Hurricanes serve the same purpose as winter storms, they vent off heat from the lower levels of the atmosphere. In summer months near the equator, ocean temperatures rise as the sun continually beats down on it. Once ocean water temperature approach 80 degrees (F) evaporation increases dramatically, causing large amounts of warm humid air to rise up from the ocean surface. As it rises, perhaps up to 15,000 ft or more, it starts to cool. The humidity condenses and falls out as cold rain. The rain in-turn cools the ocean surface and equilibrium is restored.

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But if sufficient heat and moisture is present, the rainstorm continues to push higher into the atmosphere, maybe to 22,000 ft, and a thunderstorm forms. After a few hours of good rain and lightning, equilibrium normally is restored. But sometimes so much latent heat energy is present in the ocean and atmosphere that the storm persists for days. This in combination with the presence of a pre-existing upper level circulation, pushes the moisture higher into the atmosphere, pulling more warm air up even higher and starts the system to circulate. Because the air under the thunderstorm is rising, surface air pressure drops. Air from around the storm rushes in to fill the void left by the rising air, causing wind. But all this serves to do is drag more warm moist air into the system, fueling yet more growth in term of size and height. As the storm grows it starts to circulate faster. Rotation is counterclockwise in the north hemisphere and clockwise in the south. As it grows, central pressure continues to drop and wind speed increases. The storm also moves, normally from east to west, directed by the flow and presence of high pressure systems aloft. Once wind speeds reach about 90 kts, an 'eye' can form.  This small cloud free area located in the center of the storm develops due to extreme uplifting of air, much like smoke rising up out of a chimney, clearing the center of the storm of clouds. It can range from a few miles to 40 miles in diameter.  An 'eyewall' forms around the eye, consisting of very dense clouds and the highest velocity winds.

Hurricanes are notorious for being 'unpredictable' though. They can stall, speed ahead, fall apart or strengthen quickly in response to rapidly changing events occurring in the upper atmosphere. Tropical systems with sustained wind speeds of less that 34 kts (39 mph) are called Tropical Depressions. When wind speed near the center of the storm reaches sustained speed of 34 kts (39 mph) it's called a Tropical Storm. Once sustained winds reach sustained speeds of 64 kts (74 mph) they are classified as hurricanes. Maximum sustained winds can reach as high as 175 mph. There's much reference data available on the web regarding the formation and characteristics of tropical systems (see here).

From a surf forecast perspective, hurricanes are capable of producing large waves of moderate period (13-14 seconds) if a variety of conditions are right. Typically the storm needs to be headed right towards your location, with maximum winds over a multi-day period. Fetch area is normally small compared to winter storms, with winds greater than 30 kts rarely extending out more than 250 nautical miles (nmiles) from the center of circulation. But if the storm is headed right at you with a constant heading over several days, it is possible to develop a good amount of 'virtual' fetch, effectively increasing the fetch area and improving the chances for a longer period swell (14-17 secs). But then the risk of the storm actually hitting your location increase too. It's much like playing a game of dodge-the-bullet. Conversely, if the storm is moving perpendicular or away from you, there is no probability for virtual fetch, and the chances for swell decrease dramatically.

Tropical storms can produce swells not only in the tropics, but well north too. Normally hurricanes track around the edges of large summer high pressure systems that develop in the central Atlantic and Pacific Oceans (in either hemisphere). In both the north Atlantic and Pacific, as tropical storms approach the western most point of these oceans, if they don’t hit land or die out, they turn towards the north. As they progress northward, and eventually northeastward, they loose their tropical characteristics. They move over colder waters off the north Atlantic/Pacific and transform from being warm core to cold core, and turn extra-tropical. Extra-tropical systems generally will increase in size and fetch area, though winds will back off a bit. Regardless, they can ,in-turn, take tracks towards Europe or Canada, producing nearly winter like storm surf in the early fall.

The normal atmospheric and wave models are of limited use when trying to predict strength and heading and swell sizes associated with tropical systems. Rather, building a solid historical base from past experience becomes an essential skill from predicting swell size and arrival times of tropical systems. Using a map, plot all the information you can obtain (position, wind speed, central pressure, and wind field size, distance from your location etc) for every advisory issued. Use this data to build a your own estimate of swell arrival time and size, then compare the results to what actually happens. With time, accuracy of your forecast should increase.

The Eye

The most recognizable feature found within a hurricane is the eye. They are found at the center and are between 20-50km in diameter. The eye is the focus of the hurricane, the point about which the rest of the storm rotates and where the lowest surface pressures are found in the storm. The image below is of a hurricane (called cyclone in the Southern Hemisphere). Note the eye at the center.

Located just outside of the eye is the eye wall. This is the location within a hurricane where the most damaging winds and intense rainfall is found. The image below is of a hurricane (called cyclone in the Southern Hemisphere).

Eye walls are called as such because oftentimes the eye is surrounded by a vertical wall of clouds. The eye wall can be seen in the picture above as the thick ring surrounding the eye.

Atmospheric pressure and wind speed change across the diameter of a hurricane. To demonstrate, the diagram below shows a rough profile of wind speed (blue) and surface pressure (red) across a hurricane. Between 100-200 kilometers from the eye, the winds are fast enough to qualify as tropical storm force. The atmospheric pressure here will still be relatively high compared to the storm's center at about 990­1010 millibars. However, the pressure gradually falls and the wind speed rises upon getting closer to the eye wall. It is only over the last 50­100 kilometers that the large changes in pressure and wind speed occur.

The pressure begins to fall more rapidly while the wind speed simultaneously increases. Within the eye wall, the wind speed reaches its maximum but within the eye, the winds become very light ­ sometimes even calm. The surface pressure continues to drop through the eye wall and into the center of the eye, where the lowest pressure is found. Upon exiting the eye, the wind speed and pressure both increase rapidly. The wind speed again reaches a maximum in the opposite eye wall, and then quickly begins to decrease. The wind and pressure profiles inside a hurricane are roughly symmetrical, so a quick rise in winds and pressure through the eye wall followed by a slower increase in pressure and likewise decrease in wind speed would be expected.

Apart from the storm surge, heavy rainfall causes both flash and long term flooding. Tropical storms and hurricanes are known to dump as much as a meter (about 3 feet) of rain in just a couple of days, creating big problems for residents who believe they are safe just because they do not live on or near the coast. In fact flooding kills more people than the strong winds do. Here are some of the rainfall totals which occurred in October of 1995 from the landfall of Hurricane Opal.

After a hurricane has come inland, it does begin to deteriorate. However, it still produces a lot of rainfall. Even when a tropical system is as weak as a depression, it is still a very strong storm when compared to average thunderstorms.

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Storm surge elevations in South Florida during Hurricane Andrew

Storm surge elevations in Louisiana during Hurricane Andrew landfall

One major cause of hurricane damage is storm surge. Storm surge is the rising of the sea level due to the low pressure, high winds, and high waves associated with a hurricane as it makes landfall. The storm surge can cause significant flooding and cost people their lives if they're caught unexpected.

Storm surge can be understood by looking at the video below. The strong winds blowing towards the shore help push water towards shore on the right side of the hurricane's direction of motion. This piling up contributes to most of the coastal flooding.

The intensity of a tropical cyclone is measured by the highest sustained wind speed found within it. Once it becomes a hurricane, the relative strength of that hurricane is also measured on a scale based on its greatest wind speed. This scale is named the Saffir-Simpson scale for the men who invented it. The scale is listed below.

The Saffir-Simpson scale categorizes hurricanes on a scale from 1 to 5. Category 1 hurricanes are the weakest, and 5's the most intense. Hurricanes strong enough to be considered intense start at category 3 or with sustained winds exceeding 96 knots (111 mph). For reference, there have only been two category 5 hurricanes that made landfall on the mainland U.S. (Florida Keys 1935 and Camille 1969). Recent intense hurricanes to make landfall on the United States were Opal in 1995 and Fran in 1996.

Atmospheric Pressure

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface. In the diagram below, the pressure at point "X" increases as the weight of the air above it increases. The same can be said about decreasing pressure, where the pressure at point "X" decreases if the weight of the air above it also decreases. 

Thinking in terms of air molecules, if the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases. The opposite is also true, where a reduction in the number of air molecules above a surface will result in a decrease in pressure. Atmospheric pressure is measured with an instrument called a "barometer", which is why atmospheric pressure is also referred to as barometric pressure.

View MSNBC's Comprehensive Guide to Hurricanes