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Lab 11 LAB 11--The Atmosphere (Pressure and Winds) Key Ideas Atmosphere Continentality Troposphere Marine influence Electromagnetic spectrum Atmospheric pressure Ground radiation Wind Environmental lapse rate Pressure gradient Normal environmental lapse rate Coriolis effect Temperature inversion Cyclones & anticyclones Discussion The Atmosphere The atmosphere
consists of a zone of gas molecules that surrounds the earth. The gas molecules are held there by the earth's gravitational force, which acts to pull objects toward the center of the earth. Earth's gravitational force varies in strength with the mass of the other object. Gases don't have much mass, thus they are not pulled all that close to the center of the earth. Water, which as a liquid is denser than the atmospheric gas molecules, is held somewhat closer to the center of the earth than the gases are; most of the earth's liquid water lies on the surface of the earth rather than in the air. The earth's atmosphere is commonly divided into four layers. In this class we are most interested in the troposphere
, which is the lowest atmospheric layer--the one that borders the earth's surface. The troposphere extends upwards from the surface reaching a thickness that varies between 5 and 10 miles. The troposphere consists of components of variable mixture
and components of constant mixture
. The most abundant components of variable mixture are water vapor (gaseous water) and very small airborne solids, known as particulates. Water vapor and particulates enter the atmosphere from the surface of the earth, and the amount of each varies considerably over space and time. The most abundant components of constant mixture are the gases nitrogen, oxygen, argon, and carbon dioxide. The amounts of the several other gases are so small that, to three decimal places, nitrogen, oxygen, argon, and carbon dioxide make up 100.000% of the troposphere's components of constant mixture (Table 12.1). Table 12-1 78.082% nitrogen (N
2
) 20.945% oxygen (0
2
) 0.934% argon (Ar) 0.039% carbon dioxide (CO
2
) 100.000% total
Lab 11 The relative percentages of the four principal tropospheric gases have changed considerably over long periods of geologic time. If the amount of one of these gases increases, it will then account for a larger percentage of the atmosphere, and the relative percentages of each of the other gases will decrease even though their actual amounts have not changed. The percentages of the unchanging gases must decrease because total percentage of anything is, by definition, 100%. Consider the case of the planet Vilar, with an atmosphere that consists of 20% helium, 30% xenon, and 50% chlorine. How would the percentages of xenon and chlorine change if helium were to decrease to 10% of Vilar's atmosphere? Originally, % xenon + % chlorine = 80% of Vilar's atmosphere Now, with helium having decreased from 20% to 10% of the atmosphere, the percentages of xenon and chlorine will add to 90: % xenon + % chlorine = 90% of Vilar's atmosphere. But how will the additional 10% be distributed between xenon and chlorine? Each will each increase in percentage according to their previous relative proportion to each other: xenon:
30
80
=
𝑥
90
chlorine:
50
80
=
𝑐
90
new % xenon = x = 33.75% new % chlorine = c = 56.25% Because of the reduction in helium, the atmosphere of Vilar now consists of 10% helium, 33.75% xenon, and 56.25% chlorine. Similar redistributions of the percentages of the various components happen when changes in composition occur in the earth's atmosphere. Vertical Temperature Distribution Any object with temperature (i.e., molecular motion) emits electromagnetic radiation. The hotter the object, the more electromagnetic energy it emits and the shorter the wavelengths are of the emitted radiation. Shorter wavelengths of electromagnetic energy are more intense and more dangerous than longer wavelengths of electromagnetic radiation. Short wavelength radiation, like x-rays, ultraviolet radiation, and visible light, are more dangerous than long wavelength radiation, such as thermal infrared (heat) and microwaves. The really hot sun emits its peak wavelengths in the short wavelength visible portion of the electromagnetic spectrum. The earth absorbs much of the solar radiation (sunlight) that reaches the ground surface, and this adds greatly to the earth's energy and its molecular motion. The earth, with an average global surface temperature of 50°F, is much cooler than the sun, and emits long wavelength radiation in the thermal infrared (heat), part of the spectrum. The primary source of heat in the troposphere, therefore, is this thermal infrared ground radiation
emitted by the earth. As a result, in the troposphere under normal conditions of still air, atmospheric temperature decreases with increasing altitude above the earth's surface, and it does so at a constant rate, called the environmental lapse rate
. The average value of the environmental lapse rate is the normal environmental lapse rate
, which is a cooling of 3.5°F for every thousand feet of increase in altitude, 3.5°F/1000 ft. Occasionally circumstances arise in which the air closest to the earth's surface becomes colder than air a bit higher up, rather than having a linear decrease in temperature with altitude. Colder air below and warmer air above represents a condition known as an inversion of the normal environmental lapse
Lab 11 rate, or simply a temperature inversion
. Temperature inversions can occur for several different reasons. In valleys surrounded by high mountain ranges, cold mountain air can physically flow downslope to "pond" in the valley below. On nights that are originally cold and clear, ground radiation can cool the surface so much that it starts to draw heat from the adjacent air molecules by conduction, only to radiate that heat away, too. Through this molecule by molecule transfer of heat (conduction), a thicker and thicker vertical zone of air above the surface gets colder and colder, eventually becoming colder than the air above it. Locational Influences on Temperature Many factors influence the air temperature of a given point on the surface of the earth. Latitude and time of year (position of the earth in its orbit) are fundamental factors through their impact on sun angle and length of day. For the same latitude, places at higher elevations are generally cooler than places at lower elevations. Also important is how the location of interest is situated with respect to large areas of land versus water. Land changes temperature much more readily than water does, both gaining and losing heat much faster than an equal volume of water. As a result, a place in the middle of a continent will experience hotter summers and colder winters than a place at the same latitude and elevation near the ocean. This effect of a large expanse of land on temperature trends is referred to as continentality. A location that is in the middle of a continent far from the ocean coast is described as having a continental location, or high continentality. A place so near the ocean that its atmospheric conditions are affected by the water is described as having a strong marine influence
. The marine influence moderates temperatures because water both gains and loses heat slowly. A range
of measured values is the arithmetic difference between the highest and lowest value recorded for a given variable, such as one day's highest and lowest temperature. The annual temperature range
is the difference between the average temperature of the coolest month and the average temperature of the warmest month for one year, or averaged over many years. Atmospheric Pressure Atmospheric pressure
is the weight of the molecules of the atmosphere per unit area on the earth's surface. For example, pressure might be recorded in pounds per square inch. Atmospheric pressure is often expressed in the units of inches of mercury (in. Hg). Average pressure at sea level is about 29.92 inches of mercury. The mass of air molecules over a point on the earth's surface varies with meteorological conditions. Lower than average pressure is associated with rising air because rising air expands. Higher than average pressure is associated with descending air because descending air compresses, or jams together. The reason for this is the approximately spherical shape of the earth and of the atmosphere of gases around the solid earth. The higher a point is in altitude above the earth's surface, the larger the diameter of the sphere, and the greater the area of the sphere. Expressed, in a slightly different way, think of the surface area of the approximately spherical earth. Then imagine the surface area of a spherical clear plastic bubble that completely surrounds the earth, but at 1000 ft. above the earth's surface. The outer surface area of the plastic bubble will be larger than that of the earth's surface. Gas molecules located at the surface of the bubble will have more area to occupy than gas molecules at the earth's surface. The column of air that occupies one square foot on the ground and keeps those dimensions all the way up to 1000 ft., will occupy a smaller proportion of the shell at 1000 ft. than it does on the surface of the earth. At higher altitudes, some of the rising air will spread outward beyond the location of the column at higher altitudes. It's just the opposite for descending air molecules, which get crammed into a smaller and smaller area (area of the imaginary bubble surface) as the molecules descend. Nature tends to distribute things equally. When there are more air molecules in one column of air than another, air molecules will flow parallel to the earth's surface from the area of higher pressure (more
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Lab 11 molecules) to the area of lower pressure (fewer molecules) in an attempt to equalize the distribution of air molecules. This movement of air over earth's surface from a region of higher pressure to a region of lower pressure is wind
. Wind blows out of high pressure areas and into low pressure areas
. The greater the difference in pressure from one region to another, the faster the wind speed will be. The pressure gradient is the change in atmospheric pressure from one region to another. Typically, the pressure gradient is expressed as inches of mercury (in. Hg) per some round number of distance units, like per mile, per 10 miles, per 1000 miles, and so forth. Ideally, and very close to the equator, surface winds blow straight down the pressure gradient from the area of higher pressure straight to the area of lower pressure. However, with increasing latitude away from the equator, the coriolis effect
takes over and causes winds to be turned, that is, deflected, from the ideal. The coriolis effect is a product of the earth's rotation from west to east, and it causes winds to be bent to the right in the northern hemisphere and bent to the left in the southern hemisphere. The direction, to the right or left, is right or left when looking down the pressure gradient. The amount of deflection increases with increasing latitude and with increasing wind speeds. Winds are named for directions, such as, a northerly wind, southeasterly wind, sea breeze, etc. It is important to remember that winds are always named for the direction that the blow from
. A northerly wind blows from the north to the south; a southeasterly wind blows from the southeast to the northwest; a sea breeze blows from the sea to the land. Because of the Coriolis effect, by 10° of latitude, winds are being deflected from the ideal pressure gradient, to the right of that ideal in the northern hemisphere, and to the left of that ideal in the southern hemisphere. This affects winds blowing out of high pressure areas and winds blowing into low pressure areas. Both high and low pressure areas commonly form pressure cells
, where the isobars
(lines connecting points of equal pressure) are closed, that is, are kind of circular in shape, like a bull's eye. When the rounded isobars represent higher and higher pressure toward the center of the cell, it's a known as a high pressure cell, or an anticyclone
. A low pressure cell, or cyclone
, occurs when the closed, concentric isobars represent lower pressures toward the center, with the lowest pressure at the center of the cell. Descending air at the center of a high pressure cell flows outward, and as long as it's a bit away from the equator, that outward flow will be deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This deflected outward flow of air from a high pressure cell is referred to as anticyclonic circulation
. The deflected inward flow of air to the center of a low pressure cell is cyclonic circulation
. In the northern hemisphere, the right-deflected anticyclonic circulation around a high pressure cell ends up being a clockwise outspiral of air, and the right-deflected cyclonic circulation into a low pressure cell ends up being a counterclockwise inspiral of air. Because Coriolis deflection in the southern hemisphere is to the left, anticyclonic circulation, which by definition is around a high pressure cell, is a counterclockwise outspiral, and cyclonic circulation, which by definition is around a low pressure cell, is a clockwise inspiral.
Lab 11 LAB EXERCISE 2 Name: ________ 1.
Scientists expect the amount of carbon dioxide in the atmosphere to triple over the next 150 years, and this has them concerned about global warming. As the percentage of CO
2
increases, the percentage of the other gases of constant mixture must decrease. A.
Use the information in Table 12-1 to help you calculate what the percentages of all four gases of constant mixture will be if the percentage of CO
2
were to triple from its current value. New Nitrogen (N2) Percentage: 78.082% New Oxygen (O2) Percentage: 20.945% New Argon (Ar) Percentage: 0.934% New Carbon Dioxide (CO2) Percentage: 0.117% B.
What do your results from question A, together with the level of concern about the increasing amounts of carbon dioxide in the atmosphere, tell you about the nature of carbon dioxide? The following conclusions are drawn from question A's calculations of the new gas percentages in the atmosphere in the event that carbon dioxide (CO2) were to triple: Carbon Dioxide Control: Although the proportion of carbon dioxide in the atmosphere has increased significantly, it still makes up a small portion of the atmosphere. After tripling CO2, it only makes up 0.117 percent of the atmosphere in the current scenario. Little Effect on Other Gases: The percentages of other important gases, such as nitrogen (N2), oxygen (O2), and argon (Ar), are not significantly affected by the rise in CO2. The atmosphere's composition is still dominated by these gases. Fear of Climate Change: Fear of climate change and global warming stems from CO2's function as a greenhouse gas, not just from its percentage growth. Despite making up a very minor portion of the atmosphere, CO2 is essential for retaining heat, which helps create the greenhouse effect and may even cause temperature variations on Earth. In summary, although though CO2 is still a small percentage of the atmosphere, its influence on the planet's climate makes it an important environmental factor. C.
In an hour, tourists can travel from a nearby French town at an elevation 3170 ft. up to a prominent viewing platform at 12,600 ft. for a spectacular view of Mont Blanc in the French Alps. Assuming average, normal conditions of the atmosphere, what will the daytime high temperature be at the viewing platform if the daytime high in the town is 55°F? 4.3°F 2. A scenic overlook at the upper edge of the Grand Canyon (elevation of 8350 ft.) will have a daytime high of 94°F. Under average, normal conditions, what will be the maximum air temperature that same day in the bottom of the canyon at the Colorado River (elevation of 2375 ft.)? 105.964°F
Lab 11 Figure 2.1.
(a) Pressure map ("Hg), and (b) cross-section space for a Southern Hemisphere area. 2.
Figure 2.1a shows air pressure ("Hg) for a region centered on 40°S. Some specific sites along a linear path on the ground are labelled M through R. Two other points are labelled a and b. A.
Which of the map sites M-R best represents the center of a high pressure cell? The center of high pressure cell is N, this I because the pressure levers are over 30.09 B.
Which of the map sites M-R best represents the center of a low pressure cell? The center of a low pressure cell is Q because the pressure level is down 29.87 C.
Which of the map sites M-R best represents a center of cyclonic circulation? The center of cyclonic circulation of Q because the point with low pressure air system. D.
Which of the map sites M-R best represents a center of anticyclonic circulation? The center of anticyclonic circulation is N, because the paint with the highest pressure air system
. E.
Directly on the map, draw arrows to show the general direction of air circulation (winds) that would occur around the high pressure cell and that would occur around the low pressure cell. The direction of wind circulation is clockwise for the southern hemisphere to lower pressure air system and counter clockwise to high pressure air system. F.
Draw arrows in the cross-sectional space (Fig. 2.1b) to indicate direction of air movements along the ground surface, and up into the atmosphere. In other words, show where air is rising or descending and where surface winds are moving into or away from the vertically moving air. G.
At which map site M-R (Fig. 2.1a) would you expect the fastest wind speeds to be occurring? Between M and R for the lever 29.97 H.
What is the pressure gradient in inches of Hg per 10 miles between a and b if the distance between a and b represents 75 miles on the ground?
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Lab 11 Figure 2.2
. (a) Pressure map in inches of mercury ("Hg), and (b) axes for a cross-sectional diagram. Note that points A through F on Fig. 2.2b are aligned with their position on Fig. 2.2a. 3.
Figure 2.2a is a map of air pressure for a region centered on 40°N. Points along a linear path on the ground are labelled A through F. Two other ground points of interest are labelled x and y. A.
Which one of the map sites A through F best represents the center of a high pressure cell? Site E B.
Which one of the map sites A through F best represents the center of a low pressure cell? Site B C.
Which one of the map sites A through F best represents a center of cyclonic circulation? Site B D.
Which one of the map sites A through F best represents a center of anticyclonic circulation? Site E E.
Directly on the map, draw arrows to show the general direction of air circulation (winds) that would occur around the high pressure cell and around the low pressure cell. F.
Draw arrows in the cross-sectional space (Fig. 2.2b) to indicate direction of air movements along the ground surface, and up into the atmosphere. In other words, show where air is rising or descending and where surface winds are moving into or away from the vertically moving air. G.
At which map site A-F would you expect the fastest wind speeds to be occurring? Site C H.
What is the pressure gradient in inches of Hg per 10 miles between locations x and y if the Tan(x) = (30.07-
29.99) Hg/75 miles
Lab 11 distance between x and y represents 50 miles on the ground? 29.96-29.93=0.03 inches hg
Lab 11 4.
Use what you know about the controls on temperature to match each of the four temperature charts to one of the map locations, A, B, C, or D. Indicate which site goes with which graph by writing a site letter below the graph on the blank line next to the word Location. 5.
For each map site A-D, list the main geographic factor that gives the site its distinguishing temperature characteristics, and comment on the effect of the site's latitudinal position. Site A
—
Its located near the ocean this is why the temperature is different during the day, its causes a difference in how the ocean is. Site B
—
It is located in the middle of the desert, day temperature is high but at night the temperature gets very low. Site C
—
Its located on the Great Dividing Range of Australia at a height of about 18 to 2000 M which is why the temperature at day is also low and at night temperature gets lower than the day Site D
—
It is located on the coast of Tasmania because of the ocean the temperature difference is less of a day and night.
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Lab 11