Naming Tropical CyclonesIn 1979, the World Meteorological Organization (a part of the United Nations) developed the current name listRules for North Atlantic Basin Hurricane Names:Reflect Geographic Region: Spanish, English, and FrenchShort and Distinctive: Only 21 names (no Q, U, X, Y, Z)M/F: AlternateGreek Alphabet: Only used if we run out of namesNames Repeat every 7th yearTypical North Atlantic season: 10 named tropical cyclonesIn a typical year, we make it to the letter "J"In 2005, there were 27 named storms, which forced the name list 6 letters deep into the Greek alphabet. The last named system of that year was Hurricane Zeta.Rules for retiring tropical cyclone names:Only done if the storm was particularly devastating such that using this name in the future would be insensitive.Also retire those hurricanes that are studied a lotA Tropical Cyclone is given its name once its winds have achieved a speed to categorize it as a tropical stormStages of Development and Size, Duration, and Strength1. Tropical DisturbanceCluster of tropical thunderstormsWatched or “investigated” for further developmentWinds do not exceed 24 knots150-350 miles in diameterIn Atlantic Ocean, most tropical disturbances start off the coast of Africa and move their way across the Atlantic Ocean toward the US/Mexico/Carribeean2. Tropical DepressionOrganized cluster of tropical thunderstorms with sustained winds between 22-38 mphSurface low-pressure area develops (actually a depression in the surface pressure field)Storm orbits around that low pressure area, which is a counter-clockwise direction in the northern hemisphereAssigned a number at this point3. Tropical Stormcluster of intense t-storms circulating around a deep low-pressure center with sustained winds of 39 mph-73 mphassigned a name4. HurricaneLarge, organized system of intense tropical t-storms circulating around a very deep low-pressure center74+mph sustained winds, torrential rains and storm surge 5-30+ feetSaffir Simpson Scalestandard scale that is used to rank hurricanes based upon their intensitydeveloped by Herbert Saffir, an engineer, and Bob Simpson, former director of the National Hurricane CenterTheir scale is primarily based upon maximum sustained wind speed, where "sustained wind" is defined as a 1-minute averaged wind.This is done so as to not include gusts in the hurricane ranking.Hurricane Structure and FormationFormation: What gets them started?...Goal: trigger the development of tropical thunderstorms achieved through surface convergence of air: when air collides at the surface and has nowhere to go but up. When air rises it cools, and when it cools there is condensation, which leads to clouds and precipitation. All of which is necessary for a hurricane to form.Intertropical Convergence Zone (ITCZ): a preferred location for thunderstorm formation; a major convergence zone in the topics—>a source of rising air; triggers updrafts (rising air) in a thunderstormStructure…Typical lifetime of a hurricane: 3 days to a weeks, sometimes longerTypical diameter of a hurricane: ~300-600 miles wideTypical diameter of eye: 5-50 miles widePressure: Hurricanes are intense low pressure systems, and the lowest pressure is always found at the very center of the eye. Notice on this graph that the largest drop in pressure happens in the eyewall. This rapid drop in pressure results in a very strong pressure gradient force, which gives rise to the hurricane force winds.Winds: The profile of the hurricane winds reinforces what you learned above—the fastest winds are in the eyewall, the winds are calm in the eye, and hurricane force winds (74+ mph) are confined primarily to the eyewall. Notice also in this image that the wind speed on the right-hand side of the storm is greater than on the left.Rainfall: When you examine the rainfall pattern, notice two things. First, notice the banded structure of the spiral rainbands and second, notice that as you get closer to the eyewall, the intensity of the rain increases. Finally, see that in the eye, there is no rain!Temperature: When reviewing the temperature profile, remember what we learned about the movie The Day After Tomorrow. The eye is warm because the air in the eye is descending and warming (due to compression).Storm Surge: Storm surge is simply a rise in sea level due to wind-blown water. Therefore it makes sense that the greatest storm surge will exist on the same side of the hurricane that the strongest winds are on.Coriolis ForceThe Coriolis Force acts at a 90-degree angle to the right of the motion of the air.The Coriolis Force only affects the direction the air is traveling, not its speed.If an object is standing still, there is no Coriolis Force acting upon it.The strength of the Coriolis Force is proportional to the speed of the wind.The Coriolis Force is strongest at high latitudes (near the North or South Pole) and weakest at low latitudes (nonexistent on the equator)Key Features of the HurricaneHurricane Eye—actually the calmest part of the stormLocation of the low-pressure centerPressure readings from the strongest hurricanes can often dip below 900 mb, the same pressure you would find at an altitude of 3/4 mile above sea-level on a normal day.Mostly clear skiesThe air descends in the eye and descending air warms by compression (at a rate of 10 C/km), which evaporates the clouds that would otherwise exist there.Sometimes the clouds that exist in the eye take on a really spectacular "pinwheel" structure. Check out Figure 24.5 in the text if you are interested in learning more!Winds are calmAt the center of the hurricane, the pressure gradient force is approximately equal to zero, which means there there is no wind there.EyewallMost intense part of the hurricane, although rather focused and smallMaximum extent 50–100 miles from the center of the hurricaneThe location of the heaviest rainsThe location of the fastest windsFaster on the right-hand side of the stormThe location of the greatest storm surgeSpiral RainbandsNarrow bands of intense rainfallMultiple bands of clouds that all feed into the center of the hurricaneHelp supply the eyewall with plenty of warm, moist airCan extend hundreds of miles from the eyeFour Ingredients Conductive to Hurricane DevelopmentIngredient #1: Warm Sea Surface Temperatures (>80 deg F)Measured with infrared satellites and a buoy networkThe strength of a hurricane is directly related to the sea surface temperatureEnergy within a hurricane