Effects of tropical cyclones

At sea

A mature tropical cyclone can release heat at a rate upwards of 6x10¹⁴ watts. Tropical cyclones on the open sea cause large waves, heavy rain, and high winds, disrupting international shipping and, at times, causing shipwrecks. Generally, after its passage, a tropical cyclone stirs up ocean water, lowering sea surface temperatures behind it. This cool wake can cause the region to be less favorable for a subsequent tropical cyclone. On rare occasions, tropical cyclones may actually do the opposite. 2005's Hurricane Dennis blew warm water behind it, contributing to the unprecedented intensity of Hurricane Emily, which followed it closely. Hurricanes help to maintain the global heat balance by moving warm, moist tropical air to the mid-latitudes and polar regions. Were it not for the movement of heat poleward (through other means as well as hurricanes), the tropical regions would be unbearably hot.

North American colonization

Shipwrecks are common with the passage of strong tropical cyclones. Such shipwrecks can change the course of history, as well as influence art and literature. A hurricane led to a victory of the Spanish over the French for control of Fort Caroline, and ultimately the Atlantic coast of North America, in 1565. The Sea Venture was wrecked near Bermuda in 1609 which led to the colonization of Bermuda and provided the inspiration for Shakespeare's The Tempest.

Shipping

Mariners have a way to safely navigate around tropical cyclones. They split tropical cyclones in two, based on their direction of motion, and maneuver to avoid the right segment of the cyclone in the Northern Hemisphere (the left in the Southern Hemisphere). Sailors term the right side the dangerous semicircle since the heaviest rain and strongest winds and seas were located in this half of the storm, as the cyclone's translation speed and its rotational wind are additive. The other half of the tropical cyclone is called the navigable semicircle since weather conditions are lessened (subtractive) in this portion of the storm (but are still potentially quite hazardous). The rules of thumb for ship travel when a tropical cyclone is in their vicinity are to avoid them if at all possible and do not cross their forecast path (crossing the T). Those traveling through the dangerous semicircle are advised to keep to the true wind on the starboard bow and make as much headway as possible. Ships moving through the navigable semicircle are advised to keep the true wind on the starboard quarter while making as much headway as possible.

Upon landfall

The most significant effects of a tropical cyclone occur when they cross coastlines, making landfall then it destroys ships and lives.

Strong winds

Strong winds can damage or destroy vehicles, buildings, bridges, personal property and other outside objects, turning loose debris into deadly flying projectiles. In the United States, major hurricanes comprise just 21% of all land falling tropical cyclones, but account for 83% of all damage. Tropical cyclones often knock out power to tens or hundreds of thousands of people, preventing vital communication and hampering rescue efforts. Tropical cyclones often destroy key bridges, overpasses, and roads, complicating efforts to transport food, clean water, and medicine to the areas that need it. Furthermore, the damage caused by tropical cyclones to buildings and dwellings can result in economic damage to a region, and to a diaspora of the population of the region.

Storm surge

The storm surge, or the increase in sea level due to the cyclone, is typically the worst effect from landfalling tropical cyclones, historically resulting in 90% of tropical cyclone deaths. The relatively quick surge in sea level can move miles/kilometers inland, flooding homes and cutting off escape routes. The storm surges and winds of hurricanes may be destructive to human-made structures, but they also stir up the waters of coastal estuaries, which are typically important fish breeding locales.

Heavy rainfall

The thunderstorm activity in a tropical cyclone produces intense rainfall, potentially resulting in flooding, mudslides, and landslides. Inland areas are particularly vulnerable to freshwater flooding, due to residents not preparing adequately. Heavy inland rainfall eventually flows into coastal estuaries, damaging marine life in coastal estuaries. The wet environment in the aftermath of a tropical cyclone, combined with the destruction of sanitation facilities and a warm tropical climate, can induce epidemics of disease which claim lives long after the storm passes. Infections of cuts and bruises can be greatly amplified by wading in sewage-polluted water. Large areas of standing water caused by flooding also contribute to mosquito-borne illnesses. Furthermore, crowded evacuees in shelters increase the risk of disease propagation.

Although cyclones take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they affect and bring much-needed precipitation to otherwise dry regions. Hurricanes in the eastern north Pacific often supply moisture to the Southwestern United States and parts of Mexico. Japan receives over half of its rainfall from typhoons. Hurricane Camille averted drought conditions and ended water deficits along much of its path, though it also killed 259 people and caused $9.14 billion (2005 USD) in damage.

On the other hand, the occurrence of tropical cyclones can cause tremendous variability in rainfall over the areas they affect: indeed cyclones are the primary cause of the most extreme rainfall variability in the world, observed in places such as Onslow and Port Hedland in subtropical Australia where the annual rainfall can range from practically nothing with no cyclones to over 1,000 millimetres (39 in) if cyclones are abundant.

Tornadoes

The broad rotation of a land falling tropical cyclone often spawns tornadoes, particularly in their right front quadrant. While these tornadoes are normally not as strong as their non-tropical counterparts, heavy damage or loss of life can still occur. Tornadoes can also be spawned as a result of eyewall mesovortices, which persist until landfall.

Deaths

During the last two centuries, tropical cyclones have been responsible for the deaths of about 1.9 million people worldwide. It is estimated that 10,000 people per year perish due to tropical cyclones. The deadliest tropical cyclone was the 1970 Bhola cyclone, which had a death toll of anywhere from 300,000 to 500,000 lives.

United States

Before Hurricane Katrina, the average death rate for tropical cyclones in the United States was decreasing. The main cause of storm-related fatalities was shifting away from storm surge and towards freshwater flooding. However, the median death rate per storm had increased through 1979, with a lull during the 1980-1995 period. This was due to greater numbers of people moving to the coastal margins and into harm's way. Despite advances in warning strategies and reduction in track forecast error, this increase in fatalities is expected to continue for as long as people migrate towards the shore.

Reconstruction and repopulation

While tropical cyclones may well seriously damage settlement, total destruction encourages rebuilding. For example, the destruction wrought by Hurricane Camille on the Gulf coast spurred redevelopment, greatly increasing local property values. Research indicates that the typical hurricane strike raises real house prices for a number of years, with a maximum effect of between 3 percent to 4 percent three years after occurrence. However, disaster response officials point out that redevelopment encourages more people to live in clearly dangerous areas subject to future deadly storms. Hurricane Katrina is the most obvious example, as it devastated the region that had been revitalized after Hurricane Camille. Many former residents and businesses do relocate to inland areas away from the threat of future hurricanes as well.

In isolated areas with small populations, tropical cyclones may cause enough casualties to contribute to the founder's effect as survivors repopulate their place. For example, around 1775, a typhoon hit Pingelap Atoll, and in combination with a subsequent famine, reduced the island's population to a low level. Several generations after the disaster, as many as 10% of Pingelapese have a genetic form of color-blindness called achromatopsia. This is due to one of the survivors of the depopulation brought on by the typhoon having a mutated gene, which the population bottleneck caused to be at a higher-than-usual level in succeeding generations.