Air masses often interact with one another along the 'front' sides, and it is those interactions that sometimes help generate what is known as a cyclone. There are several ways in which cyclones will form, and they represent the systems that carry precipitation, winds, and temperature across our country each day. They are often more powerful in the winter, because the temperature gradients are much stronger in the US at that time. Usually a system's strength can be gauged ahead of time based in part on how much warmth or cooling they usher in or leave behind. The first step in creating a cyclone is by having winds blow across a temperature gradient, effectively advancing warmer or cooler air someplace. This causes air motion across frontal lines, and thus an interaction between air masses.

Cyclogenesis    

The discussion of cyclones and how they form has been debated for hundreds of years and basically left up to folklore in reference to a specific place for some time frame in the future (chimney smoke, spider webs, red skies, crescent moons, etc.). It is only after the technology advances that our understanding of meteorology gets crisper. Around the time of World War I, an extraordinary team of scientists working in Norway became determined to pattern out cyclogenesis, and they did so painstakingly through reams of hand tabulated surface observations. After much toil, what follows is known as Polar Front Theory, or the Norwegian Model. This model was devised after the formation of cyclones in the Middle Latitudes of the Northern Hemisphere (note the regional restrictions- as said before there are several different ways to get cyclones to develop).

    The foundational stage for the Polar Front Theory is the "Polar Front". This is basically the separating line between the tropical winds and the polar winds. In the low and high latitudes, there's not a lot of interaction between large scale air masses, but each extend into the mid-latitudes and often interact along the polar front. Here the winds are blowing perpendicular to the front itself, thus not advancing any temperature into new ground. This current situation is stable, but always represents a potential for something to happen. The more drastic the temperature difference on each side of the front, the more potential energy is available for a more powerful cyclogenesis.
    The first thing that would happen in the development of the cyclone would be some trigger mechanism allowing for the warmer air to advance towards the colder air, and vice verse. This could be one of several things, either along the front or in the upper levels of the atmosphere. Sometimes it happens when cold air interacts with warm lake or ocean water. In any event this small movement, just like with the instability of air parcels, sets of a chain reaction of events that opens the door for a cyclone to form. Sometimes this is referred to as a "Deformation Zone", because of the breakdown of the previously stable situation. Think of this as a little ripple or wave in the ocean. Not surprisingly, this is called a "Frontal Wave". Warm air is now pushing into the cold air (and because it's less dense starts to lift over it), and colder air is digging down into the warm air (forcing it upward). The axis along which this occurs is where the air pressure is dropping. From this point onward the colder air will continually try to chase the warm air, and the warm air will continually flow in from the south and overrun the colder air above it. Clouds that can release precipitation can form at this stage.
    Next, the system gradually becomes what is known as an Open Wave, as the synergy between the overrunning warm air and the digging cold air meshes better and better. The central pressure of the system is lower now, and good soaking moisture is able to be channeled through this system from long distances away. Precipitation forms ahead of the warm front as the rushing warm air is forced to lift over the slow cold air, and also along the digging cold front as the fast-moving cold air in that region undercuts the warm air and forces it upward. Some strong thunderstorms can develop along the cold front as the rapid interaction between cold and warm air are colliding together from different directions. There is a region in between the fronts where, though in the warm air, the precipitation isn't widespread and the sun often shines through. This is called the Warm Sector. Energy for this system is compounded as the cyclone tries to attain an equilibrium. The rising warm air and the sinking cold air changes the strong potential into kinetic energy. Condensation would further supply kinetic energy as latent heat would be released each time water droplets are formed. Surface air converging towards the low pressure center would further deepen the pressure itself as it is forced upward. The deeper pressure would cause the winds to spiral in faster, causing more convergence.... You get the picture. Under the right conditions, an open wave system can turn an otherwise run-of-the-mill cyclone to the Storm of the Century overnight.
    As this open wave continues to be steered by the upper level winds and the jet stream, the colder air rushes in the rear as the deepening pressure draws it forward. It digs under more and more of the warm air, shrinking the warm sector. As the system nears its maximum power, the cold air will eventually lift up the entire pool of warm air ahead of it, and eventually exist in a cooler air mass, with warmer air still being undercut further down the chain (think of it as a partially zipped, zipper). At this point, the part of the front that has overtaken all the warm air is called an Occluded Front and it will eventually start weakening because the driving energy is now located farther away from the center of the system. The precipitation coverage of a storm that gets to this stage is quite massive, and the storms that make it into this category include practically all large scale blizzards and other dominant weather systems. The place where the warm air, cold air, and the cool air near the occlusion all meet is called the Triple Point, and since massive amounts of kinetic energy are still being developed and spent, precipitation is the strongest at this point. Keep in mind that all along the occlusion at least moderate precipitation can persist, as well as high winds, because the storm is still quite deep. From this point onward, the occlusion will gradually dissipate the system as a whole, but it could still take a while to do so.
    Just like a zipper further closing, the center of the low is further displaced by the center of the interaction between the air masses. Eventually, all the kinetic energy has been used up in one capacity or another, and the influx begins to decrease. The central pressure of the cyclone begins to raise, and the system starts to wither. The cyclone at this point elongates a bit, and covers more area as it unwinds. Some drier air can get entrained near the center, creating veins of sunshine in an otherwise gloomy weather pattern. If in the course of events, a new pool of energy is come upon (in the situation of a land-based storm nearing warm open waters), a new cyclone will rapidly deepen, beginning at the triple point. This new system would then continue in the same developmental cycle as its parent. Eventually, the dissipated low fades away, and left behind is the stationary calmness known as the "Polar Front".

Vertical Structure of Deep Pressure Cyclones

    Like a fire being stoked with wood or coal, it can only grow strong provided that it has an adequate way to use the fuel in the proportion that it's given. Put too much fuel in at once, and you may even smother the fire. Likewise, if a fire is burning hot but the level of fuel doesn't match, then the fire will eventually dwindle to an equilibrium with its fuel intake. Cyclones behave much in the same way. Cyclones have a tremendous amount of surface convergence, as the winds spiral in the moisture and heat, the storm has got to be able to lift the incoming air and  moisture to help produce more convection, storms, and kinetic energy. Therefore, in the upper levels of the atmosphere, divergence must exist for a storm to deepen. If air aloft is being dispersed away from the system, then there is more room for more convergence and more energy, and thus deeper pressure. Sometimes the divergence aloft itself is capable of creating surface convergence, like a large void yearning to be filled. Think of the upper air divergence acting like a flue, to keep the air rising and allowing for more growth at the surface. In the same way, surface areas of high pressure must have convergence in the upper atmosphere to help further the sinking air and make clearer skies, etc. If it were the case where the atmosphere became vertically "stacked", as in convergence aloft and at the surface, then a storm system would begin to die out as incoming fuel would be choked and unable to lift into kinetic energy almost as if the road up were clogged.
    Where do we find this convergence and divergence aloft? The Jet Stream is a good indicator of this, as it represents the fastest air flow in the upper levels of the atmosphere. Some days the Jet Stream blows straight from west to east, and the weather maps feature weak, fast moving systems. But on other days it has deep dips  and curves in it, and here's where the convergence and divergence exists. Anytime you have a stream of fluid and you force it to move through a narrower region, this would cause the water to move faster as a conservational response. Air moves the same way in the upper atmosphere when it's moving quite fast and yet forced to converge or diverge its path. Jet Streams can form ridges and troughs both vertically and horizontally in the atmosphere (three-dimensionally really), and it is right after the troughs that the upper air divergence is the strongest, as emerging air is able to fan out. Conversely, following a Jet Stream ridge, higher pressure would be observed at the surface, as air is converging to the trough at faster rates. In an idealized system of airflow, the interaction between air aloft and that at the surface would look somewhat like this:

    Understand that this West-East orientation is not required, and the jet stream pattern does not always flow straight across the country. Notice how the air aloft aids in the deepening or rising pressure in either system. Note also that this convergence and divergence does not need a jet stream trough or ridge to form, but either a speeding up or slowing down of the wind itself relative to ambient wind flow (normally referred to as shear), or a more obvious converging and/or diverging wind pattern. On some days a region of the jet stream with locally strong winds (called a Jet Streak) can generate this convergence/divergence system just out of the fact that wind is accelerating into it, and decelerating out of it. Often locating a Jet Streak will help meteorologists pinpoint areas of greater storm development (and weaker as well).

Thunderstorms

    If all the liquid water in the atmosphere above a single point on the Earth were squeezed out dry and precipitated, there would only be a few centimeters of rainfall. Of course, everyone can remember a day where a couple inches or more of rain fell in just a short time period, perhaps from a single storm. Just as a cyclone system is able to tap into and draw upon moisture from far away sources and drop it over a region  like a conveyor belt, so too must there be a system in place for a single storm to draw into itself sufficient moisture for a similar dangerous effect. This is called a Thunderstorm.
    A Thunderstorm predominantly begins as a Cumulus cloud, and grows and develops from there as updrafts, downdrafts, and influxes of heat and moisture are added. There are three main stages in the life cycle of a thunderstorm.

Towering Cumulus Stage: A Cumulus cloud at this point has begun to grow, powered by dominant updrafts within the core of the cloud. These clouds will approach around 20,000 feet in height. On the outskirts of the updrafts there may be some eddies developing, as vertical wind shear is present.

    Sometimes the clouds at this point just dissipate, as drier air nearby the cloud mixes in and evaporates the cloud droplets. But this at the very least puts more water vapor in place for successive updrafts to allow condensation at higher altitudes, and thus cloud growth. As the cloud builds, the transformation of water vapor into either its precipitous liquid or solid equivalent releases concentrated amounts of latent heat, which is able to keep the air inside the budding thunderstorm warmer than the air around it. Fueled by a constant updraft, clouds may grow to high heights in minutes. Since unstable air is being lifted constantly, this stage offers little time for precipitation, and lightning is not seen or heard from...yet.

Mature Stage: This stage represents the full-blown thunderstorm. Not only has this cloud forced saturated air to heights over 40,000 feet, but also has just as powerful downdrafts channeling moisture Earthward. All severe weather associated with thunderstorms occur during this stage (tornadoes, hail, flash flooding, etc.)

    As the cloud continues to build above the freezing level, the cloud particles grow larger and mass amounts of precipitation are formed. Eventually, they are to heavy for the updraft to keep everything suspended and they fall. As this happens drier air is being entrained (drawn in) to the system. The entrainment of nearby dry air evaporates some raindrops and cools it slightly. Since colder air is more dense it immediately falls, and fast. This air forms what is known as a Downdraft. Falling precipitation can enhance the downdraft, as it will bring colder air from above as it drops on by. These thunderstorms press the envelope of size, and often fan out at the top of the troposphere as the stable stratosphere prevents higher convection. An updraft-downdraft system in a thunderstorm is referred to as a Cell. Lightning and thunder are heard during this stage, and heavy precipitation and severe weather elements will occur during this stage, originating out of the turbulence created by the storm cell.

Dissipating Stage: Here the downdraft of the thunderstorm becomes dominant and effectively cuts off the updraft. Separated from the supply of warm, moist air, the cloud dissipates from the bottom up, sometimes leaving just the anvil tops. Light rain and a weak wind may linger for a time during this stage.

    Usually about a half hour into the mature stage, the thunderstorm begins to dissipate as downdrafts form throughout the cloud. Some thunderstorms, however, have their updrafts and downdrafts in such good sync, and a constant supply of moisture available to the system that it's able to support itself for an hour or more. The downdrafts cut the system off from the moist updraft balance, and it withers for lack of influx. Sometimes, a good way to sense the begging of the end of a strong thunderstorm is when precipitation is at its heaviest (but note it is not always true). A single afternoon may show several cycles of thunderstorm formation and dissipation, provided the air is still conducive for development. Also, cool downdrafts of a thunderstorm may dig into the warm, moist surface air nearby and force it upward, similar to a cold front lifting a warm air mass. The result may give the air just the extra lift it needs to destabilize and form another thunderstorm right next to the first. This is often referred to as Backbuilding.

Hurricanes

    Hurricanes are closer to thunderstorms than they are to normal storm cyclones, but in the northern hemisphere they have quite a similar counter-clockwise rotation to them. Unlike cyclones, they aren't dominated by the digging of colder air but rather, they are what is known as a 'warm core' system. They originate out of massive amounts of latent heat released through powerful convection as warm ocean water puts up a lot of energy. There are several elements that must be present for a hurricane to develop and/or strengthen:

1. A pre-existing disturbance of clouds with thunderstorms occurring or developing over open water.
2. A large coverage of ocean temperature of at least 80 degrees Farenheit.
3. Little or no change of wind direction through the upper levels of the atmosphere.
4. Preferably an upper level area of high pressure for it's corresponding divergence to encourage convergence at the surface.

    Hurricanes tend to form in the months of May through October, though they have been known to form in any month of the year. In the late summer months, not only are the water temperatures at their best, but the boundary line between equatorial and tropical air masses is elevated into latitudes that have a Coriolis force component (remember this force is zero at the equator and cyclonic organization is rare).

This interaction of air masses is called the Inter-Tropical Convergence Zone or ITCZ. Though ambient winds at low latitude are typically light, converging winds along this line can often develop a mass of thunderstorms. These storms serve to create a slight air pressure minimum as low level convergence is generated. Thunderstorms develop generously in this area due to favorably warm ocean water, and the massive amounts of latent heat will serve to warm many layers of the atmosphere. Air at this point will begin to move away from the thunderstorm mass aloft, and a system of surface convergence and upper-level divergence is created, all without the use of cold air interaction. The disturbance then continues on its path in open waters, aided by the light steering currents.

    As the dip in the surface pressure becomes more concentrated, thunderstorms are drawn in through convergence, convection is enhanced, and eventually large thunderstorms are drawn in towards the center of the primitive circulation. Through the conservation of angular momentum, these storms begin to move faster, and seas begin to get a little choppy. These choppy seas add friction to the air and thus force the winds to spiral in tighter. This becomes a feedback mechanism that will continue to draw in moisture and quicken the winds. The first declared stage of a tropical system is called a Tropical Depression.

A tropical depression requires a sustained 60-second average wind speed of more than 29 mph, and a cyclonic wind component in each quadrant of the thunderstorm cluster. In other words, the circulation doesn't have to be great, but it has to be there.  Note the thunderstorms in this picture of a tropical depression are not even concentrated around the center of the circulation. This is not a requirement, but eventually in order to show some strengthening, the convergence will eventually draw them into the center.

    Depressions can often wither as they are, if the upper-level divergence is replaced by shearing winds, or perhaps if it enters cooler waters or even landforms. By the same token, it may be less than 12 hours before this system develops further into a Tropical Storm. Once Tropical Storm status has been granted, a system then will have a name, for example: "Tropical Storm Brandon". Storm names always used to have female genders, until there was complaining; then they alternated genders until it was decided that said names were to White-Anglo-Saxon-Protestant-y (more complaining), and now we have the perfectly alternating reflection of our hemispheric diversity in the nomenclature of hurricanes (glad we got that taken care of).

A tropical storm requires a sustained 60-second average wind speed of more than 39 mph, and a well developed cyclonic circulation. Moreover, some thunderstorms must be drawn into the center and exist at all quadrants of the storm's core, referred to as an "enclosed circulation".

    A tropical storm becomes a Hurricane after it's 60-second average wind speed near the center is 74 mph or greater. A powerful and dramatic circulation is visible at this point. The convergence at the center eventually becomes so powerful that the forces on one side are balanced out by those on the opposite side. This would cause a void of precipitation and winds in the center of the circulation, commonly referred to as the Eye.

    Hurricanes can continue to develop into a more and more powerful storm, and a solid eye usually doesn't appear until the hurricane has sustained winds of over about 90 miles per hour. Just as in a thunderstorm there are cells of rising and sinking air, so too in a hurricane, whereby the most powerful winds and rains are located right outside the eye, in the Eye Wall, and sinking air actually clears out the center and calm winds and sometimes clear skies prevail. Since massive amounts of latent heat are channeled for hundreds of miles, the temperatures are quite uniform in a hurricane, practically from the ground up. Being in a hurricane resembles taking a very windy shower, as rains are just that heavy. Oddly enough, thunder and lightning are rare observations towards the eye of the storm, which is somewhat of a mystery, although it has something to do with the absence of interaction between warm and cold air, and the absence of wind shear with height as in a digging cold front cyclone. There is a powerful poetry at work in the maintenance of a hurricane over open water.
 

    As long as the upper level divergence is at least matching the strength of the surface convergence, the storm will strengthen smoothly. Another ball-park way of identifying top hurricane strength is the temperature difference between the sea surface and the top of the storm clouds. All hurricanes will immediately begin to weaken the moment it moves over land, or if it enters cooler ocean waters, or if it encounters shearing winds that would inhibit the storms desire to circulate itself at all levels. Hurricanes can even in a way hurt themselves if they move slowly enough, churning up the waters to deep depths and eventually mixing the colder ocean water up to the surface. Hurricanes will often follow warm ocean currents to feed on the energy they bring, which is why so many of them seem to hook away from the SE coastline following the gulf stream, and why hurricanes very rarely near California and its cold water temperatures.

    A Strengthening hurricane is classified and categorized under what is known as the Saffir-Simpson Scale (named after its creators). The storms are broken down by their wind speeds, and fall into categories of 1-5:

SAFFIR-SIMPSON SCALE

Category	Winds	Effects
One	74-95 mph	No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal road flooding and minor pier damage
Two	96-110 mph	Some roofing material, door, and window damage to buildings. Considerable damage to vegetation, mobile homes, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of center. Small craft in unprotected anchorages break moorings.
Three	111-130 mph	Some structural damage to small residences and utility buildings with a minor amount of curtain wall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain continuously lower than 5 feet Above Sea Level (ASL) may be flooded inland 8 miles or more.
Four	131-155 mph	More extensive curtain wall failures with some complete roof structure failure on small residences. Major erosion of beach. Major damage to lower floors of structures near the shore. Terrain continuously lower than 10 feet ASL may be flooded requiring massive evacuation of residential areas inland as far as 6 miles.
Five	greater than 155 mph	Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Major damage to lower floors of all structures located less than 15 feet ASL and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5 to 10 miles of the shoreline may be required.

    Though people remember hurricanes for their tempestuous winds and heavy rainfall, the most deadly aspect of these storms is what is known as Storm Surge. As these hurricanes move about in the ocean, their deep pressure core churns up the ocean and actually plows the water ahead of it. Normally in open water this resembles terribly rough seas, but it becomes an entirely different scenario when faced with a fixed land mass.

Here is an account of surge damage from Hurricane Camille, a powerful Category 5 Hurricane in 1969. The windswept waves and torrent aided in this damage, but the storm surge is largely responsible for erasing this apartment complex in Pass Christian, MS all the way down to the foundation. Note the pool in center.