Hurricanes Chapter 11 Tropical Weather Where are the tropics The region between 23 5 N and 23 5 S latitude Between the Tropics of Capricorn Cancer The region of the earth where the noon sun is always directly overhead somewhere Because the sun is always relatively high in the sky what kind of seasonal temperature changes would you expect Not much Not much daily change either Tropical Weather Seasons are instead marked by changes in precipitation Wind and pressure are also different than in the mid latitudes Winds are usually easterly not westerly Pressure variations at the surface are usually small Small pressure changes from place to place make isobars almost worthless for finding weather features Tropical Weather In the tropics we look at streamlines instead of isobars Lines that show wind direction Also show where wind converges or diverges Kinks in the streamlines indicate weak areas of lower pressure called tropical waves Under right conditions Hurricanes Defn an intense storm of tropical origin with sustained winds exceeding 74mph Term 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 upon Hurricanes Size varies but on avg 300 miles wide Eye varies also smaller diameter usually means more intense Hurricanes are intense areas of low pressure Strong ones have much lower pressure than midlatitude lows Wind direction at surface Counterclockwise and inward Eye Eye Wall Vertical Structure Overcast sky at first cirrostratus Little pressure change Air is moving toward the center from the W or NW Hurricane Hunter Vertical Structure Start to see showers and thunderstorms as we approach the center Wind speed picks up pressure starts to drop off more quickly Huge seas 30 50 ft Vertical Structure Intense 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 above Vertical Structure Large scale flow is 1 inward at the surface 2 then upward in the eye wall 3 outward aloft and 4 downward at the edges H L Vertical Structure Notice 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 surface H L Hurricanes So winds at surface are counterclockwise around the low Winds 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 seen Vertical Structure Smaller 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 center Formation Hurricanes generally form in regions with 1 Warm tropical waters 80 F 2 Light wind 3 High humidity through a deep vertical layer tropics In 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 surface Formation Convergence Preferably at a region of weak low pressure Starts counterclockwise rotation Recall from earlier in the semester Wind is a balance of forces Coriolis 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 equator Rotational wind is not possible Formation Convergence Usually have to be poleward of 5 So most hurricanes form between 5 20 N latitude in the N Hemisphere Where does this convergence take place Old frontal boundaries Where the northeast and southeast trade winds meet ITCZ Inter Tropical Convergence Zone Formation Even 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 system But if all goes well or badly if you d like a tropical system will develop through 4 stages Formation 1 Tropical wave Just an area of weak low pressure and convergence of surface winds Formation 2 Tropical depression Closed isobars on a weather map Winds 20 34 knots Formation 3 Tropical storm Several closed isobars Winds 34 64 knots System gets a name at this point Formation 4 Hurricane Tropical system with winds in excess of 64 knots 74 mph Hurricane Movement Hurricane Movement Hurricane Movement Hurricane Categories Hurricane 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 strongest Table 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 coast But when they do damage is severe Hurricane Damage Damage from hurricanes is due mainly to Winds Storm surge and flooding Combination of high waves heavy rain etc Causes most damage and deaths Particularly devastating when hurricane landfall coincides with high tide So most of the damage is at or near the coast Does is matter which side of the storm you re on Hurricane Damage Yes east side usually has strongest winds Depends on direction of movement also Hurricane Damage This storm has 75 knots winds and is moving north at 25 knots Looking 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 winds Hugo So when Hugo hit S Carolina the heaviest damage was on the north side of the eye wall Hugo Category 4 when it hit 138 mph winds Pressure 934 mb Surge 8 20 feet Hugo Category 4 when it hit 138 mph winds Pressure 934 mb Surge 8 20 feet Andrew Costliest natural disaster in U S history 30 billion dollars 53 deaths 41 in Florida 160 000 left homeless 200 000 homes businesses damaged or destroyed Satellites Huge help in terms of advanced warning time Try to imagine NOT having a vantage point from space Ex Hugo 49 deaths and Andrew 53 Pre satellites 1900 Galveston TX more than 6 000 deaths 1919 Keys more than 600 1928 Near Lake Okeechobee 1 836 1935 Keys 408 Landfalling Storms 162 storms hit the U S coast from 1900 1999 Only 2 Cat 5s
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