Hurricanes
What is a Hurricane? Def : A hurricane ( tropical cyclone) is an intense low-pressure system characterized by extreme high winds of more than 119 kph and torrential rainfall. They range from 100 to 1600 km in diameter . (Bleasdell pg 84) Hurricanes begin as tropical disturbances in warm ocean waters with surface temperatures of at least 80 degrees Fahrenheit (26.5 degrees Celsius). These low pressure systems are fed by energy from the warm seas. If a storm achieves wind speeds of 38 miles (61 kilometers) an hour, it becomes known as a tropical depression. A tropical depression becomes a tropical storm, and is given a name, when its sustained wind speeds top 39 miles (63 kilometers) an hour. When a storm’s sustained wind speeds reach 74 miles (119 kilometers) an hour it becomes a hurricane and earns a category rating of 1 to 5 on the Saffir-Simpson scale.
Hurricanes begin as tropical storms over warm moist waters of the Atlantic and Pacific Oceans, The birth of a hurricane requires at least three conditions. First, The ocean waters must be warm enough at the surface to put enough heat and moisture into the overlying atmosphere. Secondly, atmospheric moisture from sea water evaporation must combine with the heat and energy to form the powerful engine needed to make a hurricane. Third, a wind pattern must be near the ocean surface to spiral air inward. Hurricanes are mostly found along the ITCZ ( Inter-Tropical Convergence Zone ) The Inter Tropical Convergence Zone, or ITCZ, is a belt of low pressure which circles the Earth generally near the equator where the trade winds of the Northern and Southern Hemispheres come together. It is characterised by convective activity which generates often vigorous thunderstorms over large areas. It is most active over continental land masses by day and relatively less active over the oceans.
What makes a hurricane? 1. Winds A tropical storm is considered a hurricane when wind speeds reach 74mph Based on their wind speeds hurricanes are broken up into various categories based on the Saffir-Simpson Hurricane Wind Scale Category 1: Winds of 64-82 knots (74-95 mph) Very dangerous winds will produce some damage Category 2: Winds of 83-95 knots (96-110 mph) Extremely dangerous winds will cause extensive damage Category 3: Winds of 96-113 knots (111-129 mph) Devastating damage will occur Category 4: Winds of 114 to 135 knots (130-156 mph) Catastrophic damage will occur Category 5: Winds above 135 knots (157 mph) Catastrophic damage will occur
2.Pressure At sea level air pressure is normally around 1013.25mb (29.92 inches of mercury). Extremely strong hurricanes are accompanied by air pressure drops of between 30 and 70mb. The greater the pressure difference between a low pressure area and a high pressure area, the stronger the winds!
Case Study – Hurricane Patricia Hurricane Patricia was first a tropical cyclone formed in the Eastern Pacific and rapidly intensified into a category 5 hurricane. Even more quickly than the storm strengthened it rapidly weakened over the rugged terrain of Mexico. Patricia initially formed on October 20th 2015 and dissipated on October 24th 2015 . Just 30 hours after peaking in intensity as the most powerful tropical cyclone ever measured in the Western Hemisphere, former hurricane Patricia disintegrated into …… On October 23rd , Patricia became the most powerful tropical cyclone ever measured in the Western Hemisphere, as its maximum sustained winds reached an unprecedented 200mph (320kph) and its pressure fell to 879 millibars , 125.95 inches of mercury.
The eye of hurricane Patricia made landfall on Oct 23 at 6:15pm The eye of hurricane Patricia made landfall on Oct 23 at 6:15pm. CDT near Cuixmala in Jalisco state of southwest Mexico. Maximum sustained wind at landfall were estimated at 165mph, still firmly within the category 5 range on the Saffir- Simpson hurricane wind scale.