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    Planet earth atmosphere notes online Planet earth atmosphere notes online Document Transcript

    • The Ocean-Atmosphere SystemThe oceans and the atmosphere are the two large reservoirs of water in theEarths hydrologic cycle. The two systems are complexly linked to oneanother and are responsible for Earths weather and climate.Role of the atmosphere1. provides oxygen and moisture for life2. regulates heat by transferring energy (wind, clouds, storms, etc)3. retains enough heat to maintain acceptable surface temperatures4. protects surface from extraterrestrial bodies and solar radiation.5. Catalyzes decomposition of rock to form soil (Weathering)Weather and ClimateWeather is the condition of the atmosphere at a particular time and place.It refers to such conditions of the local atmosphere as temperature,atmospheric pressure, humidity (the amount of water contained in theatmosphere), precipitation (rain, snow, sleet, & hail), and wind velocity.Because the amount of heat in the atmosphere varies with location above theEarths surface, and because differing amounts of heat in different partsof the atmosphere control atmospheric circulation, the atmosphere is inconstant motion. Thus, weather is continually changing in a complex anddynamic manner.Climate refers to the average weather characteristics of a given region. Thefive weather variables are also used to measure the climate, but weather is ashort—term event, whereas climate is a long-term one.Climate, although it does change over longer periods of geologic time, ismore stable over short periods of time like years and centuries. The factthat the Earth has undergone fluctuation between ice ages and warmerperiods in the recent past (the last ice age ended about 10,000 years ago) istestament to the fact that climate throughout the world has been changingthrough time.The Earths weather and climate system represent complex interactionsbetween the oceans, the land, the sun, and the atmosphere. That theseinteractions are complex is evidence by the difficulty meteorologists have inpredicting weather on a daily basis. Understanding climate change is evenmore difficult because humans have not been around long enough to record
    • data on the long-term effects of these processes. Still, we do know that themain energy source for changing weather patterns and climate is solarenergy from the Sun.Composition of the atmosphereThe atmosphere is the gaseous envelope that surrounds the planet. It hasstructure. With increasing altitude, there are distinct variations in thecomposition, temperature, pressure, and humidity of the atmosphere.Air is the invisible, odorless mixture of gases and suspended particles.Because air pressure decreases with altitude, the amount of air per unitvolume (density) also varies with altitude. However the relative proportionsof the gases in the air are essentially constant regardless of altitude.Nitrogen, Oxygen, and argon make up 99.96% of the gases by volume.N2 78%O2 21%Ar 0.9%The relative amounts of the remaining gases are very small.Natural trace gases – all <0.01%Neon, Helium, Methane, Krypton, Hydrogen, RadonVariable Constituents Carbon Dioxide • ~0.036%, but increasing due to burning fossil fuels • Some transfer between oceans, organics, and soil • Much greater in urban areas (up to 0.1%) • Contributes to greenhouse Water Vapor • Up to 4% depending on availability Water - liquid or solid • Up to 1% (clouds, fog, rain, snow, etc) • (H20 is the only substance which can exist in all 3 states at the normal range of earth temperature and pressure) Ozone • < 0.7 ppm (7 x 10-6%)
    • • Much higher amounts in upper atmosphere (~10ppm) • Natural from sunlight, lightning (higher amounts after t-storms) • Vulnerable to CFC pollutants Aerosols • <0.1 ppm (1 x 10-6 %), depending on availability. • Consist of tiny liquid droplets or solid particles that are so small they remain suspended in the air. • Dust, soot, smoke particles, sea-salt, pollutants are all types of aerosols.The greenhouse effectCarbon dioxide, water vapor, methane, ozone, and nitrous oxide are thegreenhouse gases. They warm the air by preventing the heat from the sun’sradiation from escaping. They allow the sun’s visible short-wavelengthradiation in, which is re-radiated as long-wavelength infrared radiation backinto space warming the earth. The greenhouse gases absorb some of theinfrared radiation that heats the atmosphere.TemperatureTemperature is a measure of the average kinetic energy of all the atoms in abody. (Heat energy is the total kinetic energy of all the atoms in asubstance. Not all the atoms in a given sample move with the same speed,and so there is a range of kinetic energies among them…temperature is ameasure of the average of these kinetic energies).Vertical Structure of the AtmosphereGeneral trends with increasing altitude • Constituent gases tend to be lighter in upper atmosphere • Air pressure decreases • Water vapor decreases dramatically • Temperature in the troposphere, the lowest part of the atmosphere where weather occurs, decreases with increasing altitude. (Throughout the entire atmosphere temperature fluctuates and defines certain layers. There are four thermal layers of the atmosphere separated by boundaries called pauses. The four thermal layers of the atmosphere include the troposphere, stratosphere, mesosphere and thermosphere.)
    • Troposphere Details • Lowest part of the atmosphere. Extends to variable altitudes of 10 to 16km. Varies over latitude - >16km over equator where more heat per unit area reaches the earth’s surface, <9km over poles. The upper boundary of the troposphere is the tropopause. • Temperature decreases with altitude. Absorption of reradiated long- wavelength infrared rays is most effective at the base of the atmosphere where the air is most dense. It is continually warmed by the ground and ocean. • Horizontal (wind) and vertical (convection) mixing of atmosphere. Location of almost all weather. • Contains almost all atmospheric water and dust, 90% of atmospheric mass.Air PressureAir pressure decreases with altitude. As air pressure decreases, air densitydecreases. (D=m/v; with lower pressure, the mass per unit volumedecreases…the composition of the gases are the same, but the density isless).Air pressure is measured with a barometer.Air pressure decreases with altitude. At any given altitude, the air pressureis caused by the weight of air above. This means that the air near theground is compressed by the weight of the air above it. As a result, half ofthe mass of the atmosphere lies below an altitude of 5.5km and 99 percentlies below 32 km (within the troposphere). The 1- percent of theatmosphere that lies above 32 km continues out to an altitude of about500km (extent of the thermosphere).Moisture in the Atmosphere H20 is the only substance that can exist in all 3 states (solid, liquid, and gas) at the normal range of earth temperature and pressure. Whenever matter changes from one state to another, energy is either absorbed or released. In going from a more ordered state (solid) to a less ordered state (liquid or gas) heat energy is absorbed. The reverse is true when the
    • change is from a less ordered to a more ordered state – heat energy is released. The amount of heat released or absorbed per gram during a change of state is known as the latent heat. • Latent heat of condensation (from gas to liquid) releases 2260 J/g • Latent heat of evaporation (from liquid to gas) absorbs 2260 J/g • Latent heat of freezing (from liquid to solid) releases 330 J/g • Latent heat of melting (from solid to liquid) absorbs 330 J/g -Why you sweat – to evaporate water requires heat. The heat from your skin is used to evaporate the water… the loss of heat from your skin cools you down. -Why steam burns are worse…absorbed more heat to turn to the gas phase. Evaporation and condensation play vitally important roles in the weather. (1) they give rise to clouds, fog, rain (2) they are the means by which heat is moved from equatorial regions to the poles.Relative HumidityWater vapor gets into the air by evaporation. Liquid -> Gas by absorbingenergy. Because molecules in a vapor move at random in all directions, someof the gas molecules in the vapor will move back into the liquid. When thenumber of molecules that evaporate equals the number condensing back intothe liquid the vapor is said to be saturated. This is the maximumconcentration of water molecules in the vapor phase at any specifiedtemperature.Partial pressure is a measure of the volume percent of a specific gas in amixture. Every gas in a mixture will exert a certain amount of pressure andthe pressures in a mixture of gases are additive…Therefore the totalpressure of a mixture of gases is the sum of the partial pressures exertedby all the individual gases present. Water vapor is only one of the gasespresent in air. Therefore water vapor content of air is often reported as apressure rather than a percent.The saturation vapor pressure, also known as the water vapor capacity ofair at any given temperature, cannot be exceeded. The vapor pressure can,however, be lower than the saturation value. • Exercise on a dry day, the water quickly evaporates and you get cool.
    • • Exercise on a humid day, little perspiration evaporates into the air, which means little heat is carried away, and you aren’t cooled. • Swimming on a hot dry day, water quickly evaporates from your body and can carry away so much heat you get goose bumps even if the temperature is 90F.The relative humidity is the ratio of the vapor pressure in a sample of airto the saturation vapor pressure at the same temperature, expressed asa percentage. Note that the relative humidity does not refer to a specificamount of water vapor in the air….It refers to the ratio of what is presentat a given temperature to the maximum possible amount that the air couldhold at the same temperature. The amount of water vapor needed tosaturate air increases as the air’s temperature increases.Relative humidity can be changed in two ways – by addition of water vapor orby change of temperature. Decrease temperature – increase relativehumidity, Increase temperature – Decrease relative humidity. • House is dry in winter because even if the cold air outside is saturated, when brought in the house and warmed, relative humidity decreases. • Can be more humid at night- as temperature drops, relative humidity increases.Dew PointThe temperature at which the relative humidity reaches 100 percent andcondensation starts is called the dew point. If the amount of water vapor inthe air is kept constant and the temperature drops, the relative humiditywill rise. When the ground is cold and the air warm (ground cools fasterthan water), the layer of air in contact with the ground may cool sufficientlyfor the dew point to be reached, so that dew is formed. If the groundtemperature is below freezing, frost forms.Adiabatic ProcessesWarm air is less dense than cold air and therefore rises, creating aconvection cell in the process. Because it is warmest at ground level, air inthe troposphere is continually rising or, after cooling falling. However,because air pressure decreases with increasing altitude, the rising areexpands. Conversely, sinking air is compressed. • Compressional warming – when air is compressed, the temperature rises. Compression increases the temperature and density in the parcel.
    • • Expansional Cooling – when air expands, the temperature decreases. Heat is consumed in the expansion process.These are adiabatic processes…processes that occur without the additionof subtraction of heat from an external source.An ascending (rising) parcel of air tends to cool by expansion.A descending (falling) parcel of air heats by compression.NOTE that Temperature changes in both ascending and descending air occurwithout any significant heat exchange between the surrounding environmentand the vertically moving parcel of air. Adiabatic.When a parcel of unsaturated air rises and expands adiabatically, thetemperature drops. Conversely, if unsaturated air sinks toward the surface,it is compressed and the temperature. The way temperature changes withaltitude in rising or falling air is called the adiabatic lapse rate.Eventually, when air rises far enough, it will cool to the dew point where itwill become saturated and condensation will occur. This point along it’sascent is the lifting condensation level.When a parcel of air rises, reaches saturation at the lifting condensationlevel, condensation begins and clouds form.There are four principle reasons for the upward movement of air.1. Density lifting, which occurs when warm, low-density air rises convectively and displaces cooler, denser air.2. Frontal lifting, which occurs when two flowing air masses of different density meet. The boundaries between air masses of different density are called fronts. When warm humid air advances over cold air it is called a warm front. When denser, cooler air advances and displaces warm air by pushing it upward it is called a cold front.3. Orographic lifting occurs when flowing air is forced upward over a mountain range.4. Convergence lifting occurs when flowing air masses converge and are forced upward.
    • Atmospheric StabilityIn our discussion we have made two assumptionsa) certain process force air to rise from the earth’s surfaceb) that rising air does not mix substantially with the surrounding atmosphere.Once an initial lifting force ceases, the fate of a rising air parcel depends onthe state of the atmosphere through which it rises, or atmosphericstability. The relation among the dry adiabatic rate, moist adiabatic rate,and the environmental lapse rate determines the stability of theatmosphere over an area.The environmental lapse rate is the average decrease in temperature withincreasing altitude.Basically, if a parcel of air that is forced to rise stayed cooler than thesurrounding atmosphere as it expands, it would be more dense than thesurrounding air, and if allowed to do so, would sink to its original position.Air of this type, called stable air, resists vertical movement.If, however, the rising parcel of air were warmer and therefore less densethan the surrounding air, it would continue to rise until it reached an altitudehaving the same temperature. This type of air is classified as unstable air.In summary, stability is a property of air that describes its tendency toremain in its original position (stable) or to rise (unstable).Absolute StabilityThe atmosphere is considered absolutely stable if an air parcel that isforced aloft cools faster than the surrounding environment. That is, theparcel rate of cooling (the dry or moist adiabatic lapse rate) is faster thanthe environmental lapse rate. If the force lifting the parcel continued toact, condensation would eventually occur when the dew point temperature isreached, but clouds would be layered clouds without much verticaldevelopment. If the force ceased, the parcel would have a density to sink.Absolute InstabilityThe atmosphere is considered absolutely unstable if the parcel rate ofcooling is slower than the environmental lapse rate. That is, theenvironmental lapse rate is greater than the parcel’s adiabatic rate. The
    • parcel will continue to rise because it is warmer and more buoyant than itssurroundings. If the air parcel rises to its condensation level, clouds withvertical development will form as the buoyant air rises on its own.(Thunderstorms).Absolute Instability occurs most often during the warmest months and onclear days when solar heating is intense. Under these conditions, thelowermost layer of the atmosphere is heated to a much higher temperaturethan the air aloft. This results in a steep environmental lapse rate and avery unstable atmosphere.We discussed the upward movement of air and its importance in cloudformation. As important as vertical motion is, far more air is involved inhorizontal movement, the phenomena we call wind. Although we know thatair will move vertically if it is warmer and thus more buoyant thansurrounding air, what cause air to move horizontally.Simply stated, wind is the result of horizontal differences in air pressure.Air flows from areas of higher pressure to areas of lower pressure. Wind isnature’s attempt to balance larger-scale inequalities in air pressure. Becauseunequal heating of Earth’s surface continually generates these pressuredifferences, solar radiation is the ultimate driving force of wind.Wind is a horizontal air movement arising from differences in airpressure. Wind results when air flows from a place of high pressure toone of low pressure.Air pressure is related to density.High-pressure means the air is more compressed and therefore more dense.Low pressure means less compression and lower density.Wind SpeedWind Speed measurements are averaged over a specific period, commonlyfive minutes. Most places around the world have wind speeds that averagebetween 10 and 30 km/h (6 and 19 mi/h).
    • The Windchill FactorThe windchill factor measures the heat loss from exposed skin as a result ofthe combined effects of low temperature and wind speed.The human body is a heat generator that continually releases energy.Immediately adjacent to the body is a thin layer of still air called theboundary layer. This layer is still because friction prevents movement. Heatescaping from the body must pass through the boundary layer, making theboundary layer an insulator. However, as wind speed increases, thethickness of the boundary layer decreases, as does its effectiveness as aninsulator. As a result, the wind is acting to carry heat away from the bodyby constantly replacing warmer air next to the body with colder air.During a weather forecast, when mentioning the windchill factor, they areactually reporting the windchill equivalent temperature. It is thetemperature it ‘feels like’. It is the temperature at which exposed parts ofthe body would lose heat at the same rate if there were no wind.Factors affecting wind speed and directionIf the earth did not rotate and there was no friction, air would flow directlyfrom areas of higher pressure to areas of lower pressure. Because bothfactors exist, however, wind is controlled by a combination of factors.Wind is controlled by1. The air pressure gradient, which is the drop in air pressure/unit distance.2. The Coriolis effect, which is the deviation from a straight line in the path of a moving body due to the earth’s rotation.3. Friction, which is the resistance to movement when two bodies are in contact.Pressure Gradient forceTo get anything to accelerate (change in velocity) requires an unbalancedforce in one direction. The force that generates winds results fromhorizontal pressure differences. When air is subjected to greater pressureon one side than on another, the imbalance produces a force that is directedfrom the region of higher pressure toward the area of lower pressure.Thus, pressure differences cause the wind to blow, and the greater thedifferences, the greater the wind speed.
    • Variations in air pressure over Earth’s surface are determined frombarometric readings taken at hundreds of weather stations. These pressuredata are shown on surface weather maps by means of isobars. Isobars arelines connecting places of equal air pressure. The spacing of the isobarsindicates the amount of pressure change occurring over a given distanceand is expressed as the pressure gradient.You can compare the pressure gradient to the slope of a hill. A steeppressure gradient, like a steep hill, causes greater acceleration of a parcelof air than does a weak pressure gradient, like a shallow hill. Thus, therelationship between wind speed and the pressure gradient isstraightforward. Closely spaced isobars indicate a steep pressure gradientand strong winds, widely spaced isobars indicate a weak pressure gradientand light winds. The pressure gradient force is always directedperpendicular to the isobars.Pressure differences are simply a result of unequal heating of Earth’s land-sea surface.For example – surface temperatures over the ocean change only slightly on adaily basis whereas land surfaces and the air above can be substantiallywarmed during a single daylight period. As air over the land warms, itexpands, causing a reduction in density and an area of low pressure. A seabreeze then develops, as cooler air over the water moves onto the land. Atnight, the reverse may take place. The land cools more rapidly than the seaand a land breeze develops.Temperature variations create pressure differences – and hence wind – andthe greater these temperature difference, the stronger the pressuregradient and resultant wind.In summary, the horizontal pressure gradient is the driving force ofwind. It has both magnitude and direction. Its magnitude isdetermined from the spacing of isobars, and the direction of thepressure gradient force is always from areas of higher pressure toareas of lower pressure (perpendicular to the isobars).
    • Once the air starts to move, the Coriolis force and friction come into play,but only to modify the movement, not to produce it.Coriolis Effect.The Coriolis effect influences all moving bodies. Because wind is moving air,the directions of all winds are subject to the Coriolis effect.All free moving objects, including wind, are deflected to the right of theirpath of motion in the Northern Hemisphere and the left in the SouthernHemisphere.Example.Imagine the path of a rock launched from the North Pole toward a target onthe equator. If the rocket took an hour to reach its target, the earth wouldhave rotated 15 o to the east during its flight. To someone standing onEarth, it would look as if the rocket veered off its path and hit earth 15 owest of it’s target. But it was earth turning under the rocket that gave itits apparent deflection. Note that the rocket was deflected to the right ofits path of motion because of the counterclockwise rotation of theNorthern Hemisphere. Clockwise rotation produces a similar deflection inthe Southern Hemisphere, but to the left of the path of motion.Winds flowing West to East are also effected by the Coriolis effect. Thisdeflection is caused by Earth’s rotation, which changes the orientation ofthe surface over which the winds are moving. The amount of deflection isgreater at 60o latitude than at 40o latitude, which is greater than at 20 o.Furthermore, there is no deflection observed for the airflow along theequator. Therefore, the magnitude of the deflecting force (Coriolisdeflection) is dependent on latitude. It is strongest at the poles and itweakens equatorward where it eventually becomes nonexistent.• The amount of Coriolis deflection increases with wind speed because a fast-moving object covers a greater distance in a given time than a slow moving object.• The longer the trajectory , the greater the change in angular velocity and therefore the greater the Coriolis deflection.
    • In conclusion, we attribute this apparent shift in wind direction to theCoriolis force. It is hardly a ‘real’ force but rather the effect ofEarth’s rotation on a moving body. This deflecting force:1. Is toward the right in the Northern Hemisphere and to the left in the Southern Hemisphere.2. Affects only wind direction, not wind speed3. Is affected by wind speed (the stronger the wind, the greater the deflecting force)4. Is strongest at the poles and weakens equatorward, becoming nonexistent at the equator.Where airflow is concerned, the Coriolis effect is of greatest importance inlarge-scale wind systems, but of only minor importance in small scale, localwind systems.Convergent and Divergent FlowAs air near the ground flows inward from all directions toward a lowpressure center, frictional drag causes the flow direction to be across theisobars at an oblique angle. As a consequence, winds around a low pressurecenter develop an inward spiral motion. In the northern hemisphere, wherethe coriolis force deflects the flow to the right, the resulting wind blowscounterclockwise about the low.By the same process, air flow spirals outward from a high pressure area.Around a high pressure cell, the outward directed pressure gradient force isopposed by the inward directed Coriolis force, and a clockwise flow results inthe northern hemisphere.In the Southern Hemisphere, the reverse is true. Because the Coriolisforce deflects the winds to the left in the Southern Hemisphere, the flow isreversed there – clockwise around low pressure centers andcounterclockwise around high pressure centers.Centers of low pressure are called cyclones and exhibit cyclonic flow.Cyclonic flow has the same direction of rotation as earth, counterclockwisein the Northern Hemisphere and clockwise in the southern hemisphere.
    • Centers of high pressure are called anticyclones and exhibit anticyclonicflow. Anticyclonic flow has the opposite rotation as Earth, clockwise in thenorthern hemisphere and counterclockwise in the southern hemisphere.Vertical Flow Associated with Cyclones and AnticyclonesAround a surface low pressure system (cyclone) air is spiraling inward. Thisleads to an upward flow of air at the center of the low and divergence aloft.The surface convergence about a cyclone causes a net upward movement.Rising air often result in cloud formation and precipitation, and thereforethe passage of a low-pressure center is generally related to unstableconditions and stormy weather.Around a surface high pressure system (anticyclone) air is spiraling outward.This leads to a downward flow of air at the center of the high andconvergence aloft. Because descending air is compressed and warmedadiabatically, cloud formation and precipitation are unlikely in an anticyclone.Thus, fair weather can usually be expected with the approach of a high-pressure system.