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Dec 6, 2023

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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

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.

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