tropical year, time between successive vernal equinoxes; 365 days, 5 hr, 48 min, 46 sec of mean solar time (see solar time). The tropical year is the basis of the year used in the Gregorian calendar.

tropical medicine, study, diagnosis, treatment, and prevention of certain diseases prevalent in the tropics. The warmth and humidity of the tropics and the often unsanitary conditions under which so many people in those areas live contribute to the development and dissemination of many infectious diseases and parasitic infestations. Much has been achieved in combating such typical tropical diseases as yellow fever, amebic dysentery, and filariasis (elephantiasis). Better public health measures and therapeutic agents have assisted in the fight. The World Health Organization and philanthropic foundations have worked with local and international efforts to bring about medical advances. The deployment of American troops in malaria-infested regions spurred the search for more efficient synthetic antimalarial drugs. DDT, introduced in World War II to eradicate the malaria-carrying mosquito, has been found to have lingering toxic effects in the environment, and the persistent use of DDT in China for rice pests has produced DDT-resistant mosquitoes that now carry malaria. Resistance of the malarial parasite to chloroquine and Fansidar, the most widely used drugs for the protection of travelers in the tropics, has made both drugs virtually useless in most of Asia, Africa, and South America. A new drug, mefloquine, is effective in some areas but has many serious side effects. Current research is directed at making a vaccine against malaria. There have also been advances against hookworm, leprosy, and other tropical maladies. The emergence in central Africa of the Ebola virus, which exists in an as yet unknown host and is fatal to 50%-90% of those infected, has been especially challenging to medical personnel in underequipped hospitals where outbreaks have occurred.

Science of diseases seen primarily in tropical or subtropical climates. It arose in the 19th century when European colonial doctors encountered infectious diseases unknown in Europe. The discovery that many tropical diseases (e.g., malaria, yellow fever) were spread by mosquitoes led to discovery of other vectors' roles (see sleeping sickness, plague, typhus) and to efforts to destroy vector breeding grounds (e.g., by draining swamps). Later, antibiotics came to play an increasingly important role. Research institutes and national and international commissions were organized to control common tropical illnesses, at least in areas with Europeans. As colonies became independent, their governments took over most of these efforts, with help from the World Health Organization and the former colonizing countries.

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Severe atmospheric disturbance in tropical oceans. Tropical cyclones have very low atmospheric pressures in the calm, clear centre (the eye) of a circular structure of rain, cloud, and very high winds. In the Atlantic and Caribbean they are called hurricanes; in the Pacific they are known as typhoons. Because of the Earth's rotation, tropical cyclones rotate clockwise in the Southern Hemisphere and counterclockwise in the Northern. They may be 50–500 mi (80–800 km) in diameter, and sustained winds in excess of 100 mph (160 kph) are common. In the eye, however, the winds drop abruptly to light breezes or even complete calm. The lowest sea-level pressures on Earth occur in or near the eye.

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