General History of Hurricanes
Anatomy of a Hurricane

Hurricanes evolve and decay from weather anomalies through defined stages. Tropical storms are loosely defined as a warm-core, non frontal, low pressure large weather system with a defined counter clockwise wind rotation that form over tropical or subtropical waters. This distinguishes them from thunderstorms and tornadoes.        The author will use a lot of disclaimers such as generally and average as there are few absolutes and thousands of variables for this huge dynamic entity. No one should ignore the potential for damage, loss of life and instability of a hurricane; however, if not ignored there is sufficient warning to casually pack and fly to another continent if so declined.

Wind is air movement between areas of two different atmospheric pressures and flows from the high pressure to the low pressure. These pressures are generally measured in "inches of mercury"; however, the meteorologists prefer to use "millibars" which is used internationally. One millibar equals 0.02953 inch; however, 0.03 will be close enough for general use. At sea level normal pressure is 29.92 inches of mercury or 1013 millibars. When the center pressure decreases there is a finite time delay for the winds to increase throughout the hurricane.The eye is not necessarily cylindrical as it may be elongated by many forces; i.e., as it may be wider at the top in stronger hurricanes and is called the "stadium effect."

        Hurricanes, tornadoes, whirlpools, etc. are vertical spiraling circular or elliptical columns. The outer winds can be distorted from a circle by an uneven surface area, as when part of the hurricane is over land and the other over water, or from side or shear pressures of upper level air. A term often heard relating to tropical storm and hurricane development is vertical shear. In the case of a hurricane, strong upper level wind currents at or near the center of the hurricane or potential hurricane will shear or cut off the tops thus preventing a "chimney" for vertical convection. In the above image the hurricane is over smooth water and no upper level instability. 
       The direction of movement will be the total of all the forces applied. The space above the earth's surface is full of high and low pressure areas with huge rivers of wind being attracted, pushed and deflected by each other. It is a three dimensional space with many pressure areas in multiple vertical and horizontal layers. These are in a constant state of change vertically and horizontally, shifting, turning, building, decaying, etc. These are the steering and shaping forces for the huge weather system which is itself a complex and multifaceted pressure area called a storm, hurricane or whatever. To make the process even more complex, winds of all the low pressures areas spin in the counter clockwise direction and winds of the high pressure area spin in the clockwise direction. The jetstreams and wind rivers twist and turn, rise and lower through the pressure mazes.
       In the eye, or chimney, the pressure drops rapidly and the temperature rises at even a steeper rate. That is the chimney or eye of the hurricane. A temperature and pressure graph is shown to the right. The low pressure chimney pulls in warm moist air from hundreds of miles around the hurricane to fuel the hurricane. The warm moist air is carried thousands of feet up the chimney where it is spewed out in cold dry air to form clouds of moisture. Round and round, and up and down the transfer of energy perpetuates the cyclone generator. A visible eye usually forms when winds reach about 80 mph.
       Most are familiar with a whirlwind, but the huge size of a hurricane disguises the whirling appearance except when viewed from above. Larger hurricanes extend to a diameter of 100 miles of hurricane force winds. Gale force winds (over 40 mph) can extend over a diameter of 400 miles. It is very difficult to scale down the immense size, energy and power released from a hurricane - some say equivalent to 400 20-megaton hydrogen bombs, or if converted to electricity, enough to power the entire U.S. for six months.

Generally, the first phase of a potential hurricane referred to as "tropical disturbance." It will have xero to very little organization at this time. It will still not have much organization when referred to as a "tropical wave" which looks as a "wave" on the weather charts - see figure A. It will catch the eyes of forecasters if the wave becomes as pronounced as shown in figure B on the chart. If conditions are favorable the "wave" will strengthen even more coupled with other factors and may result in circular component as shown in figure C. If the winds strengthen to  39 mph, it is defined and reported as a Tropical Depression. Not all waves becomes tropical depressions and simply drift or disappear in the river of wind called the Trade Winds.

      In some months, these will be coming off the coast of Africa in what is called the "tropical convergence zone" and drift slowly towards the U.S.  In June and October many form in the western Caribbean and are pulled northward towards Florida. In reality, the atmosphere is a dynamic spatial mass of varying pressure cells and rivers of air currents of varying moisture content and varying stability. For the eastern U.S., an average of 60 per year result in an average of 10 tropical storms, many of which do not make landfall. Another average is that earlier and later hurricanes tend to be shorter in duration - June and November 8 days, and August and September 12 days. A red dust layer has been found in a Keys coral rock core sample believed to have been brought from the Sahara Desert by strong trade winds.

When a tropical wave becomes organized as a "cyclone" i.e., has a counter clockwise rotation of winds, and with winds sustained for at least one minute of less than 39 miles per hour (mph), it is a "tropical depression." This could be considered a hurricane's embryonic stage. If it strengthens with sustained winds in excess of  39 mph but less than 74 mph, it is a "tropical storm." At this time it will be given a name from a list of predetermined names which it will keep until it decreases to a tropical depression. This could be considered an infant hurricane. If it reaches 74 mph or greater it has matured to a "hurricane." From then on it is classified using the five different categories based on the Saffer/Simpson scale.

Note: The term "knot" is by definition one nautical mile per hour, I use "kts" in it plural form to equal more than one knots; however, often one will find only "kt" used. One should become accustomed to 'knots' as a unit of speed (velocity) as military, maritime and NOAA uses this unit of measure and not miles per hour (mph). I mark my tracking charts with the list of conversion listed below.
      
    In summary:
   - Tropical wave = a huge frontal activity capable of a becoming a hurricane 
   - Tropical depression = circulation and less than 39 mph (34 knots)
   - Tropical Storm = 39 to 73 mph 34 to 64 kts)
   - Hurricane, category 1 = 74 to 95 mph, storm surge = 4-5 feet (64 to 83 kts)
   - Hurricane, category 2 = 96 to 110 mph, storm surge = 6-6 feet (83 to 96 kts)
   - Hurricane, category 3 = 111 to 130 mph, storm surge = 9-12 feet (96 to 113 kts)
   - Hurricane, category 4 = 131 to 155 mph, storm surge = 13-18 feet (113 to 135 kts)
   - Hurricane, category 5 = excess of 155 mph, storm surge = 19+ feet (135 kts)
   - It will decay and disappear through the same steps.
   - (All winds must be sustained for one minute in duration at a height of 10 meters.)
   - (Storm surge depends on many factors)
   - (Some approximate conversions factors are:
         Miles per hour multiplied by 0.869 equals knots (kts),
         Knots (kts) multiplied by 1.151 equals miles per hour,
         Miles per hour multiplied by 1.61 equals kilometers per hour,
         Millibars multiplied by 0.02953 equals inches of mercury,
         Inches of mercury multiplied by 33.864 equals millibars.)
      The author suggest not to be misled by borderline hurricanes such as a category 2 hurricane with winds of 109 mph. Two mph more and it is a category 3. There is very little difference between a 109 mph Cat. 2 and 111 mph Cat. 3 hurricane. The author also would like to see the width and speed included in abbreviated reports as well as relative strength.  For example: Hurricane X, Cat 3-, W60, S16. This would translate to a weak category 3, with hurricane force winds 60 miles wide and traveling at 16 mph.
       The other significant effect is the force, and it is the force that causes damage and moves objects. Force increases as the square of the wind velocity. Wind of 100 mph exerts four times more force than 50 mph, even though 100 is only two time larger than 50. Four is the square of two. This is expressed as the force or energy of wind increasing quadratic ally to the velocity. For example, 200 mph winds have 16 times more energy as 50 mph winds.  If it were a linear relationship it would take 800 mph winds to generate the same force.
       The above parameters indicate the storm force. The following provides the speed aspect by indicating the expected time that storm force winds will make landfall. The time is usually given so one can count down. When a tropical storm or hurricane is expected to reach a coastal area within 36 hours a "watch" is issued. If not already done, when a watch is issued everyone should ascertain if their location is in the  path of the storm, or are they off to one side or the other. Never assume the forecast is totally correct. It is only meant to be the best forecast at the time it is issued. Five minutes later it could be a different situation. 
       When the storm system is 24 hours away the "watch" is changed to a "warning." Watches and warnings are usually shown in different colors, generally red for the warning as it is more serious. The weather forecasters have the prerogative to leave these warnings posted if dangerously high waters and/or waves continue even though the winds are slightly below hurricane force.
       Water can be a destructive force. The innocent rains preceding the storm soaks and loosens the soil supporting and anchoring trees, poles and foundations. The more and longer the rains fall, the weaker will be the earthen anchor when the real wind and water strikes.
       A note on the Saffer/Simpson scale. It is an excellent rating system based on barometric pressure, wind and storm surge.  However, the wind velocity is by far the dominating factor. Rainfall is not even a consideration. Storm surge is influenced by wind, but shape of the coast line, direction of the wind and the sea water bottom can magnify the pressure and wind produced storm surge significantly. The author believes that storm surge is the most important consideration for protection of life and to a lesser degree for property. However, sea water soaked property can be next to useless. In hurricanes, more deaths are the result of drowning than any factor. Few deaths are because of being over pressurized by wind. Flying/falling objects and downed power lines are also significant factors.
       Hurricanes have different faces for different places. One reason is that they change in size, magnitude, direction, shape, etc. A visible "eye" is unique for a hurricane; however, another uniqueness not to be overlooked is its variability and instability. Hurricanes are fickle. One should use caution comparing one hurricane with another. When attempted it should be done at a specific location, because a 100 miles away a completely different set of comparisons may be applicable. 
       The winds are always stronger to the right side than to the left side of its forward path of the center. The reason is that winds are spinning counter clockwise; therefore, the forward speed of the hurricane ADDS to the speed of the hurricane winds. The winds subtracts on left side of its center (eye). For example, a hurricane with 100 mph winds traveling at 10 mph will have 110 mph winds to a stationary object to the right of its forward path and 90 mph on the left side (the velocities add as two vectors). This effect becomes more significant for faster moving hurricanes, say 20 mph. 
      A small hurricane location/tracking is provided. Because of its file size, it will be slow to download. It should print on regular paper. The map had no distance scale. For an approximation, there are 70 miles between one degree of latitude. Also it is is about 440 miles from Miami to the eastern edge of Acklins Islands in the Bahamas Islands. Click to enlarge.

As for storm surge, please do not forget for every gallon of water that piles up because of the sum of these forces, a gallon must also leave some other place. Because this negative storm surge causes no damage it is seldom mentioned. The author lives on the ocean front of south Key Largo and when the water is surging, or building up in his yard, it is dropping or leaving the bay front properties one mile to the northwest. Then when the wind reverses, the opposite occurs; however, is not as noticeable since it is difficult to blow the Atlantic Ocean dry.

       Storm surge has basically four elements, but many smaller factors have their effects. One element is that a small dome of water caused solely by the lower atmospheric pressure. The vacuum of the low pressure sucks up a column of water in the same manner as a person sucks a liquid through a drinking straw. For an approximate, the dome will be about one foot for each one inch of mercury decrease in barometric pressure. The pressure for the 1935 hurricane was 26.35 inches. Therefore, with all other factors omitted, the vacuum dome would have been about three and two-thirds feet. The following factors add to the basic dome.
       Second is the favorable or unfavorable timing with the astronomical tide. According to the Key West Citizen of August 31, 1935 the low tide would have been about 8 p.m. at Upper Matecumbe on September 2, 1935; therefore, it was the optimum tide time for the hurricane to come ashore. Another example is at the authors house the average tidal swing is about three feet. A low-category 2 hurricane traveling at 10 mph and  making landfall at dead low tide has some, but little flooding effect. The same hurricane making landfall at dead high tide will bring sea water half way to the house.
      Third is the piling or pushing effect (the same force that makes wind waves) of the winds on the water. The wind strength, water depth, time and distance (fetch) effects the magnitude. The term usually applied is "fetch." Huge hurricanes as "Katrina" at the Louisiana-Mississippi coast and "Wilma" at the Keys coasts are examples of fetch producing a very wide swath of high storm surge. The author suggests the effects of "fetch" be included in forecasts (if it can be calulated) as it can be considerable for a broad and fast moving strong storm traveling consistently fast for a long distance over water.
       Fourth is the piling of water caused by the up slope of a shore line. This will generally depend on the upward's angle of the ocean's bottom, forward speed of the water mass and the water's depth. This is the same as a wave breaking on shore or against a sea wall. The water travels slower on the bottom due to bottom resistance and the upper wave advances. Generally, the water cannot build or pile up on a round island like Indian Key as much as for a long island like Key Largo. It simply passes to each side; however, in the case of Indian Key and other Florida Keys the up slope is at the reef line. All Keys experience the surge that comes over the reef.

       The above image shows the additive effect. The image does not correctly represent Hawk Channel (left), the reef (center) and the Atlantic Ocean (right) as the reef and Hawk Channel would not be exposed at a low tide. For the 1935 Hurricane, it does show how a three or four-foot dome of water with a strong wind wave and the up slope wave would fill up Hawk Channel with each wave until it it reached a reported 18 foot storm surge. It probably would not have been quite that high had the railroad bed not acted as a dam, or another up slope. Click image to enlarge.
       In the Upper Keys, Hawk Channel averages about four and a half miles wide and at average maximum depth of 20 feet out to the reef line. Outward from the reef line, in the same four and one half miles the water averages about 320 feet deep. The named reefs are only peaks of corals sometimes with ten or more miles of 15 to 30 feet of water between them. However, Hawk Channel has thousands of smaller coral peaks, some only a few feet below water. Its average tidal swing is two feet to three and half feet depending on the lunar stage, wind velocity and direction, and the duration thereof. The actual marked navigation channel averages somewhere around 15 feet deep. Lighthouses are about 30 miles apart with intermediate (light stations) in between and non lighted fixed markers as needed.
       The reef does not protect the Keys from the storm surge. It only slows the build up of the surge and at the same time delays its decay. Hawk Channel only acts as a reservoir to hold the sea water. Remember the reef is not a solid level dam but a jagged sawtooth-like ridge. The peaks seldom come completely, but very close, out of the water.  It does minimize the size of the  waves riding on top of the surge. The height of a wave is limited by the water depth and Hawk Channel is not deep enough for gigantic waves as in the ocean.
      Warnings are issued for either tropical storms and hurricanes or both.   A "Hurricane Watch" is a notice to expect hurricane conditions in the specified area with in the next 36 hours.
      A "Hurricane Warning" is to expect hurricane conditions in the specified area in the next 24 hours.

      BE PREPARED!