ATMOSPHERIC PRESSURE AND WIND #
GEOGRAPHY 101 Front Page
ATMOSPHERIC PRESSURE AND WIND
MATERIALS 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 ATLAS
GENERAL OUTLINE
V. Atmospheric Pressure and Wind
A. To paint Impact of Pressure and Wind on the Landscape.
B. To outline the Variations in Atmospheric Pressure --Vertical Variations, Horizontal,
C. To paint an Idealized World Pressure Belt.
D. Mapping Pressure Distribution--Know about Isobars and Pressure Gradient.
E. To define the Nature of the Wind--Pressure Gradients and Winds, the Coriolis Effect and Wind, Friction and Wind.
F. To compare and contrast Cyclones and Anticyclones and Convergent and Divergent Circulation.
G. To define Wind Terminology.THE MATERIAL IN THIS OUTLINE COVERS PAGES 116--130 BUT THERE IS MATERIAL NOT IN YOUR BOOK--KNOW IT--KNOW IT--KNOW IT
ANTICIPATORY SET
How do meteorologists measure air pressure? How is Air Pressure mapped and what is the pattern of Air Pressure? What are Variations in Atmospheric Pressure ? What is the Nature of the Wind? What is are the Vertical Variations in Pressure? How and Why does it move? BEHAVIORAL OBJECTIVES
To define Pressure and Atmospheric Pressure. To describe Horizontal Variations in Pressure. To identify trough, equatorial low, subtropical highs, subpolar lows, and polar highs. To identify the barometer and compare it to millibars List the four guidelines concerning pressure. (These are identified in this chapter very briefly later in the next chapter on circulation they To outline Vertical Variations in Pressure. To recall an Idealized World Pressure Belts. Remember this is a model become very important as a source of winds. To recall the development of pressurized aircraft. To identify the January conditions Siberian High, Canadian High, Icelandic Low and Aleutian Low To describe Mapping Pressure with Isobars. To contrast Cyclones and Anticyclones. To identify the July conditions Pacific High, Bermuda High or the Azores High, To recall Coriolis Effect and the Effect on the Wind. To recognize Low Pressure Cells as cyclones To identify updrafts, downdrafts, ascents, and subsidences. (These are not in your book so pay attention. To identify the element of Friction. and the Wind--Geostrophic Wind. To recognize High Pressure Cells as--anticyclones. To define wind. (Air in a hurry) To trace the movement of the wind in Cyclones and Anticyclones To portray the General Pattern of Atmospheric Pressure. The next several points are linked To recognize Convergent and Divergent Circulation To relate the Seasonal Variation in the Pattern. To portray the Direction of Movement and the factor which influence the movement. To recount Wind Terminology
Windward & Leeward
Knots & Prevailing winds.Note: The Times of the Year To relate Pressure Gradient INSTRUCTIONAL INPUT
Content Methods: LectureandClassroomdiscussionOVERVIEW: THE IMPACT OF PRESSURE AND WIND ON THE LANDSCAPE
Most climate and weather phenomenon are very easy to recognize. It can be too hot or too cold, it can be very windy and too wet as well. Pressure on the other hand can usually be felt when we take at trip to the mountains and our ears pop, but pressure is one of the invisible hands which moves weather. The role of pressure is more by proxy and can be felt when the wind blows and temperatures change. Most of these changes are very short-term, but the effects of wind, like hurricanes can cause massive property damage, not to mention the cost in lives.
I DEFINITIONS
PRESSURE
A. There are three states of matter, solids, liquids, and gases. The molecules in solids and liquids have more cohesion than gases. Solids and liquids know their place and stay put, but gases are different. Gases could be called hyperactive matter, which are in perpetual motion, always bumping into each other and into any surface they come in contact with. Imagine a red balloon that we fill up with air. The molecules are agitated and constantly banging against the walls of the balloon. This is pressure, it is the force gases exerts on a particular area. on the container walls of the balloon.
ATMOSPHERIC PRESSURE--OMNIDIRECTIONAL
B. The atmosphere, which is made up of gases, is like those gases in the balloon. Atmospheric pressure is a force which pushes against everything on the planet. At sea level it is 14.7 pounds per square inch and as you recall it is only about half of that at 18,000 feet. The force is omnidirectional and is more like a gloved hand than just a blanket. The bodies of all living things are balanced to the same pressure because of evolution.
MEASURING PRESSURE
MILLIBARS PREFERRED BY SCIENCEC. So far I’ve told you what air pressure is but have not told how it is measured. The mercury barometer is based on experiments made by Evangelista Torricelli using a sealed tube filled with mercury and open dish of mercury leaving a vacuum at the top of the tube. (Take a look at Figure 5.1 on p. 118.) Most television and newspapers still report so many inches of mercury using a barometer. Meteorology prefers a different unit of pressure called the millibar (mb.) Standard sea-level pressure is 1013.2 mb or 10 millibars. This is assuming temperature remains constant. The reason why standard sea-level pressure is important is this the standard for mapping pressure. (I’ll talk about this just a little later.)
II VARIATIONS IN ATMOSPHERIC PRESSURE
VERTICAL VARIATIONS IN PRESSURE
A. The Earth’s gravity holds the atmosphere in place, otherwise it would escape into space. It could be said that gravity is the invisible glue which keeps the universe together. The force of Earth’s gravity is stronger on low-altitude molecules than high-altitude molecules. The significance of Earth’s gravity on the lower altitudes is the gas molecules are more concentrated because of the influence of Earth’s gravity. Since molecules in the lower atmosphere are denser there are more collisions and pressures are higher.
ALTIMETER
B. Just as could be expected the air at higher altitudes is less dense and has lower pressure. No matter what level in the atmosphere, pressure and density are directly proportional to the air density at that altitude. You might ask, except for being on a test why are these things important? If you get on an airplane it might be important to know how high you are, otherwise you could crash into a mountain. The device used to measure altitude is called an Altimeter, there are two types: Aneroid Altimeter, or Barometric altimeter, which measures atmospheric pressure. It works on the same principle of mercury barometer. A small chamber is partially emptied of air and is connected to a mechanism that registers changes in pressure.
PRESSURIZED AIRCRAFT
C. Your book has one analogy of a pileup during a football concerning atmospheric pressure is a good one, read it, but I have another. The Atmospheric pressure can be thought of as a blanket of air, air has weight like a blanket. Right now I just want to talk about the link between pressure and altitude--the vertical structure of the atmosphere. As a person increases altitude there are less blankets of air, (pressure) at 18,000 feet pressure is less than half of what it is a sea level. Most of the gases making up the atmosphere are below 3.5 miles, a tad over 18, 000 feet. The decrease in pressure, less oxygen, made flying difficult at high altitudes. The brain can be damaged at 20,000 feet and prolonged flights above 15,000 feet requires an additional supply of oxygen. These problems were dealt with just before and during World War II. The airman needed an oxygen mask and very heavy cloths, remember, it gets colder the higher you go, or a special suit to supply oxygen and heat artificially. These things can be bulky and tiring on airmen, so pressurized aircraft were developed. Now commercial jets fly at 36,000 feet or higher.
III HORIZONTAL VARIATIONS IN PRESSURE
OVERVIEW:
PRESSURE, DENSITY, AND TEMPERATUREDensity and temperature can effect the pressure in the atmosphere and changes in any one of the other two can effect changes in the other two. An analogy might be the three legs of a tripod, shorten or lengthen one of the legs the other two are affected. If you just think tripod the next several points will make more sense. I know the material is new to most of you and can be difficult--THINK TRIPOD IT WILL WORK MOST OF THE TIME. (Of course there will be exceptions to the tripod model.)
THERMAL AND DYNAMIC CHANGES
A. The horizontal changes in pressure can be classified into two categories thermal caused by temperature and dynamic associated with motion. This is another key concept of pressure. I am going to talk about thermal changes first, then dynamic changes.
PRESSURE AND THERMAL CHANGES
B. In the last chapter I talked about how different surfaces heat and cool. One of the basic rules of gases is “that pressure and density of a given gas vary inversely with temperature.” During the day temperatures increase, the air expands in volume, and density decreases. The warm air rises and the surface air pressure decreases. The region around the equator is a region of low pressure. While near the poles there is an increase of air density and decrease in volume. This condition makes the air subside and high pressure. Although this might go against common sense, warm temperatures are related with low pressure and cool weather with high pressure. Pressures like temperatures are relative to each other (The next several points should explain it.) (Take a look at Figure 5.2 on p. 119.)
DYNAMIC VARIATIONS
C. It would be logical to assume that there would be a progressive increase in pressure from the equator to the poles, but real world pressure is different. There are areas of high pressure in the subtropics and areas low pressure in the subpolar regions. The zones or belts of high or low pressure are more complicated than just thermal activity. The reason for this apparent inconsistency is the dynamic (motions) action of the Earth.
DYNAMIC FACTORS
D. The dynamic factors are the rotation of the planet and the great pattern of circulation of the ocean and atmosphere. Warm air rises from the equator and moves in a poleward direction, but the Earth’s rotation deflects the air to the east. When it reaches the subtropics, the air has changed direction and is now flowing west to east. The deflection (bending) causes the air to stack up over the subtropics, which increase air pressure at those locations.
E. There are high pressure areas over the poles and subtropical zones, dynamically induced zones of low pressure are formed between them in the subpolar regions. One of the consequences is air flows (you could call it down hill) from the highs to lows, where it will rise. (Note: This is leading
to another concept called the pressure gradient)FOUR GUIDELINES CONCERNING PRESSURE
F. There are four guidelines which work most of the time when dealing with pressure and which work most of the time. Take a look at the next section and see how will the guidelines will work. Link the conditions of pressure to what type of high or low is created. Surface pressure conditions many times can be an indicator to what factor will be predominant.
1. Very warm surfaces often produce low pressure at the surface (a thermal low)
2. Strongly rising air often produces low pressure at the surface (a dynamic low)
3. Very cold surfaces often produce high pressure at the surface (a thermal high)
4. Strongly descending air often produces high pressure at the surface (a dynamic high)
VI IDEALIZED WORLD PRESSURE BELTS
A. One of the things that Geographers like to do is make models and they have constructed a hypothetical model of pressure zones of the world. (Take a look at Figure 5.2 on p. 119.) The notes and the book will compare the theory and the real world. (Does any one have some small idea on what will change the model?)
B. Straddling the equator in the model is a zone of low pressure or trough. Since this belt has the maximum amount of yearly heating, the model indicates that the low pressure, the equatorial low is caused by thermal action which causes the air to rise. (Remember the film.)
SUBTROPICAL HIGHS--LOCATIONC. North and South of the equator are the “horse latitudes” (There is a good reason for the name, but it will be in the next chapter.) They are located about 30° N and 30° S and are cells of comparative high pressure. These high pressure cells are called Subtropical highs (STH.) They are caused by the dynamic action of sinking of convection cells generated by the equatorial low. (This one is very important in the next chapter so read it again.)
D. Going towards the poles in both Hemispheres are expansive zones of low pressure ranging through these subpolar lows until about 65° latitude. Dynamic elements are the active force in the formation of subpolar lows.
E. At the poles are places of high-pressure systems called polar highs. They are caused by cold temperatures and the sinking (descending) of dense air over the poles.
IV MAPPING PRESSURE DISTRIBUTION
A. As I’ve mentioned, Geographers love to map things air pressure is no exception, it is the invisible hand which moves the weather. Compare the maps of the planet with the model. Again the pressures are converted to a common factor. Remember elevation could throw the entire map off so the common factor is sea level, using the same principles as a temperature map. Maps communicating air pressure are qualified or modified maps. This concept is even more important when charting air pressure because of the dynamic factors of the atmosphere.The pattern displays the horizontal nature of air pressure in a given region.
WEATHER STATIONS AND ISOBARS
B. The places which record air pressure are called weather stations. They record other things like temperature and precipitation as well. Weather stations record the number of millibars at a given location, dots of equal value are connected and form isolines of equal value. This type of isolines form isobars and form an isobar map.(Take a look at Figure 5.3 and Figure 5.4 on p. 121. By the way take another look I’ll be talking about them again and pay attention to the months of the year.) Like so many things in this class it is better seen than heard. The most striking feature is the nearly circular patterns of either high or low pressures. It should be understood that it is a relative condition reflecting the highs or lows in surrounding areas.
PRESSURE GRADIENT AND THE WIND
C. The highs and lows are the extremes in any given area. They can be separated by either a “ridge” of high pressure, or by a “trough” of low pressure. Remember what close lines mean on a contour map, a steep increase in elevation, the same principles apply here. These are known as the pressure gradient. It is just like the contour map, a “steepness” of the pressure slope.
Isobars, which are close together, indicate a major pressure change or a strong pressure gradient. While lines further apart indicate a weak pressure gradient.LOWS=CYCLONES
HIGHS =ANTICYCLONESD. The circular patters of pressure are called cells of either high or low pressure. Low pressure cells are usually called lows, or cyclones and cells of high pressure are called highs, or anticyclones. (Cyclones and Anticyclones will become even more important in another section.)
VI THE GENERAL PATTERN OF ATMOSPHERIC PRESSURE
LANDFORMS
A. The model implies that there will be bands of high and low pressure running east to west and are latitudinally structured. The difference between the model and real world are the land masses. The continents break up the belts of high and low pressure and form cellular pressure systems. The land masses influence the formation of high and low pressure belts in several ways. The most important difference is that land and water heat and cool at different rates. The continents slow down the movement of air in general and orographic barriers, mountain ranges are obstacles in the way of the free movement of air.
CHANGES DURING THE SEASONS
B. This one you probably can figure out for yourself if you think about it. The atmospheric pressure belts, like temperature belts, change positions because of the seasons. They meander northward in July and southward in January in the Northern Hemisphere. (What is the movement in the Southern Hemisphere?) This migration is between the Tropics of Cancer and Capricorn because of changes in temperature. (Take a look at Figure 5.3 and Figure 5.4 on p. 121.) The pressure systems do not change very much in low latitudes because of the small temperature changes. However, in high latitudes where there is a difference between hours of sunlight and the angle of the sun there will be more changes in pressure and temperature. The temperature extremes are more substantial in the Northern Hemisphere where the land takes up 40 percent of the total surface while the Southern Hemisphere is only 20 percent land.
AIR PRESSURE IN JANUARY
C. Since land cools faster than the oceans, the temperatures will be lower than nearby oceans. (Take a look at Figure 5.3 on p. 121) This situation influences the formation of high pressure cells over land.
At the same time subpolar lows materialize over the oceans since they are relatively warmer. Above east Asia there is a strongly developed anticyclone during the winter called the Siberian High and a similar but poorly developed system called the Canadian High.WINTER CONDITIONS--STORMS
D. In the North Atlantic low-pressure cell called the Icelandic Low. Another cell of low pressure develops in the North Pacific called the Aleutian Low. Since the air the two low pressure cells have relative lower pressures compared to the two subtropical or polar high systems the air moves toward these cells from the north and south . These low pressure cells are associated with cloudy, unstable weather and are the origin of winter storms in places like Colorado. (There is more to winter storms than just a Aleutian Low, but this is all you need for now. Just keep the location in mind. Later we will find out how an Aleutian Low gets to Colorado)
E. In winter in the midlatitude high-pressure cells are associated with clear, blue skies, calm, starry nights, and cold stable weather. In the winter cloudy conditions are tied to the oceanic lows, while clear weather is tied to continental highs.
AIR PRESSURE IN JULY
F. During the summer the anticyclone is very weak over the North Pole because heating of the ocean and the continents, remember the length of day increases. (Take a look at Figure 5.4 on p. 121.) The Aleutian and Icelandic lows almost disappear. Over the landmasses of Eurasia and North America low pressure cells develop, in Asia a low-pressure system is formed but it is broken into two separate cells by the Himalayas. (Take a look at Figure 6.8 on p. 144) The low-pressure system above northwest India is very strong and combines with the equatorial trough, which has shifted from its winter location. (Taking the information which you have just learned, and any other. What weather phenomenon is connected with the low pressure system over Northern India in June and July?)
G. The subtropical highs in the Northern Hemisphere are more developed over oceans than continents. They also journey northward and are extremely important factors in the climate of the continents. In the Pacific the subtropical is designated the Pacific High and has a very important part in moderating the temperatures of the West Coast of North America. In the Atlantic a similar formation serving the same function is called the Bermuda High by Americans and Azores High to Europeans.
There is a reason why the text and the notes have spent so much time on air pressure as was mentioned in the first paragraph of these notes, pressure is one of the invisible hands which moves weather. Hopefully this will become clearer in the next set of notes.
VII WINDVERTICAL MOVEMENTS OF AIRFLOW
UPDRAFTS, DOWNDRAFTS MINOR AIR MOVEMENTS
ASCENTS AND SUBSIDENCES MAJOR AIR MOVEMENTSA. As the film, the text, and the notes have pointed out many times the atmosphere is always moving. The direction of the air flow has many components. Sometimes air flow, the wind, is laid back, other times it can blow a house down. These movements have both horizontal and vertical variations. As could be expected a simple word like wind is not used in discussing the vertical movement of air. I’ll briefly define the vertical movement of air. Minor vertical movements are called updrafts and downdrafts. Major vertical movements are ascents and subsidences. These terms are only used for vertical movements.
HORIZONTAL MOVEMENT OF AIR
B. The term wind is used for the horizontal movement of air. It could be said that Wind is “air in a hurry.” It reacts to changes of air pressure. The Winds could be thought of as one form of a thermostat, to regulate the air pressure and temperature of the planet. Air pressure, temperature and density are three legs of the same tripod. In general, places below of 38° store up more solar energy than they lose, while places poleward lose more insolation than they gain.(What is the absolute location of Denver and Pueblo--Can you see a pattern?)
C. The general circulation of the atmosphere (the great wind systems) influences the ocean currents which are another thermostat for the planet. In addition to the advectional transfer of heat, the winds carry water vapor from the oceans to water the land where it condenses releases latent energy.
DIRECTION OF MOVEMENT
PRESSURE GRADIENT, THE CORIOLIS EFFECT, AND FRICTIOND. The winds are created by the unequal radiation of the Sun on separate parts of the Earth’s surface. Remember that different surfaces heat differently. This difference in heating causes temperature gradients, which create pressure gradients and set the winds into motion. The rule of thumb is the wind flows from areas of high pressure to areas of low pressure. If the Earth did not spin and friction did not exist, all flows would be from high pressure to low pressure. The course of the winds are determined by three factors: pressure gradient, the Coriolis effect, and friction.
PRESSURE GRADIENT AND WIND SPEED
E. The best way to imagine high and low pressures is to think of them as water. A high pressure area can be thought of as a hill and the low pressure area as a valley. The air in the high pressure area flows down hill just like water flowing down a hill, but the movement is horizontal in nature..
(Take a look at Figure 5.5 on p. 123.) The speed of the wind is established by the pressure gradient. The steeper the slope, the faster the wind, if the gradient is gradual, the movement is slower.CORIOLIS EFFECT AGAIN
F. Since the Earth rotates, things like air and water, which move independently, near the planet’s surface drifts to the right in the Northern Hemisphere and to the left in the Southern. This is again the Coriolis Effect. If you remember, the Coriolis effect varies depending on what part of the planet you are located. As latitude increase so does the Coriolis effect with the maximum deflection at the poles and no deflection at the equator and is a factor in navigation. (Take a look at Figure 5.6 on p. 126) (If you are very curious you can read in your book about the problems encounter by rockets from the North Pole to the equator.) Like many things in this class the wind has another influence besides the pressure gradient and the Coriolis effect.
CORIOLIS EFFECT AND GEOSTROPHIC WIND
G. The Coriolis effect is an important influence on the winds in the upper troposphere. The Coriolis Effect restricts or deflects the wind from flowing down the pressure gradient and the pressure gradient deters the Coriolis effect from turning the wind up the pressure slope. When the two factors of pressure gradient and the Coriolis Effect are in balance and the wind moves parallel to the isobars is called a geostrophic wind. However, near the surface is the factor of friction. To see the interplay between Coriolis effect, the pressure gradient, and friction. (Take a look at Figure 5.7 a and b on p. 127.)
FRICTION AND WIND
H. The third factor influencing the winds is friction. It is the drag of the Earth’s surface which slows down the winds and changes its direction. The Coriolis effect is reduced, but the pressure gradient is not changed. The winds do not flow in the same matter as upper atmosphere winds. The direction of flow is more oblique across the isobars. (Take a look at Figure 5.7 b p. 107.) What are some of the big things that can change the movements of the winds?)
I. Friction is greatest at lower elevations and decreases at higher elevations. Friction or drag varies with the surface, wind speed, time of day and year and other atmospheric conditions. Depending on the text you are looking at, the friction layer goes up to about 1650 feet, 3,300 feet according to your text book, or 5,000 feet according to my old text book. (I just point out this fact that experts can disagree on some facts.) higher than that, most winds track in a geostrophic course.
VIII CYCLONES AND ANTICYCLONES
HIGH-PRESSURE CIRCULATION PATTERNS
A. Despite the variables in wind flow like pressure gradient, Coriolis effect, and friction, there are certain patterns. These patterns form around high-pressure and low pressure centers. Centers of high-pressure are called an anticyclones in the Northern Hemisphere. In this high pressure cell the air flow is moving from the center in all directions down the pressure gradient. The moving air is defected to the right. Therefore, the wind moving out of an anticyclone in the Northern Hemisphere will move from the high pressure cell in a clockwise spiral (Take a look at Figure 5.8 on p. 128)
LOW PRESSURE CIRCULATION PATTERNS
B. The air moves down pressure gradients from all directions toward the center of a low pressure center called a cyclone. Since the air flow is defected to the right in the Northern Hemisphere, the winds move into the cyclone and move in a counterclockwise spiral. The same principle works in the Southern Hemisphere, the winds move away from an anticyclone in a counterclockwise spiral and the winds moving into a cyclone move in a clockwise spiral. Remember that in the Southern Hemisphere the circulation patterns of the wind and ocean currents are mirror images of those in the Northern Hemisphere.
CONVERGENT AND DIVERGENT CIRCULATION
C. Air does not just move in a horizontal pattern, but some times moves vertically. This can be seen with pressure centers. (Take a look at Figure 5-9 on p. 129) Air has convergent (rises) circulation in cyclones--low pressure cells, with the wind blowing toward the center. The winds of an anticyclone-- blow away from the center of high pressure and are called divergent wind circulation . Cyclonic circulation is converging no matter whether it is in the Northern or Southern Hemisphere. Anticyclonic circulation is diverging under the same concept. The spiral clockwise or counterclockwise make no difference.
WIND TERMINOLOGY
D. The winds are designated from their source . The example for midlatitude locations is the westerlies, which come from the west but are going east. Your book uses the examples of northeast and south winds the principle is the same. (This will become very important in the next chapter.)
E. Windward is the path from where the wind blows . The side facing the direct path of the wind is called the windward side . (Take a look a Figure 5.10 on p. 129) The leeward is the direction the wind is blowing and is sheltered from the wind. Although winds can come from any direction many times there is a pattern called prevailing winds. Meaning as a general rule they come from one direction more than another. (On what side of the Library is the windward side, the leeward side and what is the prevailing wind pattern in Colorado? Just look three possible Easter Eggs in one question. By the way the next chapter deals with all those same concepts.)
KNOTS--THIS IS NOT IN YOUR BOOK
G. Wind is measured in knots, (A knot is One nautical mile per hour, and is longer than a regular mile. A nautical mile is 6,076 feet while a statute mile is is 5,280 feet) most winds are around six to twelve knots. Coastal regions and lofty mountains have the most constant winds.
GUIDED PRACTICE: Questions during the lecture
INDEPENDENT PRACTICE: Readings at home
Culmination What are the gadgets use to measure air pressure which one do meteorologists use? Which one do newspapers and TV use? Which one do pilots use? Are gases hyperactive matter ? Why is a tripod a good analogy ? What are the three legs? How is Air Pressure mapped and what is the pattern of Air Pressure? Who plots Isobars and what are they? What are the Vertical Variations in Pressure? What is the Nature of the Wind and how and why does it move?