Unlike volcanoes, earthquakes, or tsunamis, that originates from the movement of plates or as a consequence of fault movement, severe weather produce natural disasters originating from giant storms, thunder storms, flashfloods, tornadoes, or lighting. The number of casualties from natural disasters that are accounted for by severe weather is 75 percent of the yearly deaths and damages (Abbott, 2004). The most severe weather is located in the mid-latitudes, given they are the transfer zones between the equatorial and polar air masses.
In order to understand natural disasters that relate to severe weather, one needs to understand the way that mid-latitude cyclones (low pressure), anticyclones (high pressure) and the effects that the jet stream has on these, depending of the season of the year, including factors such as Earth's rotation, atmospheric heating, global wind pattern, Coriolis effect, air masses, fronts, and rotating air bodies.
Due to the Earth's rotation, air and water masses deflect to the right in the northern hemisphere and to the left in the southern hemisphere. This is called the Coriolis effect. The jet stream deflects around this low and high pressures enhancing cyclonic and anticyclonic flow. An anticyclone is a clockwise rotating air mass associated with a dry air mass that descends down its core evaporating moisture from below.
Most severe weather occurs along fronts; boundaries where areas of different air masses collide varying in temperature and water vapor content. These rotating air masses produce severe weather in the form of thunderstorms, tornadoes, and hurricanes. A counterclockwise rotating air mass is referred as a cyclone. Because of its low-pressure core, winds flow inward, creating an updraft of raising air, which in cooling forms clouds and sometimes rain. A hurricane is a large tropical cyclone that can generate winds of over 150 mph by feeding moisture and heat to the eye that with the help of air updrafts, rise rapidly, condensing the water vapor, and thus increasing in intensity due to the release of heat in the process of convection.
Hurricanes begin as tropical disturbances, or low pressures poorly organized with weak surface winds. As this surface winds strengthen, they send warm moist air to the center of the cyclone, becoming more organized and change into a tropical depression. When surface winds are sustained at or exceeds 39 mph, it is a tropical storm. As the moist warm air raises and cools down when reaching its dew point, the water vapor condenses releasing latent heat causing stronger updrafts and increasing the amount of upward flow of more warm, moist air. If the sustained surface winds exceed 74 mph, the tropical storm is then characterized as a hurricane.
In the Indian Ocean, hurricanes are referred to as cyclones, while in the western Pacific Ocean they are typhoons. Regardless of the names of these low-pressure weather systems they all share the same characteristics in their formation. These cyclonic storms are dependent of warm seawater and can only originate in 5º to 20º latitude; which due to the Coriolis effect, allows for the air mass rotation to take place.
Among the most known hurricanes that have impacted the United Stated we of course must make mention of Katrina in August 29, 2005 as one of the most devastating and tragic events in recent U.S. history. Other notorious deadly hurricanes in order of the number of death include Galveston, Texas on September 8,1900; South Florida-Lake Okechobee, Florida, on mid-September, 1928; Florida Keys/ Corpus Christi, Texas, on mid-September 1919, in New England on September 21, 1938 (Abbott, 2004). For first hand accounts of severe weather affecting the New England states, including hurricanes, Nor'easters, blizzards, and floods see Watson (1990) and Caplovich (1987).
In the northeastern United States during the winter months, these mid-latitude cyclones can turn into nor'easters. As these cyclone storms move up the northeastern coastline, the counterclockwise circulation draws, in the west of the storm, the cold air from the north; while in the east draws the moisture from the Atlantic ocean feeding the moisture into the cyclone. As a consequence we have severe weather and what has been labeled as "white hurricanes", "blizzards", and "ice storms". A blizzard is defined as cyclonic storm system with sustained winds 35 miles per hour, accompanied by temperatures below 25º F, and visibility of less than a quarter mile. Among the most notorious winter storms we must include the blizzard of 1888, the "White Hurricane " between March 12 to 15, 1993; the January 6-8, 1996; and the Canadian ice storm of January 5-8, 1998.