A circular fluid motion that rotates in as closed area in the same direction as the earth is called a cyclone. A cyclone is an inward spiraling wind that rotates clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. Generally, large scale cyclones are centered in the low atmospheric pressure areas.
A rapidly rotating storm system characterized by strong winds, low pressure center and a spiral arrangement of thunderstorms is called a tropical cyclone. It produces heavy rain. It is a natural disaster which causes loss in lives and properties depending upon its strength. Cyclone has adverse effects in different parts of the world every year. It claims lives and properties in Bangladesh, India, Japan, Philippines, Mexico, and America etc. Nepal cannot be affected by cyclone directly since its landlocked geographical location.
Meteorologists give different names to cyclones. The names of some cyclones are Bhola, Katrina, Gorki, Mitch, etc. In Bangladesh, a cyclone named 'Bhola' claimed 4 lakh people in 1970 AD and 'Gorki' killed 14,000 people in 1991AD. Similarly, a cyclone named 'Katrina' claimed 1800 people and millions worth property in 2005 AD in America.
The process of developing a cyclone starts when the temperature of sea water in 50m depth from the surface of the sea is 26.5°C. In this process, water vapor with warm air rises up when the air concentrates in low pressure zone which organizes cluster of thunderstorms with weak surface winds. This process is called tropical disturbance. The continuation of tropical disturbance results in strengthening of surface winds and blowing forcefully around and into the centre of the growing storm. The storm becomes stronger and larger when the atmospheric pressure decreases. The strong storm changes into cyclone when the speed of rotating storm is more than 119km/h and may reach up to 200km/h. In cyclone, the water vapour flowing upwards cools down fast and releases heat. It is the main source of energy for a cyclone.
Although several studies argue that climate change has altered tropical cyclones; others argue that the evidence is thin. For An example, tropical cyclone intensity has increased over the past 40 years as the climate has warmed. However, this recent upward trend is still within natural variability and longer term records do not reveal changes in underlying frequency or severity. The historic record may simply not be long enough and clear enough to detect how climate may be affecting tropical cyclones; nor is the physical understanding of the phenomenon sufficient to project how future activity might change with climate. In particular, there remains significant debate about how rising greenhouse-gas concentrations affect tropical cyclones.
There is also evidence that the damage from extreme events and specifically tropical cyclones is increasing over time. One explanation for this trend is that there are just more people and assets in harm's way. Until the influence of rising vulnerability from income and population is properly controlled, it is difficult to know whether the trend in damage is due to a trend in the underlying hazards.
Tropical cyclones variously defined as hurricanes, typhoons, and cyclones regularly impact human populations and periodically produce devastating weather-related natural disasters. The epidemiology of tropical cyclones is fundamentally determined by the physical forces of massive cyclonic systems intersecting with patterns of human behavior. The destructive forces of cyclonic winds, inundating rains, and storm surge are frequently accompanied by floods, tornadoes, and landslides. Human factors include land use and settlement patterns, building design and construction, forecasting and warning systems, risk perception, evacuation, and sheltering. Preparedness and mitigation strategies for minimizing harm include family disaster planning, stocking of hurricane supplies, protection of home sites, timely response to public warnings, and alertness to post storm hazards.
Public health consequences associated with tropical cyclones include storm-related mortality, injury, infectious disease, psychosocial effects, displacement and homelessness, damage to the health-care infrastructure, disruption of public health services, transformation of ecosystems, social dislocation, loss of jobs and livelihood, and economic crisis. These outcomes disproportionately befall developing nations, and human factors strongly influence the observed disparities.