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Lecture 3

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Environmental Science
Tanzina Mohsin

Lecture 3 Hurricanes  Hurricane – North American term from the Taino language of Central America, meaning “god of evil”  Typhoon – Term used in the Western Pacific from the Chinese word “taifung” meaning big wind‟  Cyclone – Term used in Australia and in the Indian Ocean  Tropical Cyclone – Term used in the scientific community  What is a tropical storm? A tropical storm is defined as a storm occurring in the tropical region (20 N to 20 S) in which the sustained winds range from 18 to 33 m/s.  What is a hurricane? It is a tropical storm in which the sustained winds range from 33 m/s to 50 m/s.  What is a major hurricane? It is a tropical storm which has sustained winds which exceed 50 m/s  Saffir-Simpson Scale (Herbert Saffor, Robert Simpson) o Rating from 1-5 and is based on wind speed and the central pressure of the hurricane. Average air pressure is 1013 mb o Category One Hurricane  119-153 km/h (33 – 42.5 m/s), > 980 mb o Category Two Hurricane  154-177 km/h (42.5 - 49 m/s), 965 – 979 mb o Category Three Hurricane  178-209 km/h (49 – 58 m/s), 945 – 964 mb o Category Four Hurricane  210-249 km/h (58 – 69 m/s), 920 – 944 mb o Category Five Hurricane  Greater than 249 km/h (> 69 m/s), < 920 mb Dynamics  How do hurricanes form? o Tropical storms fueled by sea surface temperatures and latent heat release  26.5 C threshold is necessary but not sufficient  What is latent heat? o The energy released due to a change of phase of matter o Heat is released when matter changes from gas to liquid and from liquid to solid o To make the opposite conversion, heat is required o Heat is released when a gas condenses to a liquid o Fuel for the storm  Tropical storm development o Begin as a tropical wave in the ITCZ near equator o o o Storm spawns between latitudes 5 -20 in either of northern or southern hemispheres o Groups of thunderstorms becomes organized and self-sustaining that can last up to 2 weeks  Requirements for the formation of tropical cyclone o Convergence at surface o Divergence aloft to balance surface convergence o Rising air releases latent heat which causes the upper atmosphere to warm and expand resulting in divergence o Eye forms in the middle where air is sinking  What affects the length and strength of a tropical cyclone o SST – warmer the surface, stronger and longer the storms o Upper wind structure – strong upper level winds inhibit tropical cyclone longevity o Landfall – cuts of cyclone from both sources of energy (SST and water vapor) – increases surface roughness Distribution  Tropical cyclones occur in North Atlantic, the northeastern Pacific, the Indian Ocean and both sides of Australia  Temperature distribution in South Atlantic and Southern Eastern Pacific have summer temperatures less than the 26.5 C threshold Canadian Hurricanes  Eastern provinces of Canada as far west as Great Lakes (once every 5 years)  N
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