of the most notorious typhoons in American military history hit Okinawa in October, 1945, two months after the end of World War II. A large segment of the U.S. naval task force that had been assembled for the invasion of Japan was still anchored in Buckner Bay on the east coast of Okinawa. Typhoon Louise, which had developed south of Guam, took a sudden unexpected turn and headed straight for Okinawa, giving the fleet no advance warning and no time to put to sea. The typhoon struck with sustained winds of 100 MPH (160 KPH), gusting to 120 MPH (192 KPH). Waves in the bay rose to 35 ft (10.7m). The fleet task force was devastated. 12 ships were lost, 222 went aground, and more than 30 were badly damaged. 83 sailors were dead or missing, and another 100 badly injured. It was fortunate for the Allies that the surrender had already been signed. The crippled task force would have been hard pressed to carry out its mission had it been called upon to do so. Damage on the island, where 200,000 troops had been massed for the invasion of Japan, was equally severe. Roads were washed out. Supply depots were blown down, scattered, and drenched by seawater blowing across the island. Aircraft and landing strips were badly damaged. Most islanders and many soldiers took refuge in Okinawa’s many caves.

Typhoons changed the course of history in 13^th century Asia. The Mongolian leader Kublai Khan ruled all of mainland Asia, including Mongolia, China, and what is now Korea. The only Asian nation Kublai Kahn hadn’t conquered was Japan. In 1274, he assembled a fleet of hundreds of ships and thousands of soldiers and set out to invade the Japanese islands. Off the coast of Japan a typhoon struck the invading force. Most of the wooden ships were demolished and the rest retreated to the mainland. The Japanese called the typhoon Kamikaze, or divine wind. In 1281, Kublai Khan tried again, this time with thousands of ships and a hundred thousand soldiers. Once again a typhoon intervened, wrecking the invading fleet. Kublai Khan made no further attempt to conquer Japan. Twice, the Kamikaze divine wind had saved the Japanese empire. The Kamikaze pilots of World War II were named after the wind that saved Japan.

This is an updated revision of one of this website’s earlier articles.