Convection
Abbot describes the process of convection as, "the transmission of heat in flowing water (or air)" (Abbot, 274). Or in simpler terms, convection is the process of hot fluid air rising and cold falling.
Water Condensation Boosting
There is hot and rising water vapor in the atmosphere, when the air reaches lower pressure zones, the temperature drops. This drop in temperature causes water within it to condense out. As a result, heat is released and all of the heat that was present has now been eliminated.
Upwelling Air
Air rises and expands, because the air rises, the temperature drops but there is no release of heat. Once the temperature goes down, because the air has risen, water vapor condenses out and releases heat. The heat that is released makes the air rise even faster because the upwelling effect is sucking air in behind it at a faster rate.
Coriolis Effect
The Coriolis effect can be easily defined as the movement north from the equator, where an object is always going to turn towards the East, its right side. For example, if an object was standing at the equator trying to move north, it will always veer towards its right side as it tries to move away from the equator. As the object is moving, it has the speed of the equator air behind it and it is moving much faster than the air to the E or the right of it. As a result, the object begins to veer right as the objects underneath it are moving at a slower rate.
Severe Weather
The weather channel defines severe weather as any destructive weather event, but usually applies to localized storms such as blizzards, intense thunderstorms, or tornadoes. For this unit, I will concentrate on nor'easters, thunderstorms, hurricanes and tornadoes. All of these are powerful storms that have cost large amounts of money as well as caused numerous fatalities.
Nor'easters
Winters storms, otherwise known as nor'easters, are one of the most common forms of severe weather that we see living in the northeast. The weather channel defines a nor'easter as a mid-latitude cyclonic storm occurring off the east coast of North America. A nor'easter gets its name from the continuously strong northeasterly winds that blow in from the ocean ahead of the storm and over coastal areas. A nor'easter is formed when, "a low-pressure system moves up the northeastern United States coastline, its counterclockwise circulation on its western or landward side draws cold air down from the north. Meanwhile, its eastern or seaward side picks up moisture from the Atlantic Ocean to feed into the cyclone," (Abbot, 279). In other words, a nor'easter is formed when cold Canadian air and warm Atlantic moist air low pressure zones brings them together and crashes into one another. An air mass can be described as a large region above the Earth with a uniform temperature and moisture level. Most winter storms are formed in the northeast when an air mass of cold, dry Canadian air moves south and interacts with a warm, moist air mass moving north from the Gulf of Mexico.
For a winter storm to develop three things need to be present: cold air mass, moist warm air mass and lift. Cold air is required for a snow storm to develop. There needs to be freezing temperatures in the clouds as well as near the ground in order to make snow and/or ice. Lift is required to raise the moist air to form the clouds and in return cause precipitation to form. Last, but of course not least, moisture needs to be present to help form the clouds and precipitation.
There are a couple of different stages that we can see during a nor'easter. For example, the storm can consist of snow, sleet and freezing rain. In one storm we can see all of these elements and in others we may see only one or two. Each storm is uniquely different and depending on the levels of the three ingredients present we will see different types of precipitation. Snow is formed in the top layer of the storm because it is usually cold enough to create snowflakes. Snowflakes are made out of a collection of ice crystals that cling to each other as they fall down towards the Earth. Snow will continue to fall as long as the temperature stays at or below 0 degrees Celsius from the base of the cloud to the ground. As with anything, there are varying types of snow ranging from light to heavy accumulation. Snow flurries are the lightest form of snow that falls, leaving behind no accumulation because it only lasts for a very short duration of time. Next on the scale are snow showers, these leave us with a small amount of accumulation and last for brief intervals of time where there are light and then heavy snow falling. Next are snow squalls, which consist of heavy snow showers that last for a brief amount of time. Although the showers do not last long, they can leave behind a significant amount of snow because they are accompanied by strong and gusty winds. Then there are blowing snow showers which reduce visibility because they are, as the name implies, wind-driven. Last, but not least, are blizzard showers. These consist of snow showers as well as blowing snow showers. As a result, the winds accompanied by this storm can be as strong as 35 mph and can last for at least 3 hours!
Another type of weather condition we can see during a nor'easter is sleet and hail. Sleet forms from snowflakes that have only partially melted when they fall through a shallow layer of warm air. As a result, when they hit the next layer of colder air they instantly re-freeze and hit the ground as frozen rain drops that bounce off the ground.
The last type of weather we can see within a nor'easter is freezing rain. Freezing rain is formed when snowflakes completely melt because they have fallen through an even warmer layer of air. When the drops continue to fall towards the ground, they do not have time to refreeze before hitting the next layer of cold air. Since they did not have enough time to refreeze, they instantly freeze when they come into contact with anything that is less than 0 degrees Celsius. As a result, extremely dangerous and slippery conditions occur.
Thunderstorms
A thunderstorm can easily be defined as a rain shower in which you hear thunder. Thunderstorms fall under the classification of severe weather when they contain one or more of the following things: three-quarter inch or greater size hail, winds that gust in an excess of 57.5 mph and/or a tornado (www.nssl.noaa.gov). A thunderstorm, on the average, lasts for about 30 minutes and is usually 15 miles in diameter (www.nssl.noaa.gov). There are approximately 2,000 thunderstorm taking place, all around the world, at any given moment in time. As a result, there are approximately 100,000 thunderstorms throughout the year (www.nssl.noaa.gov). Out of these 100,00, only 10% are actually categorized as severe weather thunderstorms (www.nssl.noaa.gov). For a thunderstorm to develop, there are three basic ingredients that need to be present. These ingredients are: moisture, rising unstable air and a lifting mechanism to provide the "nudge". (To find a full description of water condensation and upwelling air, please refer back to the section on background information).
A thunderstorm goes through three stages of development. They are: the developing stage, the mature stage, and the dissipating stage. Stage one, as I just mentioned, is the developing stage. During this stage a cumulus cloud is pushed up by a rising column of air. The cloud begins to look like a tower as it continues to be pushed upwards. During this stage there is little, if any, rain but there can be some lightning. This is a very short stage of the thunderstorm, only lasting 10 minutes. The next stage in development is the mature stage. In this stage, the updraft continues to feed the growing thunderstorm and precipitation begins to fall. As a result, a downdraft begins to form, where air is being pushed downward. During this stage, we are must likely going to see hail, heavy rain, frequent lightning, strong winds and/or tornadoes. When you look into the sky during this period, you will notice that the storm takes on a black or dark green appearance. The final stage of development is the dissipating stage. During this final stage, a large amount of precipitation is produced letting the downdraft overcome the updraft. The rain begins to decrease in intensity but the amount of lightning still remains at very dangerous levels. (www.nssl.noaa.gov)
There are 4 different types of thunderstorms that can develop. The first type of thunderstorm is the single cell storm. These are the weakest form of thunderstorms. They only last for 20-30 minutes and do not normally cause any severe damage. Although they usually do not cause severe damage, they can produce some unpleasant weather conditions. During this type of storm we will see the elements of thunderstorms such as rain or hail but just at a weaker degree. The second type of thunderstorm is the multicell cluster storm. These are the most common form of thunderstorms that we will see. As the name suggests, this storm is comprised of a cluster of cells that move along as one single unit. Within the cluster each cell is at a different phase of the thunderstorm life cycle. The mature cells are the ones that can be found in the center of the storm while the dissipating cells can be found on the outer regions of the storm. Each cell within the cluster stays active for about 20 minutes. The entire cluster can stay active for several hours. During these storms we can expect to see hail, flash floods and possibly even weak tornadoes. The third type of thunderstorm is called the multicell line storm or also known as the squall line storm. This storm is comprised of a long line of storms that have a continuous and well-developed gust front at the leading edge of the line. The line may be solid or it can have gaps and/or breaks in it. During these storms we can expect to see hail that can be the size of a golf-ball, heavy rainfall and weak tornadoes. These storms are especially known for producing strong downdrafts. These strong downdrafts can cause a portion of the squall line to pass right by other parts of the line. The last type of thunderstorm is also the most powerful and life threatening, it is called the supercell storm. Although these types of storms are rare, they are highly organized and can cause a huge amount of damage. The supercell, just like the single-cell, has one main updraft. But unlike the single-cell, the supercell's updraft is extremely strong. It can reach speeds of 150 to 175 mph (www.teacher.scholatic.com). The rotation of these storms are also very unique and set them apart from other thunderstorms. During these storms, winds come in from different directions and cause the rotation in which precipitation is produced. When this precipitation is produced in the updraft, the strong upper-level winds blow it downward. As a result, very little precipitation falls back into the updraft and therefore expands the life of the storms, allowing them to survive for long periods of time. During these storms we can expect to see giant hail and violent tornadoes.
One of the major contributors of damage during thunderstorms are the strong winds that can be produced by them. In fact they "account for half of all severe reports in the lower 48 states and is more common than damage from tornadoes," (www.nssl.noaa.gov). Wind speeds during a thunderstorm can reach up to 100 mph but those that reach 50-60 mph and above are classified as damaging winds. There are 7 different types of damaging winds that we can see during thunderstorms, they are: straight-line winds, downdrafts, downbursts, microburst, gust front, derecho and bow echo. The first type of winds are called straight-line. The National Severe Storms Laboratory defines them as, "any thunderstorm wind that is not associated with rotation, and is used mainly to differentiate from tornadic winds," (www.nssl.noaa.gov). The next type of winds are downdrafts which can be described as a small-scale column of air that quickly sinks toward the ground. These winds are a direct result of a strong downdraft. The third type of winds are downbursts, which are strong downdrafts consisting of horizontal dimensions greater than 2.5 miles resulting in damaging winds that are on or near the ground (outward burst). These winds can start out as microburst and then spread out over a wider area. The fourth type of winds are microbursts which are small concentrated downbursts which produce an outward burst of damaging winds at the surface (www.nssl.noaa.gov). These winds are generally small in size and last a short period of time (5-10 minutes) but can reach speeds of up to 168 mph. The fifth type of winds are called gust front. These winds are the leading edge of rain cooled air the come and clash with warmer thunderstorm inflow. Gust fronts are comprised of wind shifts, temperature drops and gusty winds which are out ahead of the thunderstorm. Next are derecho winds. Derecho is of Spanish origin and means "straight ahead" (www.nssl.noaa.gov). Derecho winds develop when a new thunderstorm forms along the edge of an outflow boundary. An outflow boundary is, "a surface boundary formed by horizontal spreading of thunderstorm cooled air," (ww.nssl.noaa.gov). These winds are usually produced during the summer months when thunderstorms form over the plains and northern plain states. These winds are especially dangerous because they are capable of covering a large area and can last for an extended period of time. The seventh and final type of winds are bow echo winds, which consist of radar echo winds that are linear but then bend outward resulting in the bow shape. These winds can be over 300km in length and last for hours while producing large amounts of damage at the ground level.
Lightning is another dangerous and damaging feature found in thunderstorms. The National Severe Storms Laboratory defines lightning as, "a gigantic electrostatic discharge between the cloud and the ground, other clouds, or within a cloud" (www.nssl.noaa.gov). The creation of lightning is a very complicated process to explain. There are two different theories that seem to support how clouds build up electrical charges that in turn produce lightning, they are: precipitation and convection theories. Precipitation theory describes the creation of lightning as different size raindrops, hail or graupel, having either a positive or negative charge, that collide with heavier particles that carry negative charge to the cloud bottom. As a result, lightning occurs. On the other hand, according to convection theory, it is believed that updrafts transport positive charges close to the ground up through the clouds while downdrafts carry the negative charges down to the ground. Within clouds there are negative and positive areas that grow creating an electric field between the oppositely-charged thunderstorm base and top (www.nssl.noaa.gov). A large amount of charge has to build before this electric field can overpower the atmosphere's insulating properties. Currents of electricity force a path through the air until it makes a connection with something. The current is then discharged as a flash of lightning. Most lightning occurs within the storm cloud, in fact it accounts for 75-80% of all lightning that is created (www.nssl.noaa.gov). There are two different types of lightning that occur: ground flashes and cloud flashes. Ground flashes consist of flashes of lightning that hit the ground. On the other hand, cloud flashes, are lightning flashes that occur within the clouds.
Tornadoes
Tornadoes will be the next severe weather phenomenon covered in my unit. Tornadoes descend from thunderstorms and consist of violently rotating columns of air. They can reach speeds of up to 300 mph. In order for a tornado to form there are 3 types of air that must be present in the environment. First, there must be instability within the atmosphere. Close to the ground, we find a layer of warm and humid air with strong south winds. In the upper atmosphere, we find colder air and strong west or southwest winds. This means that the air closer to the surface is much less dense than the air higher in the atmosphere. If we gave the warm air an initial push to move upwards into the atmosphere, it would continue rising, sending it to mix with the much colder air in the higher part of the atmosphere. As a result, the combination of the two different types of air would cause the tornado's parent thunderstorm. The second element needed is a change in the speed and direction of wind. The cold front dives underneath the warm front, or in other words, they slide by each other. Air then becomes trapped between the two fronts and gets stuck. As a result, the air begins to tumble and turn. The storm then grasps the air and pulls it up causing an upwelling of air. This in turn causes a low pressure zone at the bottom because the storm is sucking air off the ground. Since the air is turning in-between the two fronts and now more air is being picked up, the tube begins to spin faster. The wind shear then tilts the storm allowing more time for the storm to get tighter and last longer. Last, a layer of hot, dry air between the upper and lower layers is needed. When all of these elements combine a thunderstorm is formed and from a severe thunderstorm a tornado is produced.
When compared to other countries, more tornadoes occur in the United States. Australia is also known to have a large amount of tornadoes, but still less then the United States. The make-up or geography of the United States makes it an ideal place for a tornado to develop because it brings all of the necessary elements together. For example, the Rocky Mountains are to the west, the Gulf of Mexico is to the south and there's a terrain that slopes downward from west to east. This area of land is known as "Tornado Alley". Approximately 500 tornadoes occur within this area each year (www.teacher.scholastic.com). The average for the United States is only 1,000 per year (www.teacher.scholastic.com). So, as you can see, most of the tornadoes that occur in the United States, occur within the boundaries of "Tornado Alley". The states with the highest risk of experiencing a tornado are also those that reside within "Tornado Alley." They are: Arkansas, Iowa, Kansas, Louisiana, Minnesota, Nebraska, North Dakota, Ohio, Oklahoma, South Dakota and Texas. Tornadoes typically occur between April and June because of the unseasonable warm and humid spring weather.
Tornadoes are measured using the Fujita Tornado Damage Scale developed by Dr. T. Theodore Fujita, also referred to as "Dr. Tornado." The scale was created in 1971 and it is used to estimate the strength of a given tornado based on the winds associated with it. The scale ranges from F0 to F5, going from lowest danger to highest danger. A tornado given a score of F0 means the winds reached speeds from 40 to 72 mph. On the other hand, a tornado given a score of F5 means the winds reached speeds from 261 to 318 mph.
Hurricanes
The final type of severe weather that will be addressed in my unit are hurricanes.
Hurricanes are formed over tropical water and can be range from 60 to 1,000 miles in diameter. They start as a cluster of stronger thunderstorms, known as a tropical disturbance, that move across the ocean. The tropical wave begins by spinning around a center of low pressure known as a tropical depression. When the winds with that tropical depression reach a speed 0f 40 mph or higher, the storm then changes into a tropical storm. At this point in time, the storm is given a name. Scientists give hurricanes names so that people are able to track the storm with greater ease and tell one storm from another. Official storm trackers have a list of names readily available which are used on a 6 year rotation schedule. The list, complied from the National Hurricane Center, can be viewed at: http://www.nhc.noaa.gov/aboutnames.shtml. Once the 21 names that have been chosen for the year have been used, the Greek alphabet is used to help name the storms. Names are retired once they have been given to a hurricane that has caused an enormous amount of damage. Once the winds reach a speed of 74 mph or greater, the storm is called a hurricane or a "tropical cyclone". The conditions in the atmosphere must be just right for the tropical storm to develop into a full blown hurricane. In order for a hurricane to occur there are a couple of conditions which need to be met. First of all, water temperature within the tropical ocean must be at least 80 degrees F. The warmer water gives the hurricane the energy it needs to keep growing and moving. There also needs to be a low wind shear from the top to the bottom of the atmosphere. In other words, there cannot be light winds on the ocean's surface and stronger winds at high altitude over the storm. If these conditions exist then the hurricane can be ripped apart and will stop developing. The last element needed is something to get the tropical wave spinning. Most of the time a low-pressure system, also known as a front, moving from the land to over the ocean helps develop the storm. Hurricanes have an eye which consists of calm winds and low pressure. Surrounding the eye of the storm is an eyewall consisting of internal thunderstorms which have high winds and heavy rain.
Hurricane season, as it is referred to as, lasts for 5 months (June 1 to November 30). Most hurricanes occur within in these months because of the rising water temperature in the ocean. Typically hurricanes last from 2 to 14 days, moving from east to west and reaching speeds up to 30 mph. The size of a hurricane is measured by the Saffir-Simpson Scale and range in intensity from 1 to 5. The scale measures 3 types of activity within a hurricane: wind speed, air pressure and storm surge. "The storm surge is a 50 to 100-mile-wide dome of water that sweeps across the coastline near where a hurricane makes landfall," (www.teacher.scholastic.com/activities/wwatch). Hurricanes which range in categories 3, 4, and 5 are referred to as intense or major hurricanes. These hurricanes are responsible for over 70% of the damage in the United States, although they only account for 20% of all hurricane strikes. Hurricanes die when they move over cold water or over land where they get cut off from its source of energy. Hurricanes rely on water vapor that evaporates from warm ocean water for their energy. Once that energy source is gone, they are no longer able to survive.
