HURRICANE
"Huracan" ... "Carib God of Evil"
Warm-core low pressure system of tropical origin, that has surface winds of 74+ mph (65 knots or greater, that also weakens rapidly with altitude (esp. > 3000 m; 9800')
In upper atmosphere (49,000'), usually find anti-cyclonic air flow (divergence)
Hurricane season June 1st till November 30th ... usually peaks from August through October (approx. 90% of all hurricane days) ... peak date is Sept. 10th. (Earliest hurricane on record was "Allison" on 6/5/1995 which made landfall in Apalachee Bay, FL.)
Tropical basins or "breeding grounds" for tropical storms must have warm water (>78 F; >25.5 C) and this water must be warm through a depth of at least 200' (60 m) ... this insures a high rate of evaporation and tremendous latent heat release ... a hurricane is an efficient "heat engine".
Must be far enough away from the Equator to have a sufficiently strong Coriolis Force to engage a circulation (>= 5 deg latitude) ... usually between 5 & 20 deg latitude.
Must have weak winds throughout troposphere, blowing in a uniform direction ... in other words, no vertical wind shear .. high-level westerlies (250 mb) can shear top off tropical storms.
Atlantic Basin: 9-10 T.S. And 5-6 Hurricanes on average.
Eastern Pacific Basin: 16 T.S. And 8-9 Hurricanes on average.
Western & North Pacific Basins: 30+ T.S. and many Hurricanes ... year round.
Atlantic & Eastern Pacific ... Hurricanes
Central & Western Pacific ... Typhoons
Indian Ocean & Australia ... Tropical Cyclones
Atlantic & Eastern Pacific ... TPC (NHC) in Miami, FL
Central Pacific (140 W to International Date Line) ... HNL, HI
West of International Date Line ... Joint Typhoon Warning Center, Guam
Hurricane "Recipe"
Easterly Wave: a possible "spark" ... a cluster of thunderstorms that break away from the ITCZ, usually moving from east to west ... can be found through a streamline analysis (i.e., low-level wind convergence .. watch for strong waves that cross Africa and emerge off the west coast, south of the Cape Verde Islands.
Tropical Disturbance: disorganized clusters of tropical thunderstorms ...latent heat release forms a "warm core low" ... surface winds increase in speed and converge around the low ... the thunderstorms increase in number and intensity ... resulting in more latent heat release, which in turn, further strengthens the low ... and, on and on it goes.
Tropical Depression: sustained low-level winds around LOW reaching 23 mph (20-33 knots) ... a "chimney effect" causes an upper level HIGH as latent heat warmed air rise and diverges aloft ... a near perfect FEEDBACK mechanism ... under optimal conditions, latent heat release causes drop in surface pressure, which increases surface winds.
Tropical Storm: sustained low-level winds of at least 39 mph (39-73 mph; 34-63 knots) ... up until 1950, WW II names were used (Able, Baker, Charlie, Dog, etc.) ... in 1953, female names were used to name Tropical Storms ... in 1979, a 6-year rotating list of alternating female & male names was introduced, with an "ethnic" flavor to the particular basin ... names of deadly and/or destructive hurricanes are "retired" for future research purposes, e.g., Camille(1969), Agnes(1972), Hugo(1989), Andrew(1992), etc.
HURRICANE: sustained winds of at least 74 mph (64 knots) ... winds are cyclostrophic, which is a balance between the pressure gradient force and the centrifugal force of curved flow (remember, the Coriolis Force is weak) ... conservation of angular momentum would suggest an infinite wind speed as parcels circulate into the center ... the maximum wind speed is dictated by the water temp (90+ F water could produce up to a 200 mph sustained wind) ... an "eye" becomes visible on satellite photos when the sustained winds reach at least 78 mph (68 knots) ... the "eye wall" is a fierce "doughnut" of thunderstorms surround the "eye" and contains the strongest winds ... the "eye" itself is almost cloudless due to subsiding air and has light or calm winds ... the "eye" has an average diameter of 12-40 miles and can take up to an hour to pass over a particular location ..."spiral bands" of clouds pinwheel cyclonically inward to the "eye" like "tentacles" of thunderstorms ... the outflow at upper levels spiral outward anti-cyclonically.
Lethal to Hurricanes: LAND (greater roughness & friction, reduced source of warm water for latent heat release), COLDER WATER (greatly slows evaporation rates), STRONG WESTERLIES (wind shear).
Hurricanes transport only 2% of the total energy out of the tropics annually ... but can be as much as 30% during the peak Hurricane Season.
Hurricane Hazards
Landfall & Storm Surge: in the "eye", the lower central pressure can cause sea level to rise 1-2 feet (0.5 m) due to a 60 mb drop in pressure ... the storm surge is the most destructive weapon of a Hurricane ... it is highest on the right-hand side of the eyewall of a land-falling Hurricane ... can cause a rise of ocean water up to 30 feet (9 m) ... is highest at high tide ... is accompanied by very heavy rain, powerful and destructive winds and sometime, tornadoes.
Flooding due to Heavy Rain: due to the enormous amounts of water carried by Hurricanes, rainfall can be excessive ... better measured in feet of rain, rather than inches ... the slower the Hurricane moves, the greater the threat of flooding ... mountainous terrain can enhance the rainfall ... Hurricane Mitch dumped up to 5 feet of rain over parts of Central America ... especially hard-hit was Honduras, where an extinct volcanic crater filled up with water and collapsed .. the resulting land/mud-slide killed thousands, literally wiping out entire towns ... 10-15% of the annual rainfall over the southeastern U.S. In July-October is provided by tropical systems.
Tornadoes: usually in the right-front quadrant, as Hurricane comes ashore or as it moves up along a coastline to its left side ... these tornadoes are caused by a sudden increase in friction (water to land), which causes the surface winds to slow and cross isobars at a greater angle ... as the winds slow, both friction and Coriolis diminish ... the pressure gradient force becomes dominant and can spin up F0 & F1 twisters.
Wind Damage: the sudden increase of sustained winds (with higher gusts) with land-falling Hurricanes, can cause considerable wind damage ... again, the slower a Hurricane moves, the longer the sustained winds are over a given area, causing various structures to weaken and eventually collapse and blow away.