UPPER AIR WINDS AND THE JET STREAM

Winds at the top of the troposphere are generally pole ward and westerly in direction. Figure 7q-1 describes these upper air westerlies along with some other associated weather features. Three zones of westerlies can be seen in each hemisphere on this illustration. Each zone is associated with either the Hadley, Ferrel, or Polar circulation cell.

Figure 7q-1: Simplified global three-cell upper air circulation patterns.

The polar jet stream is formed by the deflection of upper air winds by coriolis acceleration (see Figure 7q-3 below). It resembles a stream of water moving west to east and has an altitude of about 10 kilometers. Its air flow is intensified by the strong temperature and pressure gradient that develops when cold air from the poles meets warm air from the tropics. Wind velocity is highest in the core of the polar jet stream where speeds can be as high as 300 kilometers per hour. The jet stream core is surrounded by slower moving air that has an average velocity of 130 kilometers per hour in winter and 65 kilometers per hour in summer.

GLOBAL SCALE CIRCULATION OF THE ATMOSPHERE

Simple Model of Global Circulation

We can gain an understanding of how global circulation works by developing two simplified graphical models of processes that produce this system. The first model will be founded on the following simplifying assumptions:

• The Earth is not rotating in space.
• The Earth's surface is composed of similar materials.
• The global reception of solar insolation and loss of longwave radiation cause a temperature gradient of hotter air at the equator and colder air at the poles.

Based on these assumptions, air circulation on the Earth should approximate the patterns shown on Figure 7p-1. In this illustration, each hemisphere contains one three-dimensional circulation cell.

Figure 7p-1: Simplified one-cell global air circulation patterns.

As described in the diagram above, surface air flow is from the poles to the equator. When the air reaches the equator, it is lifted vertically by the processes of convection and convergence. When it reaches the top of the troposphere, it begins to flow once again horizontally. However, the direction of flow is now from the equator to the poles. At the poles, the air in the upper atmosphere then descends to the Earth's surface to complete the cycle of flow.

LOCAL AND REGIONAL WIND SYSTEMS

Thermal Circulations

winds blow because of differences in atmospheric pressure. Pressure gradients may develop on a local to a global scale because of differences in the heating and cooling of the Earth's surface. Heating and cooling cycles that develop daily or annually can create several common local or regional thermal wind systems. The basic circulation system that develops is described in the generic illustrations below.

Figure 7o-1: Cross-section of the atmosphere with uniform horizontal atmospheric pressure.

In this first diagram (Figure 7o-1), there is no horizontal temperature or pressure gradient and therefore no wind. Atmospheric pressure decreases with altitude as depicted by the drawn isobars (1000 to 980 millibars). In the second diagram (Figure 7o-2), the potential for solar heating is added which creates contrasting surface areas of temperature and atmospheric pressure. The area to the right receives more solar radiation and the air begins to warm from heat energy transferred from the ground through conduction and convection. The vertical distance between the isobars becomes greater as the air rises. To the far left, less radiation is received because of the presence of cloud, and this area becomes relatively cooler than the area to the right. In the upper atmosphere, a pressure gradient begins to form because of the rising air and upward spreading of the isobars. The air then begins to flow in the upper atmosphere from high pressure to low pressure.

Figure 7o-2: Development of air flow in the upper atmosphere because of surface heating.

Figure 7o-3 shows the full circulation system in action. Beneath the upper atmosphere high is a thermal low pressure center created from the heating of the ground surface. Below the upper atmosphere low is a thermal high created by the relatively cooler air temperatures and enhanced by the descending air from above. Surface air temperatures are cooler here because of the obstruction of shortwave radiation absorption at the Earth's surface by the cloud. At the surface, the wind blows from the high to the low pressure. Once at the low, the wind rises up to the upper air high pressure system because of thermal buoyancy and outflow in the upper atmosphere. From the upper high, the air then travels to the upper air low, and then back down to the surface high to complete the circulation cell. The circulation cell is a closed system that redistributes air in an equitable manner. It is driven by the greater heating of the surface air in the right of the diagram.

Figure 7o-3: Development of a closed atmospheric circulation cell because of surface heating.

FORCES ACTING TO CREATE WIND

Wind can be defined simply as air in motion. This motion can be in any direction, but in most cases the horizontal component of wind flow greatly exceeds the flow that occurs vertically. The speed of wind varies from absolute calm to speeds as high as 380 kilometers per hour (Mt. Washington, New Hampshire, April 12, 1934). In 1894, strong winds in Nebraska pushed six fully loaded coal cars over 160 kilometers in just over three hours. Over short periods of time surface winds can be quite variable.

Wind develops as a result of spatial differences in atmospheric pressure. Generally, these differences occur because of uneven absorption of solar radiation at the Earth's surface (Figure 7n-1). Wind speed tends to be at its greatest during the daytime when the greatest spatial extremes in atmospheric temperature and pressure exist.

Figure 7n-1: Formation of wind as a result of localized temperature differences.

Wind is often described by two characteristics: wind speed and wind direction. Wind speed is the velocity attained by a mass of air traveling horizontally through the atmosphere. Wind speed is often measured with an anemometer in kilometers per hour (kmph), miles per hour (mph), knots, or meters per second (mps) (Figure 7n-2). Wind direction is measured as the direction from where a wind comes from. For example, a southerly wind comes from the south and blows to the north. Direction is measured by an instrument called a wind vane (Figure 7n-2). Both of these instruments are positioned in the atmospheric environment at a standard distance of 10 meters above the ground surface.

GLOBAL SURFACE TEMPERATURE DISTRIBUTION

If the Earth was a homogeneous body without the present land/ocean distribution, its temperature distribution would be strictly latitudinal (Figure 7m-1). However, the Earth is more complex than this being composed of a mosaic of land and water. This mosaic causes latitudinal zonation of temperature to be disrupted spatially.

Figure 7m-1: Simple latitudinal zonation of temperature.

The following two factors are important in influencing the distribution of temperature on the Earth's surface:

• The latitude of the location determines how much solar radiation is received. Latitude influences the angle of incidence and duration of day length.
• Surface properties - surfaces with high albedo absorb less incident radiation. In general, land absorbs less insolation that water because of its lighter color. Also, even if two surfaces have the same albedo, a surface's specific heat determines the amount of heat energy required for a specific rise in temperature per unit mass. The specific heat of water is some five times greater than that of rock and the land surface (see Table 7m-1 below). As a result, water requires the input of large amounts of energy to cause a rise in its temperature.

Table 7m-1: Specific Heat of Various Substances.

Substance	Specific Heat
Water	1.00
Air	0.24
Granite	0.19
Sand	0.19
Iron	0.11

Mainly because of specific heat, land surfaces behave quite differently from water surfaces. In general, the surface of any extensive deep body of water heats more slowly and cools more slowly than the surface of a large land body. Other factors influencing the way land and water surfaces heat and cool include:

• Solar radiation warms an extensive layer in water, on land just the immediate surface is heated.
• Water is easily mixed by the process of convection.
• Evaporation of water removes energy from water's surface.

DAILY AND ANNUAL CYCLES OF TEMPERATURE

Daily Cycles of Air Temperature

At the Earth's surface quantities of insolation and net radiation undergo daily cycles of change because the planet rotates on its polar axis once every 24 hours. Insolation is usually the main positive component making up net radiation. Variations in net radiation are primarily responsible for the particular patterns of rising and falling air temperature over a 24 hour period. The following three graphs show hypothetical average curves of insolation, net radiation, and air temperature for a typical land based location at 45° of latitude on the equinoxes and solstices(Figures 1, 2, and 3).

Insolation

Figure 1: Hourly variations in insolation received for a location at 45° North latitude over a 24 hour period. In the above graph, shortwave radiation received from the Sun is measured in Watts. For all dates, peak reception occurs at solar noon when the Sun attains its greatest height above the horizon.

Net Radiation

Figure 2: Hourly variations in net radiation for a location at 45° North latitude over a 24 hour period.

Units in Figure 2 are the same as the insolation graph above. The net radiation graph indicates that there is a surplus of radiation during most of the day and a deficit throughout the night. The deficit begins just before sunset when emitted longwave radiation from the Earth's surface exceeds solar insolation and longwave radiation from the atmosphere.

Temperature

Figure 3: Hourly variations in surface temperature for a location at 45° North latitude over a 24 hour period.

THE CONCEPT OF TEMPERATURE

Temperature and Heat

Temperature and heat are not the same phenomenon. Temperature is a measure of the intensity or degree of hotness in a body. Technically, it is determined by getting the average speed of a body's molecules. Heat is a measure of the quantity of heat energy present in a body. The spatial distribution of temperature in a body determines heat flow. Heat always flows from warmer to colder areas.

The heat held in a object depends not only on its temperature but also its mass. For example, let us compare the heating of two different masses of water (Table 7k-1). In this example, one mass has a weight of 5 grams, while the other is 25 grams. If the temperature of both masses is raised from 20 to 25° Celsius, the larger mass of water will require five times more heat energy for this increase in temperature. This larger mass would also contain contain 5 times more stored heat energy.

Table 7k-1: Heat energy required to raise two different quantities of water 5° Celsius.

Mass of the Water	Starting Temperature	Ending Temperature	Heat Required
5 grams	20° Celsius	25° Celsius	25 Calories of Heat
25 grams	20° Celsius	25° Celsius	125 Calories of Heat

Temperature Scales

A number of measurement scales have been invented to measure temperature. Table 7k-2 describes important temperatures for the three dominant scales in use today.

Table 7k-2: Temperature of absolute zero, the ice point of water, and the stream point of water using various temperature measurement scales.

Measurement Scale	Steam Point of Water	Ice Point of Water	Absolute Zero
Fahrenheit	212	32	-460
Celsius	100	0	-273
Kelvin	373	273	0

The most commonly used scale for measuring temperature is the Celsius system. The Celsius scale was developed in 1742 by the Swedish astronomer Andres Celsius. In this system, the melting point of ice was given a value of 0, the boiling point of water is 100, and absolute zeros -273. The Fahrenheit system is a temperature scale that is used exclusively in the United States. This system was created by German physicist Gabriel Fahrenheit in 1714. In this scale, the melting point of ice has a value of 32, water boils at 212, and absolute zero has a temperature of -460. The Kelvin scale was proposed by British physicist Lord Kelvin in 1848. This system is often used by scientists because its temperature readings begin at absolute zero and due to the fact that this scale is proportional to the amount of heat energy found in an object. The Kelvin scale assigns a value of 273 for the melting temperature of ice, while the boiling point of water occurs at 373.

GLOBAL HEAT BALANCE

INTRODUCTION TO HEAT FLUXES

Figure 7j-1 illustrates the annual values of net shortwave and net long-wave radiation from the South Pole to the North Pole. On closer examination of this graph one notes that the lines representing incoming and outgoing radiation do not have the same values. From 0 - 35 ° latitude North and South incoming solar radiation exceeds outgoing terrestrial radiation and a surplus of energy exists. The reverse holds true from 35 - 90° latitude North and South and these regions have a deficit of energy. Surplus energy at low latitudes and a deficit at high latitudes results in energy transfer from the equator to the poles. It is this meridional transport of energy that causes atmospheric and oceanic circulation. If there were no energy transfer the poles would be 25° Celsius cooler, and the equator 14° Celsius warmer!

Figure 7j-1: Balance between average net shortwave and long-wave radiation from 90° North to 90° South.

The redistribution of energy across the Earth's surface is accomplished primarily through three processes: sensible heat flux, latent heat flux, and surface heat flux into oceans. Sensible heat flux is the process where heat energy is transferred from the Earth's surface to the atmosphere by conduction and convection. This energy is then moved from the tropics to the poles by advection, creating atmospheric circulation. As a result, atmospheric circulation moves warm tropical air to the polar regions and cold air from the poles to the equator. Latent heat flux moves energy globally when solid and liquid water is converted into vapor.

NET RADIATION AND THE PLANETARY HEAT BALANCE

Shortwave radiation from the Sun enters the surface-atmosphere system of the Earth and is ultimately returned to space as longwave radiation (because the Earth is cooler than the Sun). A basic necessity of this energy interchange is that incoming solar insolation and outgoing radiation be equal in quantity. One way of modeling this balance in energy exchange is described graphically with the use of the following two cascade diagrams.

Figure 7i-1: Global shortwave radiation cascade.

The Global Shortwave Radiation Cascade describes the relative amounts (based on 100 units available at the top of the atmosphere) of shortwave radiation partitioned to various atmospheric processes as it passes through the atmosphere. The diagram indicates that 19 units of insolation are absorbed (and therefore transferred into heat energy and longwave radiation) in the atmosphere by the following two processes:

• Stratospheric Absorption of the Ultraviolet Radiation by Ozone 2 units; and

• Tropospheric Absorption of Insolation by Clouds and Aerosols 17 units.

23 units of solar radiation are scattered in the atmosphere subsequently absorbed at the surface as diffused insolation. 28 units of the incoming solar radiation are absorbed at the surface as direct insolation. Total amount of solar insolation absorbed at the surface equals 51 units. The total amount of shortwave radiation absorbed at the surface and in the atmosphere is 70 units.

THE GREENHOUSE EFFECT

The greenhouse effect is a naturally occurring process that aids in heating the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by absorbing longwave radiation emitted from the Earth's surface. Without the greenhouse effect life on this planet would probably not exist as the average temperature of the Earth would be a chilly -18° Celsius, rather than the present 15° Celsius.

As energy from the Sun passes through the atmosphere a number of things take place (see Figure 7h-1). A portion of the energy (26% globally) is reflected or scattered back to space by clouds and other atmospheric particles. About 19% of the energy available is absorbed by clouds, gases (like ozone), and particles in the atmosphere. Of the remaining 55% of the solar energy passing through the Earth's atmosphere, 4% is reflected from the surface back to space. On average, about 51% of the Sun's radiation reaches the surface. This energy is then used in a number of processes, including the heating of the ground surface; the melting of ice and snow and the evaporation of water; and plant photosynthesis. 

The heating of the ground by sunlight causes the Earth's surface to become a radiator of energy in the longwave band (sometimes called infrared radiation). This emission of energy is generally directed to space (see Figure 7h-2). However, only a small portion of this energy actually makes it back to space. The majority of the outgoing infrared radiation is absorbed by the greenhouse gases (seeFigure 7h-3 below).

Figure 7h-3: Annual (1987) quantity of outgoing longwave radiation absorbed in the atmosphere.

(Image created by the CoVis Greenhouse Effect Visualizer).

Absorption of longwave radiation by the atmosphere causes additional heat energy to be added to the Earth's atmospheric system. The now warmer atmospheric greenhouse gas molecules begin radiating longwave energy in all directions. Over 90% of this emission of longwave energy is directed back to the Earth's surface where it once again is absorbed by the surface. The heating of the ground by the longwave radiation causes the ground surface to once again radiate, repeating the cycle described above, again and again, until no more longwave is available for absorption.

GLOBAL PATTERNS OF INSOLATION RECEIPTS

The following image describes the annual pattern of solar radiation absorption at the Earth's surface for the year 1987.

Figure 7g-1: Annual (1987) pattern of solar radiation absorbed at the Earth's surface.

(Image created by the CoVis Greenhouse Effect Visualizer).

The combined effect of Earth-Sun relationships (angle of incidence and day length variations) and the modification of the solar beam as it passes through the atmosphere produces specific global patterns of annual insolation receipt as seen on Figure 7g-1 above (and see the NASA WWW links below). After examining these patterns, the following trends can be identified:

• Highest values of insolation received occur in tropical latitudes. Within this zone there are localized maximums over the tropical oceans and deserts where the atmosphere has virtually no cloud development for most of the year. Insolation quantities at the equator over land during the solstices are approximately the same as values found in the middle latitudes during their summer (seeNASA WWW links below).
• Outside the tropics, annual receipts of solar radiation generally decrease with increasing latitude. Minimum values occur at the poles. This pattern is primarily the result of Earth-Sun geometric relationships and its effect on the duration and intensity of solar radiation received.
• In middle and high latitudes, insolation values over the ocean, as compared to those at the same latitude over the land, are generally higher (see NASA images). Greater cloudiness over land surfaces accounts for this variation.

ATMOSPHERIC EFFECTS ON INCOMING SOLAR RADIATION

Three atmospheric processes modify the solar radiation passing through our atmosphere destined to the Earth's surface. These processes act on the radiation when it interacts with gases and suspended particles found in the atmosphere. The process of scattering occurs when small particles and gas molecules diffuse part of the incoming solar radiation in random directions without any alteration to the wavelength of the electromagnetic energy (Figure 7f-1). 

Scattering does, however, reduce the amount of incoming radiation reaching the Earth's surface. A significant proportion of scattered shortwave solar radiation is redirected back to space. The amount of scattering that takes place is dependent on two factors: wavelength of the incoming radiation and the size of the scattering particle or gas molecule. 

In the Earth's atmosphere, the presence of a large number of particles with a size of about 0.5 microns results in shorter wavelengths being preferentially scattered. This factor also causes our sky to look blue because this color corresponds to those wavelengths that are best diffused. If scattering did not occur in our atmosphere the daylight sky would be black.

Figure 7f-1: The process of atmospheric scattering causes rays of sunlight to be redirected to a new direction after hitting a particle in the atmosphere. In this illustration, we see how three particles send light rays off into three different directions. Scattering does not change the striking light ray's wavelength or intensity.

If intercepted, some gases and particles in the atmosphere have the ability to absorb incoming insolation (Figure 7f-2). Absorption is defined as a process in which solar radiation is retained by a substance and converted into heat energy. The creation of heat energy also causes the substance to emit its own radiation. In general, the absorption of solar radiation by substances in the Earth's atmosphere results in temperatures that get no higher than 1800° Celsius. According to Wien's Law, bodies with temperatures at this level or lower would emit their radiation in the longwave band. Further, this emission of radiation is in all directions so a sizable proportion of this energy is lost to space.

Figure 7f-2: Atmospheric absorption. In this process, sunlight is absorbed by an atmospheric particle, transferred into heat energy, and then converted into longwave radiation emissions that come from the particle.

THE OZONE LAYER

The ozone layer is a region of concentration of the ozone molecule (O3) in the Earth's atmosphere. The layer sits at an altitude of about 10-50 kilometers, with a maximum concentration in the stratosphere at an altitude of approximately 25 kilometers. In recent years, scientists have measured a seasonal thinning of the ozone layer primarily at the South Pole. This phenomenon is being called theozone hole.

The ozone layer naturally shields Earth's life from the harmful effects of the Sun's ultraviolet (UV) radiation. A severe decrease in the concentration of ozone in the ozone layer could lead to the following harmful effects:

• An increase in the incidence of skin cancer (ultraviolet radiation can destroy acids in DNA).
• A large increase in cataracts and Sun burning.
• Suppression of immune systems in organisms.
• Adverse impact on crops and animals.
• Reduction in the growth of phytoplankton found in the Earth's oceans.
• Cooling of the Earth's stratosphere and possibly some surface climatic effect.

Ozone is created naturally in the stratosphere by the combining of atomic oxygen (O) with molecular oxygen (O2). This process is activated by sunlight. Ozone is destroyed naturally by the absorption of ultraviolet radiation, 

O3 + UV >>> O2 + O

and by the collision of ozone with other atmospheric atoms and molecules. 

O3 + O >>> 2O2 

O3 + O3 >>> 3O2

Since the late 1970s, scientists have discovered that stratospheric ozone amounts over Antarctica in springtime (September - November) have decreased by as much as 60%. Satellite measurements (NIMBUS 7 - Total Ozone Mapping Spectrometer) have indicated a 2% decrease in ozone between 65 degrees North - 65 degrees South per decade since 1978 (Figure 7e-1). A reduction of about 3% per year has been measured at Antarctica where most of the ozone loss is occurring globally. During the late 1990s, large losses of ozone were recorded above Antarctica year after year in the months of September and August. In some years, spring levels of stratospheric ozone were more than 60% lower than the levels recorded months prior to the seasonal development of the hole.

Figure 7e-1: This image shows the global (from 65° N latitude to 65° S latitude) monthly average total ozone amount from 1979 to 2001. The green line shows measurements from Nimbus-7 TOMS instrument. The red line shows the results from the Meteor-3 TOMS instrument. The blue line shows the results from the Earth Probe TOMS instrument. (Source: NASA, TOMS Multimedia)

It appears that human activities are altering the amount of stratospheric O3. The main agent responsible for this destruction was human-made chlorofluorocarbons or CFCs. First produced by General Motors Corporation in 1928, CFCs were created as a replacement to the toxic refrigerant ammonia. CFCs have also been used as a propellant in spray cans, cleaner for electronics, sterilant for hospital equipment, and to produce the bubbles in styrofoam. CFCs are cheap to produce and are very stable compounds, lasting up to 200 years in the atmosphere. By 1988, some 320,000 metric tons of CFCs were used worldwide.

In 1987, a number of nations around the world met to begin formulating a global plan, known as the Montreal Protocol, to reduce and eliminate the use of CFCs. Since 1987, the plan has been amended in 1990 and 1992 to quicken the schedule of production and consumption reductions. By 1996, 161 countries were participating in the Protocol. The Montreal Protocol called for a 100 % reduction in the creation and use of CFCs by January 1, 1996 in the world's more developed countries. Less developed countries have until January 1, 2010 to stop their production and consumption of these dangerous chemicals.

ATMOSPHERIC PRESSURE

Air is a tangible material substance and as a result has mass. Any object with mass is influenced by the universal force known asgravity. Newton's Law of Universal Gravitation states: any two objects separated in space are attracted to each other by a force proportional to the product of their masses and inversely proportional to the square of the distance between them. On the Earth, gravity can also be expressed as a force of acceleration of about 9.8 meters per second per second. As a result of this force, the speed of any object falling towards the surface of the Earth accelerates (1st second - 9.8 meters per second, 2nd second - 19.6 meters per second, 3rd second - 29.4 meters per second, and so on.) until terminal velocity is attained.

Gravity shapes and influences all atmospheric processes. It causes the density and pressure of air to decrease exponentially as one moves away from the surface of the Earth. Figure 7d-1 below models the average change in air pressure with height above the Earth's surface. In this graph, air pressure at the surface is illustrated as being approximately 1013 millibars (mb) or 1 kilogram per square centimeter of surface area.

Figure 7d-1: Change in average atmospheric pressure with altitude.

Measuring Atmospheric Pressure

Any instrument that measures air pressure is called a barometer. The first measurement of atmospheric pressure began with a simple experiment performed by Evangelista Torricelli in 1643. In his experiment, Torricelli immersed a tube, sealed at one end, into a container of mercury (see Figure 7d-2 below). Atmospheric pressure then forced the mercury up into the tube to a level that was considerably higher than the mercury in the container. 

PHYSICAL BEHAVIOR OF THE ATMOSPHERE AND THE GAS LAW

In the previous topic, we learned the atmosphere is composed of a mixture of many different gases. This mixture behaves in many ways as if it were a single gas. As a result of this phenomenon, the following generalizations describe important relationships between temperature, pressure, density and volume, that relate to the Earth's atmosphere.

(1) When temperature is held constant, the density of a gas is proportional to pressure, and volume is inversely proportional to pressure. Accordingly, an increase in pressure will cause an increase in density of the gas and a decrease in its volume.

(2) If volume is kept constant, the pressure of a unit mass of gas is proportional to temperature. If temperature increase so will pressure, assuming no change in the volume of the gas.

(3) Holding pressure constant, causes the temperature of a gas to be proportional to volume, and inversely proportionalto density. Thus, increasing temperature of a unit mass of gas causes its volume to expand and its density to decrease as long as there is no change in pressure.

These relationships can also be described mathematically by the Ideal Gas Law. Two equations that are commonly used to describe this law are:

Pressure x Volume = Constant x Temperature

and

Pressure = Density x Constant x Temperature

CITATION

Pidwirny, M. (2006). "Physical Behavior of the Atmosphere and the Gas Laws". Fundamentals of Physical Geography, 2nd Edition. 30/11/2011. http://www.physicalgeography.net/fundamentals/7c.html

THE LAYERED ATMOSPHERE

The Earth's atmosphere contains several different layers that can be defined according to air temperature, Figure 7b-1 displays these layers in an average atmosphere.

                    

Figure 7b-1: Vertical change in average global atmospheric temperature. Variations in the way temperature changes with height indicates the atmosphere is composed of a number of different layers (labeled above). These variations are due to changes in the chemical and physical characteristics of the atmosphere with altitude. 

According to temperature, the atmosphere contains four different layers (Figure 7b-1). The first layer is called the troposphere. The depth of this layer varies from about 8 to 16 kilometers. Greatest depths occur at the tropics where warm temperatures causes vertical expansion of the lower atmosphere. From the tropics to the Earth's polar regions the troposphere becomes gradually thinner. The depth of this layer at the poles is roughly half as thick when compared to the tropics. Average depth of the troposphere is approximately 11 kilometers as displayed in Figure 7b-1.

About 80% of the total mass of the atmosphere is contained in troposphere. It is also the layer where the majority of our weather occurs (Figure 7b-2). Maximum air temperature also occurs near the Earth's surface in this layer. With increasing height, air temperature drops uniformly with altitude at a rate of approximately 6.5° Celsius per 1000 meters. This phenomenon is commonly called the Environmental Lapse Rate. At an average temperature of -56.5° Celsius, the top of the troposphere is reached. At the upper edge of the troposphere is a narrow transition zone known as the tropopause.

Figure 7b-2: Most of our planet's weather occurs in the troposphere. This image shows a view of this layer from an airplane's window (Photo © 2004 Edward Tsang).

Above the tropopause is the stratosphere. This layer extends from an average altitude of 11 to 50 kilometers above the Earth's surface. This stratosphere contains about 19.9% of the total mass found in the atmosphere. Very little weather occurs in the stratosphere. Occasionally, the top portions of thunderstorms breach this layer. The lower portion of the stratosphere is also influenced by the polar jet stream and subtropical jet stream. In the first 9 kilometers of the stratosphere, temperature remains constant with height. A zone with constant temperature in the atmosphere is called an isothermal layer. 

From an altitude of 20 to 50 kilometers, temperature increases with an increase in altitude. The higher temperatures found in this region of the stratosphere occurs because of a localized concentration of ozone gas molecules. These molecules absorb ultraviolet sunlight creating heat energy that warms the stratosphere. Ozone is primarily found in the atmosphere at varying concentrations between the altitudes of 10 to 50 kilometers. This layer of ozone is also called the ozone layer . The ozone layer is important to organisms at the Earth's surface as it protects them from the harmful effects of the Sun's ultraviolet radiation. Without the ozone layer life could not exist on the Earth's surface.

Separating the mesosphere from the stratosphere is transition zone called the stratopause. In the mesosphere, the atmosphere reaches its coldest temperatures (about -90° Celsius) at a height of approximately 80 kilometers. At the top of the mesosphere is another transition zone known as the mesopause.

The last atmospheric layer has an altitude greater than 80 kilometers and is called the thermosphere. Temperatures in this layer can be greater than 1200° C. These high temperatures are generated from the absorption of intense solar radiation by oxygen molecules (O2). While these temperatures seem extreme, the amount of heat energy involved is very small. The amount of heat stored in a substance is controlled in part by its mass. The air in the thermosphere is extremely thin with individual gas molecules being separated from each other by large distances. 

Consequently, measuring the temperature of thermosphere with a thermometer is a very difficult process. Thermometers measure the temperature of bodies via the movement of heat energy. Normally, this process takes a few minutes for the conductive transfer of kinetic energy from countless molecules in the body of a substance to the expanding liquid inside the thermometer. In the thermosphere, our thermometer would lose more heat energy from radiative emission then what it would gain from making occasional contact with extremely hot gas molecules.

CITATION

Pidwirny, M. (2006). "The Layered Atmosphere". Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/fundamentals/7b.html

ATMOSPHERIC COMPOSITION

Nitrogen and Oxygen are the main components of the atmosphere by volume. Together these two gases make up approximately 99% of the dry atmosphere. Both of these gases have very important associations with life. Nitrogen is removed from the atmosphere and deposited at the Earth's surface mainly by specialized nitrogen fixing bacteria, and by way of lightning through precipitation. The addition of this nitrogen to the Earth's surface soils and various water bodies supplies much needed nutrition for plant growth. Nitrogen returns to the atmosphere primarily through biomass combustion and denitrification.

Oxygen is exchanged between the atmosphere and life through the processes of photosynthesis and respiration. Photosynthesis produces oxygen when carbon dioxide and water are chemically converted into glucose with the help of sunlight. Respiration is a the opposite process of photosynthesis. In respiration, oxygen is combined with glucose to chemically release energy for metabolism. The products of this reaction are water and carbon dioxide.

The next most abundant gas on the table is water vapor. Water vapor varies in concentration in the atmosphere both spatially and temporally. The highest concentrations of water vapor are found near the equator over the oceans and tropical rain forests. Cold polar areas and subtropical continental deserts are locations where the volume of water vapor can approach zero percent. Water vapor has several very important functional roles on our planet:

• It redistributes heat energy on the Earth through latent heat energy exchange.
• The condensation of water vapor creates precipitation that falls to the Earth's surface providing needed fresh water for plants and animals.
• It helps warm the Earth's atmosphere through the greenhouse effect

The fifth most abundant gas in the atmosphere is carbon dioxide. The volume of this gas has increased by over 35% in the last three hundred years (see Figure 7a-1). This increase is primarily due to human induced burning from fossil fuels, deforestation, and other forms of land-use change. Carbon dioxide is an important greenhouse gas. The human-caused increase in its concentration in the atmosphere has strengthened the greenhouse effect and has definitely contributed to global warming over the last 100 years. Carbon dioxide is also naturally exchanged between the atmosphere and life through the processes of photosynthesis and respiration.