HurricanesTropical WeatherSlide 3Slide 4Slide 5Slide 6Vertical StructureSlide 8Slide 9Slide 10Slide 11Slide 12Slide 13FormationSlide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Hurricane MovementSlide 23Slide 24Hurricane CategoriesHurricane DamageSlide 27Slide 28HugoSlide 30Slide 31AndrewSatellitesLandfalling StormsHurricanesChapter 11Tropical WeatherWhere are the tropics??-The region between 23.5º N and 23.5º S latitude-Between the Tropics of Capricorn/Cancer-The region of the earth where the noon sun is always directly overhead somewhereBecause the sun is always relatively high in the sky, what kind of seasonal temperature changes would you expect?-Not much-Not much daily change eitherTropical WeatherSeasons are instead marked by changes in precipitationWind and pressure are also different than in the mid-latitudes-Winds are usually easterly (not westerly)-Pressure variations at the surface are usually smallSmall pressure changes from place to place make isobars almost worthless for finding weather featuresTropical WeatherIn the tropics we look at “streamlines” instead of isobars-Lines that show wind direction-Also show where wind converges or divergesKinks in the streamlines indicate weak areas of lower pressure called “tropical waves”-Under right conditions...HurricanesDefn. - an intense storm of tropical origin, with sustained winds exceeding 74mphTerm “hurricane” is reserved for storms is the N. Atlantic and Eastern N Pacific-Elsewhere - “cyclones”, “typhoons”, etc.“Tropical Cyclone” is a generic term everyone agrees uponHurricanesSize varies but on avg:-~ 300 miles wide-Eye varies alsosmaller diameter usually means more intenseHurricanes are intense areas of low pressure-Strong ones have much lower pressure than mid-latitude lowsWind direction at surface?-Counterclockwise and inwardEyeEye WallVertical StructureOvercast sky at first (cirrostratus)Little pressure changeAir is moving toward the center from the W or NWHurricane HunterVertical StructureStart to see showers and thunderstorms as we approach the centerWind speed picks up - pressure starts to drop off more quicklyHuge seas - 30-50 ft!Vertical StructureIntense wind and rain as we enter the eye wall-Strongest winds and heaviest rain in the entire storm!Then we pop out of the bad weather and into the eye-Lowest pressure, light winds, higher temperature, and maybe even clear sky aboveVertical StructureLarge scale flow is:-1) inward at the surface, 2) then upward in the eye wall, 3) outward aloft, and 4) downward at the edgesLHVertical StructureNotice the high pressure aloft and low pressure at the surface-Ideal setup - strong high pressure aloft allows for maximum amount of air to move out of the vertical column lowest pressure possible at surfaceLHHurricanesSo winds at surface are counterclockwise around the lowWinds aloft are clockwise around the upper level high-Satellite loops often show this….looks as if it’s spinning the wrong way-Upper level clouds are the ones seenVertical StructureSmaller scale flow is:-1) Upward in the “spiral bands” of thunderstorms, 2) downward in between, and 3) sinking in the eye (clear skies)-Depth of the flow increases toward the centerFormationHurricanes generally form in regions with-1) Warm tropical waters (~80ºF)-2) Light wind-3) High humidity through a deep vertical layer….. tropicsIn the N. Atlantic and Eastern N. Pacific, these conditions prevail in the summer and early fall-Hurricane season is June - November-One other thing we need-4) Convergence of air at the surfaceFormationConvergence-Preferably at a region of weak low pressure-Starts counterclockwise rotationRecall from earlier in the semester:-Wind is a balance of forcesCoriolis, PGF, etc.-From what you know, can a hurricane form at the equator? That’s about as tropical as you can get right?NO! Coriolis force = zero at the equatorRotational wind is not possibleFormationConvergence-Usually have to be poleward of 5º-So most hurricanes form between 5º - 20º N latitude in the N. HemisphereWhere does this convergence take place?-Old frontal boundaries-Where the northeast and southeast trade winds meet (ITCZ - Inter Tropical Convergence Zone)FormationEven with all of the ingredients in place, tropical development may not happen-Inversions may cap the rising air-Strong winds and shear aloft can destroy a developing tropical systemBut, if all goes well (or badly if you’d like), a tropical system will develop through 4 stagesFormation1) Tropical waveJust an area of weak low pressure and convergence of surface windsFormation2) Tropical depressionClosed isobars on a weather mapWinds 20-34 knotsFormation3) Tropical stormSeveral closed isobarsWinds 34-64 knotsSystem gets a name at this pointFormation4) HurricaneTropical system with winds in excess of 64 knots (74 mph)Hurricane MovementHurricane MovementHurricane MovementHurricane CategoriesHurricane strength is rated by wind speed on the Saffir-Simpson Scale-Categories range from 1 - 5 with 1 being the weakest and 5 being the strongestTable 11.2 on page 307-Category 3 or higher (winds >111 mph) indicates a “major hurricane”Less than 1 major hurricane per year hits the U.S. coastBut when they do, damage is severeHurricane DamageDamage from hurricanes is due mainly to:-Winds-Storm surge and floodingCombination of high waves, heavy rain, etc.Causes most damage and deathsParticularly devastating when hurricane landfall coincides with high tideSo, most of the damage is at or near the coastDoes is matter which side of the storm you’re on?Hurricane DamageYes - east side usually has strongest windsDepends on direction of movement alsoHurricane DamageThis storm has 75 knots winds and is moving north at 25 knotsLooking at wind direction:-East side - winds moving northward at 75 knots and + speed to north at 25 knots = 100 knot winds-West side -wind moving south at 75 knots - speed to north at 25 knots = 50 knot windsHugoSo when Hugo hit S. Carolina, the heaviest damage was on the north side of the eye wallHugoCategory 4 when it hit-138 mph winds-Pressure 934 mb-Surge 8-20 feetHugoCategory 4 when it hit-138 mph winds-Pressure 934 mb-Surge 8-20 feetAndrewCostliest natural disaster in U.S. history-$30 billion dollars-53 deaths (41 in
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