GEOGRAPHY 101 Front PageAIR MASSES & ATMOSPHERIC
DISTURBANCESMATERIALS NEEDED FOR CLASS:
Daily Lesson Plans. H.K.C--Take roll and announcements.TEXT: Essential of Physical Geography: 6th Edition, Robert E. Gabler, Robert J. Sager, Daniel L. Wise, and James Peterson. Saunders College Publishing, Harcourt Brace College Publishers 1999. Rand McNally GOODE's WORLD ATLASGENERAL OUTLINE:
VIII. Air Masses and Atmospheric Disturbances
A. To classify Air Masses and to draw their Trajectory.
B. To list the Air Masses of North America.
C. To describe and map Fronts.
D. To list the different types of Fronts.
E. To relate Atmospheric Disturbances and how it differs from storms.
F. To describe transient Cyclones and Anticyclones their nature, size, and
appearance on maps.
G. To describe Thunderstorms. (I hope you get a bang from this section.)
H. To draw the development of Tornadoes and to explain why they are so
common in parts of North America.
I. To detail the development of Hurricanes.THE MATERIAL IN THIS OUTLINE COVERS PAGES 183-- 208 BUT THERE IS MATERIAL NOT IN YOUR BOOK--KNOW IT--KNOW IT--KNOW IT
ANTICIPATORY SET What are Air Masses? You have heard of them before Cyclones and Anticyclones. What are the Air Masses of North America? What are the Major Midlatitudes Disturbances? What are Fronts? What are the Minor Atmospheric Disturbances? What are the two major Atmospheric Disturbances? What is the Major Tropical Disturbances?
BEHAVIORAL OBJECTIVES To define the Characteristics of Air Masses. To identify the movement and Modification in the weather made by Air Masses To depict Weather Forecasting.To depict Anticyclones. To map the Origin of Air Masses--the Source region. To classify North American Air Masses. To recall Thunderstorms NOTE---NOTE--NOTE It would be a very good idea to know the Simplified Classification of Air Masses and the coding system. This is a big one know the different types and Simplified forms. Refer back to the list. Many of them come in to play. To evaluate Tornadoes as the most destructive of all storms. I'll list them here, but you need to know them. Knowing the codes is the key. Little letter big letter. To portray the Nature of Fronts in three dimensions. To map the trail of Tornadoes. To identify:
Maritime Equatorial (mE)
Equatorial oceans.To identify surface of discontinuity and frontal lifting. To compare the Relationships of Cyclones and Anticyclones Maritime Tropical (mT)
Tropical & subtropical oceans.THERE ARE PICTURES TO KNOW ON THESE FRONTS. To investigate Major Tropical Disturbances--
The Hurricanes.Continental Tropical (cT)
Deserts, plateaus in subtropics.To contrast Warm and Cold Fronts To identify the different names for Tropical Disturbances. Maritime Polar (m P)
Seas between 40š & 60šTo identify Stationary Fronts and Occluded Fronts To recognize the Characteristics of Hurricanes Continental Polar (cP)
Subpolar lands.To describe Atmospheric Disturbances (Storms.) To pin point the Origin of Hurricanes. Continental Arctic (cA)
The areas very near the poles.To identify storm tracks. To map the Movement of Hurricanes To plot the trajectory of an Air Mass. To trace the Movements of and recount Cyclones and Anticyclones. To discern the Damage and Destruction of Hurricanes To describe to recognize their Characteristics of Cyclones and Anticyclones. To delineate Minor Atmospheric Disturbances.
INSTRUCTIONAL INPUT
Content Methods: LectureandClassroomdiscussionOVERVIEW:
In normal weather pattern of the midlatitudes there coexists another pattern, a wheel within a wheel. These are the flow of major air masses and storms. They are short lived events and are part of the general pattern of the atmospheric circulation. In some places like the Gulf Coast or the Midwest they play an important role in the weather and affect the climate.
THE IMPACT OF STORMS ON THE LANDSCAPE
Most of the time the atmosphere is rather dull and the only questions are do I need a coat or short sleeves. However, what seems to come out of thin air are the storms, (atmospheric disturbances) which could be called Nature's great special effects. Even a trivial display can make us come indoors. The landscape can change in just a few minutes, from just wetting the streets and ruining a picnic to major property damage and even deaths. Major storms are just part of the routine of the planet and can bring moisture to some places in their path and are major means of energy exchange within the the atmosphere.
I AIR MASSES
CHARACTERISTICS
A. The atmosphere is far from being a consistent body of air, it has many different parcels, which are like many invisible balloons. These large parcels are called air masses. To meet the definition an air mass has three characteristics. First, an ordinary air mass is big more than 1000 miles across and several miles deep it can be subcontinental in size. Air masses must always be thought of in three dimensions. Second, an air mass has an almost homogeneous nature. At any given altitude it has the same temperature, humidity, and stability. Third an air mass is identifiable from the surrounding air even when it moves it keeps the fundamental attributes that it started with.
ORIGIN
B. An air mass forms by taking on the same characteristics of a surface area, like the land or sea. The air mass must hang around long enough (stationary,) just a few days, to acquire the characteristics of that particular land or sea area. Such air is stagnate and stable (high pressure) and most air masses are related with anticyclonic conditions.
SOURCE REGIONS
C. Source regions must be physically the same and tied to stationary air or anticyclonic. Only a few places in the world make good source regions the key is a homogeneous surface. In a little less technical terms, a source region is a large uniform land or water surface which can be the birthplace for air masses. Such regions are ocean surfaces, flat land areas like snow fields, forests, or deserts. (Take a hard look at Figure 8.1 on p. 186. I said a hard look I've mention some of them in passing before,) The map gives details on the major air masses for North America, warm air masses can develop any time of the year over the Gulf of Mexico and Caribbean Sea, the southern North Atlantic and Pacific, and in the summer over the Southwest United States and Northwest Mexico. Cold air masses form in the North Atlantic and Pacific and over the snow fields of north central Canada. (I'll go into detail on each one of these but it would be a good idea to begin to know them.) According to some scientists air masses can form in high or low latitudes but infrequently in the midlatitudes.
SOURCE REGION GIVES ITS NAME TO AIR MASSES
KNOW THE GENERAL PATTERND. The source region gives its name to the air mass over it. The latitude of source region, and the temperature of the air mass are directly link to each other. The surface area also affects the humidity of the air mass. This is how it works, low-latitude air masses are warm or hot, while high-latitude air masses are cool or cold. An air mass over a continent is likely to be dry, while if it forms over an ocean it is moist. This is just common sense, if you know what the climate is below an air mass you can figure out the air mass.
CLASSIFICATION OF AIR MASSES--KNOW THEM--KNOW THEM--!!!!!!!!!!-------TABLE 8.1
E. This idea has given birth to a code of one or two letters to name air masses. KNOW THE TABLE ON PAGE 185 FOR THE CODES AND WHERE THEY ARE FORMED (TABLE 8-1.) This will help you in understanding what I am talking about, if that is not enough reason. . . . Well you are smart people and can figure out that much. This section deals with the names and codes. The key to breaking the code is simple. On two letter codes, the little letter is the place of origin, the big letter is the type of air mass.
The source region is in the Behavior Objectives I'll talk about them as well look at the Table from time to time. THERE WILL BE MORE THAN ONE QUESTION CONCERNING THIS TABLE. I'll go into detail on some of these. A statement in the text might run something like this: In classifying air masses the cold, dry ones are termed continental polar, cP. (Of course, there would be other choices when test time comes.) Look at the type of weather each air mass brings.
1. Maritime Equatorial--mE 4. Maritime Polar --m P 2. Maritime Tropical--mT 5. Continental Polar--cP 3. Continental Tropical --cT 6. Continental Arctic--cA To unlock the key the letters tell you every thing. There are two points to understand air masses. FIRST: As you may have figured out the little m stands for maritime (wetter) and c for continental (drier air). SECOND: To recognize temperature or latitude these symbols are used A (arctic), P (polar), T (tropical), E (equatorial).
Warning! your text just use the symbol so be careful. I'll use both because this is the first time you have ever been exposed to this material. I do want you to know both the symbols and and what they mean.
STABILITY OF AIR MASSES AND TRAJECTORY
F. Some air masses seldom move and stay where they were formed and do not often change the weather in other areas. It is vital to understand those air masses which move, especially when they move over the midlatitudes. When an air mass begins to move over Earth's surface along a path it is known as trajectory. An air mass keeps its homogeneous nature for the most part.
MOVEMENT AND MODIFICATION
G. As could be expected when an air mass moves it changes, the further it moves the more it changes. The changes are caused by thermal modification (heating or cooling from below) and dynamic modification, (uplift, subsidence, convergence,) and turbulence from the winds. Sometimes moisture is added or subtracted.
THE ALBERTA CLIPPER
H. When an air mass departs its source, it takes the traits of the source region with it. The best and perhaps most famous example is one that I referred to once in class. This event happens several times in the winter. A blast of continental polar air, cP, of dry cold air, from northern Canada comes south down central North America like a fast freight train called the Alberta Clipper. Just look at the temperatures at the source of --50š F near Great Bear Lake. On the journey south it does warm a little, but it delivers very cold weather to those places it visits. (I wish it would stay put, I don't like cold weather) During the summer it can cool parts of central North America. Temperature is only one part of the picture changed, there are some shifts in humidity and stability.
II NORTH AMERICAN AIR MASSES
A. Most of us are familiar with the weather of Colorado and perhaps have lived in other places and know about the weather there. We might have watched the news and are familiar with weather in other parts of North America especially storms like hurricanes and tornadoes. The weather patterns of North America are good examples of weather patterns for most midlatitude locations.
FIVE TYPES OF AIR MASSES IN NORTH AMERICA
B. There are five types of air masses which affect the weather of North America. They are:
1. Continental Arctic cA, 2. Continental Polar, cP, 3. Maritime Polar, mP, 4. Continental Tropical, cT, 5. Maritime Tropical, mT. (Take a look at Table 8.1 on p. 185 again also look at the map Figure 8.1 on p. 186 for the locations of the air masses in North America.)
GEOLOGICAL REASONS FOR CONFLICTING AIR MASSES
C. North America is an excellent example of conflicting air masses. The principal reason is geological, the mountains run north and south which allows polar air to rush southward and tropical air to slide northward. Air Masses coming in from the west, the Pacific air masses have several mountain ranges to cross which changes their impact on climate. The mountains and the westerlies also usually protect the West Coast from most continental polar cP.
CONTINENTAL ARCTIC, cA, AIR MASSES
SOURCE AREA THE NORTH POLED. Continental Arctic, cA, air masses which are bitter cold and dry comes from the pole and contains stable air. The source area is the Arctic Ocean. Most of the time, even in winter when this air mass is most developed, it knows its place and stays put. Sometimes it does move south and temperatures drop very dramatically. If Continental Arctic, cA, stays in the Midwest for a protracted time crop damage can be extensive. In the Midwest record-setting cold temperatures often result when a Continental Arctic, cA, invades the region.
CONTINENTAL POLAR, cP, AIR MASSES
SOURCE AREA--NORTHERN CANADAE. Continental Polar, cP, air masses are formed in central and northern North America, Canada is the source region for this air mass. This air mass is cold, dry and stable. Continental Polar, cP, is only formed in the Northern Hemisphere and most developed in winter. Since the only thing stopping a Continental Polar, cP, from going south is a few bard wire fences between Canada the United States. The temperature and humidity rise when it reaches the Midwest. They can reach as far south as the Gulf states causing freezing temperatures. If you need to you can read the description in MOVEMENT AND MODIFICATION in Roman Numeral One. Both Continental Arctic cA and Continental Polar cP are similar in nature and the most important characteristics during the winter months.(Why does Continental Arctic cA or Continental Polar, cP, sometimes move much further south than usual? If you do not know read on and you can put the facts together?)
MARITIME POLAR mP AIR MASSES
SOURCE AREA--NORTHERN PACIFIC AND NORTH ATLANTICF. These air masses are found over water between 40š and 60š north latitude. You should remember that water cools slower than the land and Maritime Polar mP is warmer than Continental Polar, cP. The Maritime Polar, mP air masses begin their life as cold dry, cP air in Siberia (Ask the class what cP means?)but picks up moisture as it moves slowly across the Northern Pacific where it changes to a Maritime Polar mP air mass, but this is only at low-altitudes. At these altitudes the air is moist and relatively unstable. Remember the high altitudes are part of the jet stream and are cold, dry and stable. These are part of the westerly wind system remember how storms can track under the Jet Stream.
This is what I was talking about before, the only thing that is more specific is more information about the nature of the air masses themselves.G. Pacific Maritime Polar, mP, increases cloud cover and drops a lot of water on the windward side of the mountains and the coast, but because of the rain shadow effect and distance it is changed by the time it reaches most of the interior of North America. This is the basic pattern but sometimes even after crossing the mountains of the west Pacific Maritime Polar, mP, can be the source of a Midwestern snow. In summer it does not affect the weather pattern very much.
H. Maritime Polar, mP, air masses also develop over the North Atlantic but does not cause much weather in North America and are more a concern for Europe. Except when there is reversal of winds which can hit New England and the Canadian Maritime provinces. The condition is called a "northeaster" and cool damp winds, rain, and snow, can make life and navigation difficult.
MARITIME TROPICAL mT AIR MASSES
SOURCE AREA SUBTROPICAL ATLANTIC & GULF OF MEXICOI. Maritime tropical, mT, air masses are formed in the subtropical Atlantic and over the Gulf of Mexico. This air mass is warm, moist, and unstable. The weather and climate east of the Rockies, in the United States, most of Mexico, and southern Canada are greatly influenced by this air mass. It is the major source of precipitation in the region. It is more dominant in summer because of increased insolation and longer days. As this air mass moves north during the summer instability increases and becomes a source of thunderstorms in the southeastern and eastern United States. It is the cause for the muggy humid heat in the region. It is convective precipitation and according to some authorities is monsoon-like. (Why is this not a true monsoon?)
PACIFIC mT AIR MASSES
SOURCE AREA PACIFIC WEST OF MEXICOJ. Pacific mT begins over the water west of Mexico and only influences the Southwest United States, and north west Mexico. This air mass is a little cooler than the one on the Atlantic side because it is cooled because of the California, a cold water current. This is the reason for the dry summers of southern California. (What type of climate does Southern California have?)
CONTINENTAL TROPICAL cT AIR MASSES.
SOURCE AREA DESERTS OF NORTHWEST MEXICO
AND SOUTHWESTERN UNITED STATESK. Continental Tropical, cT, is the least important to the weather of the United States. The deserts of the northwestern Mexico and southwestern United States is the source for this air mass. It is a small air mass. For the most part it is not very significant for the weather in North America. However, sometimes in summer it can rush out of the Southwest traveling eastward and could bring heat and drought conditions. The air is hot, very dry, and unstable.
III FRONTS
NOTE IN THIS NEXT SECTION THE BOOK CALLS ATTENTION TO TWO FIGURES MORE THAN ONE ALWAYS LOOK BACK I MIGHT NOT CATCH EVERYTHING. ALSO KNOW THE DIFFERENCES BETWEEN THE TWO!
THE NATURE OF A FRONTS--NEXT TWO POINTS
A. When different air masses meet they do not mix and create a zone of surface discontinuity between them called a front. In simple terms they "Duke it out." The battle ground in North America is the area east of Rockies where tropical and polar air masses change the weather of North America. Latitudes affected by fronts have the most dynamic and changeable day-to-day weather on Earth. Remember air masses keep the characteristics of their source region and the differences in density, air pressure, temperature, and humidity. When these different air masses meet a narrow zone of contact is the front with sloping boundaries. (This is a good time to take a Figure 8.2 on p. 188 and Figure 8.3 on p. 189 and compare them.)
FRONTS ARE THREE-DIMENSIONAL
B. When you turn on the weather on TV or look at a weather map fronts are pictured as nice sharp one-dimensional lines. However, this can be a little misleading because fronts are three dimensional zones of surface discontinuity with swiftly moving air. Fronts can be as small as one or two miles or as wide as ninety miles. You might think of it as blockers on a football team, but in three dimensions with warm air always rising. A basic difference between different fronts is temperature, a front breaks up warm air from cool air. This happens at all altitudes and latitudes. In all fronts warm air is on one side while there is cool air on the other with a steep temperature gradient through the front. (This is a good time to take another look at Figure 8.2 on p. 188 and Figure 8.3 on p. 189 and compare them.) Air masses can also have different levels of density, humidity, stability, and also have steep gradients in the fronts. In short a front is a zone where air masses of different temperature and moisture characteristics meet.
FRONTAL LIFTING
C. The idea of fronts was proposed during the First World War because it seemed that the battle between air masses was like the European battle fields. Except for some minor mixing within the leading edges of the fronts, both air masses keep their distinct character as one pushes the other out of the way. A front has a sloping surface (the slope will vary depending on the kind of front) with the warmer (a warm front) being lifted or rising over the cooler front (a cold front). This is called frontal lifting and is the major source of precipitation in midlatitude regions.
THIS IS NOT TO BAD IF YOU LOOK AT THE FIGURES
KNOW HOW DIFFERENT FRONTS LEAND. A major characteristic of fronts is they lean or are inclined, (This is a good time to take another look Figure 8.2 on p. 188 and Figure 8.3 on p. 189 and compare them.) this leaning lets air masses to be uplifted where adiabatic cooling takes place. They incline so much that they are almost horizontal rather than vertical. This inclination is such that warmer air is always over cooler air (Look again at Figure 8.2 on p. 188.) In a cold front the air slopes backward and at its lower altitudes the "leading edge" comes before the higher altitude, the "trailing edge." In a warm front the "leading edge" is at the higher altitude (Take a look at Figure 8.3 on p. 189) and is forward of the lower altitude "trailing edge," No matter what air mass is advancing the warmer air always rises over the cool air. It may be only common sense but sometimes things obvious need telling, a steeper slope increases frontal lifting assuming all other things are equal.
WARM AND COLD FRONTS
E. Fronts are labeled either warm or cold fronts. The term is relative to each other. The weather which happens depends on which one is moving either a warm or cold front. The front that is moving is called the "aggressor".
COLD FRONTS A WEDGED LIKE EFFECT
MOVE FASTER THAN WARM FRONTSF. A Cold Air Front brings cold air, surface friction delays the lower part of the front and makes the slope steeper as it advances. In fact, the average cold front is twice as slanted as most warm fronts. This wedge like effect causes several movements across the frontal path so pay attention to the action. Cold fronts also move quicker than warm fronts. A rapidly moving cold front, with its steeper slope forces the warmer air above it to an unstable condition and the consequences are blustery winds and violent weather like thunderstorms. (Take a look at Figure 8.2 on p. 188) Vertical clouds are commonplace with atmospheric turbulence and showery precipitation. The precipitation and the clouds are centralized behind the leading edge of the frontal system. The precipitation is heavier, but not long lasting.
CHARACTERISTICS OF WARM FRONTS
LOOK AT THE CLOUDSG. A warm front brings warm air and can have a gentle slope of 1:200. One meter in the vertical for every 100 or even 200 meters of horizontal distance. The warm air slowly climbs over the departing cool air but the temperature of the warm air subsides adiabatically. Clouds and precipitation form, (Take a hard look at Figure 8.3 on p. 183.) Since the uplift is slow the formation of clouds is incremental, but steady. The first indication of a warm front is high-flying cirrus clouds which may be several hours before the front reaches the new location. The closer the front the clouds become lower, thicker,and more expansive.
A warm front causes many hours of steady rain before the front passes. Precipitation falls over an extensive range, such precipitation has a long duration and is gentle.STATIONARY AND OCCLUDED FRONTS
H. Stationary Fronts, like the name, indicates they stay put and are not predictable. There can be some precipitation. When two air masses have converge and formed a frontal boundary but neither is moving a stationary front has been formed. The weather conditions stick around until the two air masses change or the general circulation of the atmosphere moves them. A fourth kind of front is an occluded front, when a faster cold front overtakes a warm front. This happens in the late development of midlatitude cyclonic storm. Stated another way an occluded front is formed by the merger of a cold and warm front.
SIDE BAR OR HOW TO READ A WEATHER MAP--REMEMBER MAPS
I. Just know the general picture, since this is an introductory class and there is no follow up all you to know is the general picture. I could have you chart the weather for a week and know all the symbols and take a look at Weather Maps on p. 198-199, but I'm not. This is what I want you to know.They are different colored symbols which describe different kinds of fronts. (Take a look at Figure 8.4 on p. 189) Since you do watch to know what to wear look at the map and you will see these symbols. The map is also in the newspaper. Just to reinforce these potential questions.
COLD FRONT --A line with solid triangles on one side. If the map is in color the line and the triangles are blue. See the symbols know the symbols.
WARM FRONT--A line with semicircles on one side. If the map is in color the line and semicircles are red. See the symbols know the symbols.
STATIONARY FRONT--A line with triangles on one side and semicircles on the other side. If the map is in color, the triangles are blue and the semicircles are red. See the symbols know the symbols.
OCCLUDED FRONT--A line with triangles and semicircles on the same side. If in color it is purple. See the symbols know the symbols.
IV ATMOSPHERIC DISTURBANCES
VARIATIONS IN THE NORMAL PATTERN
A. The next topic is atmospheric disturbances which occur in the general wind circulation pattern. They are more than just storms and are variations in the normal circulation pattern. Many times these atmospheric disturbances are called extra tropical disturbances in the middle latitudes. Many of them are savage displays of nature's fury and are called storms. However, sometimes they can create the opposite effect, the weather can be calm, clear, and quiet. Both the storms and the calm weather have certain common components of atmospheric disturbances.
1. Both kinds of disturbances are limited compared to the general pattern.
Their size varies a great deal.2. They move over the surface a great deal and are fleeting in nature.
3. While they are occurring they have a short duration and only last a
few minutes, hours, or a few days.4 They are generally predictable as far as weather is concerned and
have distinctive patterns.Highs, lows, front, and conflicting air masses frequent the prevailing westerlies, which is the wind pattern for midlatitude locations. These atmospheric disturbances are a wheel within a wheel.
MAJOR MIDLATITUDE STORMS
B. It is the midlatitudes where the action is as far as changing weather is concerned. The midlatitudes is where tropical and polar air masses clash with different fronts. Storms develop at a front and migrate (track) along it. These storms of polar and tropical air cause the changes in weather in places like the United States. The key word is change it is not uncommon in places like Colorado to have a nice warm day in the morning and face blizzard conditions by afternoon. Perhaps you might remember the late October blizzard of 1997. The two most important of midlatitude disturbances are midlatitude cyclones and anticyclones because of their size and widespread occurrence.
MIDLATITUDE CYCLONES AND ANTICYCLONES---LOCATION
C. In another chapter I've talked about the semipermanent cyclones and and anticyclones which are centers of low and high pressure. Extend the idea to much smaller systems also called cyclones and anticyclones, which are cells of low or high pressure and they move across the midlatitudes because of the westerlies.
MIDLATITUDE ANTICYCLONES--HIGH PRESSURE CELLS
D. Midlatitude anticyclones are other important disturbances in the circulation pattern of the westerlies. Many times they are called a high, H, and are moving high-pressure cells of the midlatitudes. Like Midlatitude cyclones they are migratory in nature, but are larger than midlatitude cyclones and move in the same westerly direction. Midlatitude anticyclones have no clashes of air masses and hence no fronts and are technically outside the high-pressure system. The weather in these systems are clear and dry and do not form clouds. During the winter they have properties of very low temperatures.
MIDLATITUDE CYCLONES--LOW PRESSURE CELLS
E. The most important of atmospheric disturbances are midlatitude cyclones. It is their presence which monopolize weather maps and are the agents of change for most of the weather and precipitation for the populated parts of the planet. They are moving, migratory low-pressure systems, sometimes called lows or wave cyclones in the United States and are called depressions in Europe. Midlatitude cyclones are zones of clashing air masses in regions 35š and 70š of latitude and mostly occur in the westerly wind system. These systems move from west to east since each system is a little different the characteristics are for a typical system.
THIS ANALOGY WORKS
F. Sometimes it is better to find some sort of analogy to realize things in nature. Your book has a good one to describe cyclones and anticyclones by using landforms and wash basin. A cyclone is like a wash basin, as the basin is filled the steeper the sides the deeper the depression. A anticyclone is like a hill with water flowing down the slope increased height and a steeper slope will increase the speed of the water. (Take a look at Figure 8.5 on p. 192)
GENERAL MOVEMENTS
G. Both systems are powerful disturbances in the westerlies, crossing the midlatitudes from west to east. They are driven by the upper air westerlies which can have large oscillations. (They drift from time to time. I'll more about this later.) People living in midlatitudes like the United States look to the west to know about upcoming weather. Storms move around 20 to 30 miles per hour with an average speed of about 23 miles per hour and can cross the United States in about three or four days. Since cold fronts move faster then warm fronts they move faster in winter.
JET STREAM CONDITIONS
H. No two cyclones or anticyclones are exactly alike but they do form patterns and follow general paths know as storm tracks. (Take a look at Figure 8.6 on p. 192.) When conditions are zonal the air is in the normal pattern and this is the condition during most of the summer. However, this is not the condition many times in the winter. The jet stream drifts from the north to the south in a meandering fashion. This is called meridional airflow. When there is upper-level changes in speed or wind direction it almost always means the jet stream has wandered.
V CYCLONES
A. As I've mention several times warm air masses and cool air masses do not mix and where they meet is the polar front. They do not join hands and sing Kumbaya, but move in opposite directions with each air mass bulldozing each other for position. Although the causes for cyclonic formation is not well understood it is believed that a long wave may develop in the upper troposphere and a cyclone is born. In not technical terms something happens in the Jet Stream which are influenced by the Rossby waves. Most cyclones, low pressure cells, are centered below the polar front jet stream. For this concept like many things in the course pictures sometimes work better than words so pay very close attention to (Figure 8.7 and their stages on p. 193. A very good look and follow each stage.)
B. The warm air (a warm front) coming from the Gulf of Mexico heading poleward and the cold air (a cold front) from Canada moving to the equator truly "duke it out" over North America. If you have not figured it fully, the air starts spinning. This is why sometimes it will stop snowing or raining for a time, but in a few hours it starts raining or snowing again. Depending on whether the air is the stable or unstable and the moisture content of the air determines for what type of precipitation may occur.
CYCLOGENESIS
C. Most midlatitude cyclones have a birth cycle of three to six days and disappear in about the same amount of time. Scientists think that the cause of cyclogenesis, the birth of cyclones is upper-air conditions near the polar jet stream. (It seems to me that just in passing I've talked about this once or twice.) There is a interrelationship between the jet stream and ground-level disturbances. When the jet stream is more or less west to east the condition is called zonal, there is little chance for cyclonic activity. In short and in non technical jargon, there will not be a major storm.
ALBUQUERQUE LOWS
D. Cyclogenesis, the birth of cyclones, can happen in another way. This type of birth happens with some regularity on the eastern sides of mountainous areas like in northeastern New Mexico and affects the weather of Colorado a great deal. This is called the Albuquerque Low. A low pressure system floats with the westerlies and diminish in power as it climbs over the Rockies. As it rises the column of air compresses and spreads and slows down its counterclockwise movement. After low pressure system drops on the leeward side, the column of air expands vertically and contracts horizontally. This shape shift makes the cyclone rotate faster and may start a major cyclone. It happens more on the eastern side of the Rockies but can happen on the eastern side of the Appalachians. These systems move east and northeast and can drop heavy rain or snow.
NOTE-- NOTE
I am going off text a little bit but your book talks about Cyclones and Local Weather and you should read it just for reinforcement. The thing I do want you to know is the cutline on Figure 8.8 on pp. it goes into more detail than I just did. It also gives you a bird's eye view on most general cyclones.
E. The ordinary midlatitude cyclone has a diameter of 1000 miles and is an expansive cell of low-pressure and has an oval shape. Their circulation pattern is counterclockwise in the Northern Hemisphere. (What causes the counterclockwise flow in the Northern Hemisphere?) The winds generated by a midlatitude cyclone draws cool air from the north and warm air from the south. The intersection of these two different air masses makes two fronts: a cold front which extends to the southwest from the center and a warm front which extends to the east from the center. These separate fronts break up the cyclone into a cool zone in the north and a warm zone in the south. Remember air masses come in three-dimensions, in most midlatitude cyclones there is a distinctive trough from the southwest which sees a drop in air pressure. Do your remember the device used to measure air pressure and how it is measured? (Why do weathermen mention air pressure when there is a cyclone?)
VI THUNDERSTORMS
A. There are various types of minor atmospheric disturbances in the world. Many occur are only in a localized area and many are destructive, but none of them are as large as midlatitude cyclones and anticyclones.
THUNDERSTORMS
B. A lot of this material is review from the last unit so bare with me for a little while. A thunderstorm is violent and usually a convective storm accompanied by thunder and lighting. It is a brief and vertical event. They are very common in middle and lower latitudes. Lighting is an intense discharge of electricity. Lighting is an extraordinary light show when positive and negative electrical charges are generated in a cloud. The theory is that positive charges are in the upper part of a cloud while negative charges are generated in the lower part of a cloud. Don't get shocked over the next statement but the heat generated is over 45,000 š F. Thunder is a shock wave created by the rapidly expanding air. A great deal of humidity and instability creates a cumulonimbus cloud and usually produces precipitation, for the most part they are summer events.
REMEMBER THE STAGES
C. Thunderstorms often accompany hurricane, tornadoes, and fronts and (especially cold fronts,) but these thunderstorms maybe formed by other means than convection. The warm moist air frees latent heat of condensation to reinforce the continuing rise of air. The first stage of a thunderstorm is called the cumulus stage, where updrafts are dominate and clouds mature. The upper freezing levels and the supercooled water droplets and ice crystals coalesce and fall because of their own weight. The water in the descent creates precipitation and a downdraft. At this stage the storm has is in the mature stage where updrafts and downdrafts are coexisting. The cumulonimbus clouds have an anvil top and the whole system is a typical thunderstorm. After the storm has spent itself, downdrafts are the most prominent and the storm is in the dissipating stage.
OROGRAPHIC THUNDERSTORM
D. There is one form of thunderstorm is common when air is forced to rise over mountains. This is called an orographic thunderstorm. The mountains give the air the beginning push which leads to convectional cells. These cells play an important part in the monsoons of South and Southeast Asia. In the Rockies and the Sierra Nevada orographic thunderstorms can cause problems especially during the summer. Small planes try not to fly in the afternoons during these conditions. (What is the very large mountain range that affects the monsoons of South Asia?)
E. Thunderstorms as a rule are unknown poleward of 60š and they occur more over land than the oceans because summer temperatures are higher over land. (I think I remember that from another chapter.) If you remember any summer in Colorado thunderstorms are tied with cold air fronts which makes warm air rise. Forming often ahead of Cold front is the squall line which is part of frontal lifting.
VII TORNADOES
A. Tornadoes seem to pop up out of thin air and are very random. They start just a few hundred feet above ground level becoming visible when water vapor is shaped into the distinctive funnel shape cloud. In the United States they move in unpredictable track from the southwest to the northeast. The United States has the most reported tornadoes in the world. About 90% of all sightings are in the central and southeastern states. Tornadoes are so common that part of the United States Kansas, Texas and Oklahoma are called "Tornado Alley." (Take a look at Figure 8.11 on p 202 and Figure 8.12 on 203) The reason for this is the flat nature of the landscape which allows Canadian, cP, air to come into contact with Gulf, mT, air masses. Like I've told you before unlike air masses do not get along.
DYNAMITE IN SMALL PACKAGES
B. Sometimes things in small packages can cause the most damage. Tornadoes are small and localized. They are the most devastating of all atmospheric disturbances. They are small cyclonic storms with very low air pressure. Most tornadoes are only about 400 yards wide, about a quarter of a mile and have the most extreme pressure gradient, as much as 100 millibars from the center to the outside of the funnel cloud. In general, the Tornado has the lowest pressure of all cyclonic disturbances.
DOPPLER RADAR
C. It is the vortex in a deep low-pressure cell encircled by a very, very fast cylinder of wind. Until very recently the specific wind speed was unknown because the few times an anemometer, a device used to measure wind speed was smashed to smithereens. Estimates place the wind speed some where between 200 to 500 miles per hour with winds of these speeds a tornado could take Dorothy and her little dog to Oz. Because of their small size they were very difficult to predict, let alone, to get wind speeds. However, new technology, the Doppler Radar has made analyzing storms and detecting air turbulence much more accurate.(Take a look at Figure 8.13 on p. 204) This radar can even detect pollution, insects, and has other uses besides storm tracking. (Where else could Doppler radar be very useful and what is the condition that it is looking for?)
FUJITA INTENSITY SCALE
D. Prior to Doppler Radar the winds of a tornado could only be estimated by the damage that they caused. A scale was developed to assess the damage, which is called the Fujita Intensity Scale. (Take look at Table 8.2 on p. 205) The tornado funnel brushes the ground annihilating everything in its trail. The trajectory is like a pin ball. Their devastation zone is very narrow from about 150 feet to 330 feet wide, it is not usual for one side of a street to be in ruins and the other side to just have busted windows. Most of the time the bottom of the funnel only touches ground for a few minutes and they move about thirty miles per hour. About two-thirds of tornadoes in the United States are "weak," thirty per cent "destructive" and two per cent are "violent."
TORNADO MYTHS??
E. The dark funnel shaped cloud is colored from dust and debris vacuumed up by the winds of the vortex. According to some books, it is more than folk-tales about buildings exploding because of the low-pressure of the vortex of the tornado passing over a shut-up building with relatively high pressure.
F. Like other storms the mechanics are not quite understood. They begin when there is warm, unstable air with a midlatitude cyclone. Tornadoes are often born along a squall line that comes before a rushing cold front. Many times thunderstorms and tornadoes go hand in hand and tornadoes come out of the thunderhead, however most thunderstorms give birth to tornadoes.
SPRING AND EARLY SUMMER ARE THE MOST LIKELY FOR TORNADOES
G. Although tornadoes can develop in any month, there are certain months and times of day which are more favorable for their formation. Spring and early summer and afternoons are the best times for tornado formation, (Take a look at Figure 8.12 on p. 203.) June is the peak month, and mid afternoon is the prime time, but some nasty tornadoes in recent years have come in the spring and in the early morning.
VIII HURRICANES
For ease of reference unless I am being specific all tropical cyclones will be called hurricanes, BUT REMEMBER Hurricanes are tropical cyclones.
CHARACTERISTICS--HURRICANES AND WIND SPEED
A. Hurricanes are low-pressure centers with very circular formations and have steep pressure gradients from the center. (Take a look at Figure 8.16 p. 207) As could be expected a hurricane has strong winds which spiral inward and must reach 64 knots, 74 miles per hour, to be classed as a hurricane. Many times those wind speeds are doubled and sometimes tripled.
STORM SURGE
B. The major killer in a hurricane is not so much the wind but the storm surge which can flood whole communities.The winds and the rains of a hurricane can cause a great deal of damage, but the real power of the storm and the greatest loss of life are the high-seas. Since a hurricane is a low pressure system, the rising water levels, this is called the storm surge of wind driven water can rise as much as twenty-five feet over normal levels. (What monthly event can even make the storm surge even worse?)
THE EYE OF A HURRICANE AND EYE WALL KNOW THE DIFFERENCE
C. Hurricanes have a diameter between one hundred and six hundred miles, which is much smaller than midlatitude cyclones. The exceptional part of a hurricane is the non stormy eye which is in the center of the storm. (Take a look at Figure 8.16 p. 207) The eye has a diameter between ten to twenty-five miles, and is the only place where there is a calm in a violent storm. Within the eye, there is no rain and low clouds and scattered high clouds and light winds the movement of the winds in the eye is downward and intermittent sunlight. Sailors have reported even seeing birds flying in the eye of a hurricane. In the eye wall of a hurricane things are very different. It is in this zone where the winds are the strongest and are at the edge of the eye.
SAFFIR-SIMPSON HURRICANE SCALE
D. All hurricanes are formed the same way, but not all hurricanes are equal. Some are more intense then others. The United States has a scale based on wind speed, pressure, and storm surge, of course they are interrelated, which rates the power of these storms. The name of the scale is the Saffir-Simpson Hurricane Scale and the categories run from one to five. (Take a look at Table 8.3 on p. 208 for damage reports.)
ORIGIN AND FUEL
E. Hurricanes are tropical phenomenons and develop over the warm oceans of the world. Since the Coriolis effect is negligible near the equator, no hurricane has been sighted within 3š of it and no hurricane has ever crossed it. More than 80 percent of all hurricanes originate on the the poleward side of the intertropical convergence zone. A violent storm like a hurricane need fuel, a supply of energy, to make it run. This fuel is provided by the latent heat freed when water vapor condenses. In less technical terms, the major energy source of hurricanes is warm sea water. The energy produced by just an average hurricane is equivalent to all the electric power plants in the United States. (What has changed in recent years that could make some storms bigger or lesser killers? This is a two part question)
F. The names of tropical storms, come from a list in the three tropical storm tracking centers; Miami, Honolulu, and Guam. Each center has separate zones in the world's oceans to watch. (What movement pressured the government to add male names to the list because they thought only female names were sexist?)
MOVEMENT AND DAMAGE AND DESTRUCTION
G. The general location of hurricanes is predicable. They are formed is about six locations of low-latitude, and are most frequent in the North Pacific east of the Philippines and west of southern Mexico and Central America. The third belt is the North Atlantic and the Gulf of Mexico/Caribbean. Another zone is the South Pacific and the Indian Ocean. They do not exist in the South Atlantic and the southeast part of the Pacific because the water is too cold and are zones of high pressure.
OTHER NAMES FOR HURRICANES
H. Most of the weather over the tropics is just plain dull. It is hot and humid and the temperate only varies a degree or two in winter and summer, however, it does rain a lot. The only break in the boring routine are fleeting atmospheric disturbances called tropical cyclones. These tropical cyclones go by other names in different parts of the world. In the Americans they are called hurricanes, typhoons, in the western and North Pacific, baguious in the Philippines,. and cyclones in the Indian Ocean and Australia. No matter what they are called, they are severe, spinning, water laden, moving, devastating, storms, which take place in the tropics and subtropical areas in the world. All tropical cyclones are formed in the same way, like hurricanes.
I. The largest and strongest tropical cyclones, hurricanes, are associated with the South China Sea and are from off the coast or East Asia. The track is erratic compared to the trade winds and a specific course is hard to follow. On average directional movement of hurricanes is east to west and around one third steer a straight course due east, but two thirds start east, then turn veer toward the poles. When they hit a continent they either dissipate or get caught up in the westerlies. When they encounter land, their source of power, warm sea water, is cut off.
J. No matter what course a hurricane takes they usually last for about a week, with a month being the longest life of a hurricane. As I referred to before in passing there is a hurricane season in North America, for the most part they start in late spring and decrease in the late fall with September usually being the peak month.
KILLER STORMS
K. These storms are killers North America's greatest hurricane disaster took place in Galveston Texas in 1900 which killed 6,000 people. Hurricanes have cost thousands of lives in the flat low lying river deltas of the Ganges and Brahmaputra deltas of Bangladesh.
L. Despite the destructive nature of these storms many regions like northwest Mexico, northern Australia, and southeast Asia depend on a hurricanes rains for agriculture. It depends on the track on the storm. However, sometimes it does not pay to mess with Mother Nature.
GUIDED PRACTICE Questions during the lecture. INDEPENDENT PRACTICE Readings at home Culmination: What is the nature of an Air Mass? List the Air Masses of North America? Do Fronts like each other or do they Duke it Out? The two major Atmospheric Disturbances are? Thunderstorms and Hurricanes have what type of pattern?