Severe Weather in Haiti

The following post will discuss tornadoes and hurricanes. Tornado and hurricane formation and frequency on the Continental US will be compared to Haiti's tornado and hurricane frequency.

 Tornadoes: 

    Tornadoes are created during intense thunderstorms. Tornadoes are air rotating rapidly around a narrow region of low pressure. This center of low pressure can be as low as 900mb. A tornado's wind speed can be between 70-300 mph. There a three steps to tornado development. The first step involves forming a rotating body of air at the surface (vortex tube). Vertical wind shear helps the vortex tube to start spinning. The change in wind speeds with increasing height helps to keep the storm strong. During the next step, the horizontal vortex tube is lifted off of the ground by a thunderstorm's updraft. (The updraft lifts the rotating column of air similar to human lifting a cat.) The rotating column of air is pulled into the thunderstorm. A mesocyclone develops in the updraft of the thunderstorm once the rotating column of air is vertical. During the third step, a the rotating vertical column of air's circumference contracts, increasing wind speeds. The vertical column of air then descends from the mesocyclone's wall cloud. The mesocyclone becomes a thunderstorm once the vertical column of air touches the ground (Small, J.; Lecture 18). 

    In the US, tornadoes can travel in any direction. However, many tornadoes travel southwest to northeast, or from west to east. Robert Edwards of the Storm Prediction Center at NOAA, claims that tornadoes tend to travel in a dominant direction due to "an increased frequency of certain tornado-producing weather patterns (say, hurricanes in south Texas, or northwest-flow weather systems in the upper Midwest" (The Online Tornado FAQ, NOAA).

    While tornadoes are common on the Continental Untied States, they are a rarity in the Caribbean. As of 2000, no known tornadoes have occurred on the island of Hispaniola. (However, tornadoes have occurred on other islands of the Caribbean, such as Cuba.) (The Tornado Project). According to NOAA, the yearly average number of tornadoes in the US is over 1,000 (NOAA), while the  yearly average number of hurricanes in Haiti is zero (The Tornado Project).  

A map of Global tornado occurrences (Wikipedia).

        Above is a global map of tornado occurrences, featuring areas denoting tornado "hot spots" shaded in orange. Hispaniola and the entire Caribbean are not shaded, insinuating that the region is not a tornado "hot spot." Therefore, the region rarely, if ever, experiences tornadoes. Almost the entire Continental United States, however, is shaded orange. According to a graph released by NOAA, the US Annual Tornado Count has been steadily increasing. This might, however, not be true, as technology has improved to detect tornadogenesis has dramatically improved over the past 30 years. Prior to such advancements, scientists could not possibly know about every single tornado that hit the US (including uninhabited areas). The increase in number of observed tornadoes may be due to the fact that scientists can now detect, track, and study tornadoes in more detail now than they previously could.

Hurricanes:

   Hurricanes need three "ingredients" in order to form: warm ocean temperatures greater than 27°C (>80°F), the Coriolis effect, and a deep layer of ocean water (about 200 m in depth). Therefore, the prime areas for hurricane formation occur between 20 and 5 N/S, respectively. 

  Hurricanes are called by different names in different formation regions. In the Atlantic and Pacific (1 and 2 on the map below), these storms are called hurricanes. In the Indian Ocean and in the areas surrounding Australia and La Réunion (4,5,6,7 on the map), these kinds of storms are called cyclones. Typhoons form off of China's coast (3, on the map). The term used internationally for hurricanes is Tropical Cyclone (Small, J.; Lecture 20). 

A map showing the formation regions of tropical cyclones, with labeled RSMCs (Wikipedia).

An exhaustive map showing tropical cyclone paths. Note how tropical cyclones in the Northern Hemisphere tend to curve to the left upward, while tropical cyclones in the Southern Hemisphere curve to the opposite way. (Wikipedia)

      In the United States, hurricanes tend to travel Northward up the East Coast. Tropical storms in the Atlantic tend to develop off of the Western Coast of Africa, travel to the Caribbean, and turn up the US Atlantic Coast (see the above map for marked paths of tropical cyclone storms). Hurricanes tend to travel up the Atlantic Coast because the Gulf Stream (aka North Atlantic Drift) flows upward from the Gulf of Mexico along the East Coast. Hurricanes, fueled by the warm water beneath, are directed from above by the West-Easterly flow of the region's Jet Streams and Westerlies up the East Coast (Small, J.; Lecture 20).

    Like the US East Coast, hurricanes have impacted Haiti in the past, and will affect Haiti in the future. (Click here to learn more about how hurricanes impact the well-being of Haitian citizens.) Many hurricanes travel over or by Haiti before going north and making landfall on the Continental United States (please see the exhaustive map of tropical cyclone paths above). The average number of hurricanes that appear each year in the Atlantic basin is 6.3. Interestingly, the average number of hurricanes that make landfall on the Continental US each year is 1.7 (Hurricane Research Division, NOAA), while the average annual number of hurricanes that make landfall on Hispaniola is one (Klose, C.).