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MAR 110: Introductory Oceanography The atmosphere and ocean Oceans and climate, part 1 • In the middle latitudes, westerly winds moderate the climates of continents on eastern sides of the ocean basins. – Western Europe’s climate is milder than that of Eastern North America in that the difference between summer and winter temperatures is less. • SSTs change little over the course of the year. • Gulf stream adds heat and water vapor to atmosphere above, especially during winter. • Cork, Ireland (51° 54′ N): 4.5 °C in January; 15.5 °C in July. • Saskatoon, Saskatchewan (52° 8′ N): -18.5 °C in January; 18.0 °C in July. Rev. 12 October 2006 The atmosphere and ocean 2 Oceans and climate, part 2 • What would happen if the influence of the westerlies weakened? – Winters were much colder in Western Europe during some periods of the past. – In the Little Ice Age (1400 to 1850 CE), sea ice expanded over the North Atlantic, montane glaciers advanced, and growing seasons shortened. Rev. 12 October 2006 The atmosphere and ocean 3 Oceans and climate, part 3 • Proxy evidence shows a number of long cold spells in the last 10,000 years. – Average temperatures have been as much as 7 °C colder. – Weakening circulation in the North Atlantic may have played a role. – Wallace Broecker (of Lamont-Doherty Earth Observatory) first proposed that changes in thermohaline circulation caused by fluctuating amounts of fresh water in the North Atlantic may trigger major climate swings (as in the movie The Day After Tomorrow. Rev. 12 October 2006 The atmosphere and ocean 4 Earth-atmosphere system • We will examine radiational heating and cooling of the earth-atmosphere system. • The oceans support the atmosphere by playing a vital role in the Earth’s heat budget, transporting solar energy from the tropics – where there is a surplus – to the poles – where there is a deficit. Rev. 12 October 2006 The atmosphere and ocean 5 Weather • Weather is the instantaneous state of the atmosphere at a given place and time. – Weather may be described in terms of variables such as temperature, humidity, cloudiness, precipitation, and wind speed and direction. – Weather varies continuously from place to place and time to time. • “If you don’t like the weather, wait a minute.” – Meteorology is the study of the atmosphere and the processes that cause weather. Rev. 12 October 2006 The atmosphere and ocean 6 Climate, part 1 • Climate refers to the pattern or average of weather conditions over a long period, encompassing mean characteristics, variability, and extremes. – By international convention, climate variables are averaged over a 30-year period beginning with the first year of a decade (e.g., 1971-2000). The averaging period is shifted forward 10 years with the beginning of a new decade. – These 30-year averages (normals) of monthly temperature and precipitation variables are used to describe climate. • Seasonal variables, length of growing season, percent of possible sunshine, and number of days with dense fog among other important variables. Rev. 12 October 2006 The atmosphere and ocean 7 Climate, part 2 • Climate is probably the most important environmental control, affecting agriculture, water supply, heating and cooling requirements for buildings, weathering and erosion processes, and much, much more. • Climatology is the study of climate, its controls, and its spatial and temporal variability. Rev. 12 October 2006 The atmosphere and ocean 8 The role of the sun • How does energy flow into and out of the Earthatmosphere system? – The sun drives the atmosphere, providing the energy that powers atmospheric and oceanic circulation and storms. • The sun emits electromagnetic energy which strikes the Earth’s atmosphere – some of which is absorbed and converted into other forms of energy, like heat and the kinetic energy of wind and water currents. – First law of thermodynamics: Energy can be transferred and transformed, but not created nor destroyed. Rev. 12 October 2006 The atmosphere and ocean 9 Electromagnetic spectrum • The energy from the sun that bathes the Earth is part of the electromagnetic spectrum, which is energy that possesses electrical and magnetic properties. • Electromagnetic energy exhibits properties of waves, thus wavelength or frequency are effective measures to use to classify energy bands within the spectrum. – Electromagnetic energy ranges from radio waves to gamma waves. – One energy band grades into another. Rev. 12 October 2006 The atmosphere and ocean 10 Rev. 12 October 2006 The atmosphere and ocean 11 Wave properties • Wavelength is the distance between successive wave crests or successive wave troughs. • Frequency is the number of waves that pass a given point in a specified amount of time. – Passage of a complete wave is called a cycle. – The typical measure of frequency is the number of cycles per second, or hertz (Hz). • Wavelength and frequency are inversely proportional. – The higher the frequency, the shorter the wavelength. Rev. 12 October 2006 The atmosphere and ocean 12 Rev. 12 October 2006 The atmosphere and ocean 13 Radiation bands, part 1 • There are seven primary bands of electromagnetic radiation. – Gamma rays, x-rays, and ultraviolet radiation: These three bands occur naturally, but can also be produced artificially. • Gamma rays have the shortest wavelengths (and highest frequencies) and contain the most energy. • X-rays have slightly longer wavelengths (and shorter frequencies) • Ultraviolet waves are the short-wavelength band adjacent to the visible spectrum. • All three are too short to be seen by the human eye, and they can cause considerable damage to living organisms if they reached the surface, but the atmosphere filters them out. Rev. 12 October 2006 The atmosphere and ocean 14 Radiation bands, part 2 • Seven primary bands (continued): – The visible spectrum, ranging from wavelengths of from 0.4 to 0.7 micrometers, makes up only 3 percent of the electromagnetic spectrum, but it represents a large portion of solar energy. • Visible light is necessary for many activities of plants and animals, such as photosynthesis and daylength-induced control of reproduction. Rev. 12 October 2006 The atmosphere and ocean 15 Radiation bands, part 3 • Seven primary bands (continued): – Infrared light, the portion of the electromagnetic spectrum with wavelengths slightly longer than the visible spectrum, cannot be seen by the human eye. • It is emitted by hot objects and is thus sometimes called heat rays. • Radiation emitted by the Earth radiation is entirely in the infrared region, but it represents only a small fraction of total solar radiation – Microwave and radio waves are low wavelength, low energy waves (radio waves have the longest wavelengths) that are useful in communications, cooking, and weather radar.. Rev. 12 October 2006 The atmosphere and ocean 16 Blackbody • Blackbody: A blackbody is a perfect absorber and emitter of energy. • At a constant temperature, blackbodies absorb all wavelengths of energy incident upon them, and emit all wavelengths that they absorb. – Emissivity is a measure of how closely an object resembles a blackbody. Rev. 12 October 2006 The atmosphere and ocean 17 Wien’s displacement law, part 1 • Wien’s displacement law: The wavelength of the most intense radiation emitted by an object is proportional to its temperature. – max = C/T, where • max is the wavelength of most intense radiation emitted by an object • C is a proportionality constant, and equals 2,897 if max is expressed in micrometers • T is the temperature expressed in degrees Kelvin – In other words, hot objects emit radiation that peaks at short wavelengths, while cool objects do the opposite. Rev. 12 October 2006 The atmosphere and ocean 18 Wien’s displacement law, part 2 • The spectrum emitted by the sun is similar to that of a blackbody at about 6,000 K. • The spectrum emitted by the Earth is similar to that of a blackbody at 288 K. Rev. 12 October 2006 The atmosphere and ocean 19 Rev. 12 October 2006 The atmosphere and ocean 20 Rev. 12 October 2006 The atmosphere and ocean 21 Stefan-Boltzmann law • Stefan-Boltzmann law: The total energy flux, or flow, emitted by a blackbody across all wavelengths (E) is proportional to the absolute temperature of the object raised to the fourth power: – E = T4, where the temperature is given in degrees Kelvin. • In other words, a small change in the temperature of a blackbody results in a large change in the total amount of energy emitted by the object. Rev. 12 October 2006 The atmosphere and ocean 22 Inverse square law • Inverse square law: The intensity of radiation falling on an object diminishes with increasing distance from the source. – Doubling the distance reduces the intensity by 75 percent. Rev. 12 October 2006 The atmosphere and ocean 23 Global radiation equilibrium The global radiation equilibrium is a balance between radiation received by the Earth-atmosphere system and that energy lost from the Earth-atmosphere system. Rev. 12 October 2006 The atmosphere and ocean 24 The sun, part 1 • The sun is more than 80 percent hydrogen (by mass), with most of the rest of its mast as helium. – Its internal temperatures may exceed 20 million degrees C. – It produces energy by the fusion of four hydrogen atoms to form one helium atom. • The mass of the four hydrogens exceeds that of one helium by 0.7 percent; the remaining mass is converted to energy. – E = mc2, where • E is energy • m is mass • c is the speed of light (300,000 km per second) Rev. 12 October 2006 The atmosphere and ocean 25 The sun, part 2 • The visible surface of the sun is called the photosphere. – At about 6,000° C, the photosphere is much cooler than the sun’s interior. – Irregularly shaped convection cells, or granules, give the photosphere its honeycomb-like appearance. • Granules are about 1,000 km in diameter, with supergranules up to 50,000 km in diameter. – The photosphere also features dark, cool areas called sunspots. • They may be as much as 1,800° C cooler than the photosphere. Rev. 12 October 2006 The atmosphere and ocean 26 The sun, part 3 • The photosphere (continued): – Faculae are bright areas near sunspots. • The chromosphere, consisting of hydrogen and helium ions, extends outward from the photosphere. – Chromosphere temperatures range from 4,000° C to 40,000° C. • The outermost portion of the sun is the corona, a region of highly ionized gases with temperatures as high as 4 million degrees C. – The corona extends millions of kilometers into space. Rev. 12 October 2006 The atmosphere and ocean 27 The sun, part 4 • The solar wind originates in the corona and sweeps the solar system. • Solar flares that originate in the photosphere intensify the solar wind. • The Earth intercepts only a portion of the energy emitted by the sun. Rev. 12 October 2006 The atmosphere and ocean 28 Solar altitude • The intensity of solar radiation striking the Earth is a function of the solar altitude, or the angle that the sun is above the horizon. – The higher the angle of the sun, the more intense the radiation, with the most intense radiation striking areas where the sun is directly overhead (at a 90° angle). – It is the primary determinant of the amount of solar radiation reaching a particular place on Earth. Rev. 12 October 2006 The atmosphere and ocean 29 Rev. 12 October 2006 The atmosphere and ocean 30 Rev. 12 October 2006 The atmosphere and ocean 31 Daylength • Daylength is important because the longer the day, the more solar radiation can be received and the more heat can be absorbed. – Middle and high latitudes have pronounced seasonal variations in day length, while tropical areas have little variation. Rev. 12 October 2006 The atmosphere and ocean 32 Atmospheric obstructions • Atmospheric obstructions – such as clouds, particulate matter, and gas molecules – absorb, reflect, or scatter solar radiation. – How much effect they have depends on path length, the distance a ray must travel. • Because angle of incidence determines path length, atmospheric obstruction reinforces the pattern established by the varying angle of incidence. • Because they must pass through more atmosphere than high-angle rays, low-angle rays are subject to more depletion through reflection, scattering, and absorption. Rev. 12 October 2006 The atmosphere and ocean 33 Rev. 12 October 2006 The atmosphere and ocean 34 Latitudinal differences • The latitudinal radiation balance occurs because the belt of maximum solar energy swings back and forth through tropics as the direct rays of sun shift northward and southward in course of a year. – Low latitudes (about between 28° N and 33° S) receive an energy surplus, with more incoming than outgoing radiation. – There is an energy deficit in latitudes north and south of these low latitudes. • This simple latitudinal pattern is interrupted principally by atmospheric obstruction. Rev. 12 October 2006 The atmosphere and ocean 35 Rev. 12 October 2006 The atmosphere and ocean 36 Earth’s revolution, part 1 • The tropical year is the time it takes Earth to complete one revolution around the Sun; for practical purposes it can be simplified to 365.25 days. • Earth’s revolution is an ellipse, along which varies the Earth-Sun distance. – The varying distance between Earth and the Sun is not an important determinant of seasonal temperature fluctuations. Rev. 12 October 2006 The atmosphere and ocean 37 Earth’s revolution, part 2 • The two “end” points of the ellipse are the perihelion and aphelion. – The perihelion is the point in an orbit that takes a planet nearest to the Sun (for Earth, it is 147,166,480 kilometers or 91,455,000 miles, on January 3). – The aphelion is the point in an orbit that takes a planet furthest away from the Sun (for Earth, it is 152,171,500 kilometers or 94,555,000 miles, on July 4). Rev. 12 October 2006 The atmosphere and ocean 38 Rev. 12 October 2006 The atmosphere and ocean 39 The seasons, part 1 • The plane of the ecliptic is the imaginary plane that passes through the Sun and through every point of Earth’s orbit around the Sun. – It is not perpendicular to Earth’s rotation axis, which allows for seasons to occur. • Inclination is the degree to which Earth’s rotation axis is tilted (about 23.5 degrees away from the perpendicular). Rev. 12 October 2006 The atmosphere and ocean 40 The seasons, part 2 • Polarity, also called parallelism, occurs because Earth’s axis always points toward Polaris, the North Star, no matter where Earth is in its orbit. • Insolation is incoming solar radiation. – The angle at which the Sun’s rays strike Earth determines the amount of insolation reaching any given point on Earth. • That angle is a result of the combined effect of rotation, revolution, inclination, and polarity. Rev. 12 October 2006 The atmosphere and ocean 41 The seasons, part 3 • Solstices – A solstice is one of two times during year in which the Sun’s perpendicular (vertical) rays hit the northernmost or southernmost latitudes (23.5°). • On or about December 21 (called the winter solstice in Northern Hemisphere). • On or about June 21 (called the summer solstice in Northern Hemisphere). Rev. 12 October 2006 The atmosphere and ocean 42 The seasons, part 4 • Solstices (continued) – The tropic of Cancer is the parallel of 23.5° north latitude, which marks the northernmost location reached by the vertical (perpendicular) rays of the Sun; occurs on or about June 21. – The Tropic of Capricorn is the parallel of 23.5° south latitude, which makes the southernmost location reached by the vertical (perpendicular) rays of the Sun; occurs on or about December 21. Rev. 12 October 2006 The atmosphere and ocean 43 The seasons, part 5 • Solstices (continued) – The Arctic Circle is the parallel of 66.5° north latitude; experiences 24 hours of either light (circa June 21) or dark (circa December 21). – The Antarctic Circle is the parallel of 66.5° south latitude; experiences 24 hours of either light (circa December 21) or dark (circa June 21). Rev. 12 October 2006 The atmosphere and ocean 44 Rev. 12 October 2006 The atmosphere and ocean 45 Rev. 12 October 2006 The atmosphere and ocean 46 The seasons, part 5 • Equinoxes – The Equinoxes are the times of year when the perpendicular rays of the Sun strike the equator, the circle of illumination just touches both poles, and the periods of daylight and darkness are each 12 hours long all over Earth. • On or about March 20 (called vernal equinox in Northern Hemisphere). • On or about September 22 (autumnal equinox in Northern Hemisphere). – The equinoxes represent the midpoints in the shifting of direct rays of the Sun between the Tropic of Cancer and the Tropic of Capricorn. Rev. 12 October 2006 The atmosphere and ocean 47 Rev. 12 October 2006 The atmosphere and ocean 48 Rev. 12 October 2006 The atmosphere and ocean 49 The seasons, part 6 • Changes in daylight and darkness – The period of daylight varies throughout the year, increasing everywhere north of the equator from the shortest day of the year on the December solstice until the longest day of the year of the June solstice. Then days begin to shorten again in Northern Hemisphere. (Southern Hemisphere experiences an opposite effect.) Rev. 12 October 2006 The atmosphere and ocean 50 The seasons, part 7 • Changes in daylight and darkness (continued) – Both day length and the angle at which the Sun’s rays strike Earth are principal determinants of the amount of insolation received at any particular latitude. • Tropic latitudes are always warm/hot because they always have high Sun angles and consistent days close to 12 hours long. • Polar regions are consistently cold because they always have low sun angles. Rev. 12 October 2006 The atmosphere and ocean 51 Rev. 12 October 2006 The atmosphere and ocean 52 Rev. 12 October 2006 The atmosphere and ocean 53 Rev. 12 October 2006 The atmosphere and ocean 54 Rev. 12 October 2006 The atmosphere and ocean 55 Rev. 12 October 2006 The atmosphere and ocean 56 Rev. 12 October 2006 The atmosphere and ocean 57 The solar constant, part 1 • The solar constant is the fairly constant amount of solar insolation received at the top of the atmosphere at a place perpendicular to the solar beam. • The solar constant – which actually fluctuates slightly during the year – is about 1.97 Langleys per minute, or 1368 Watts per square meter (W/m2). – A Langley is a unit of measure of radiation intensity that is 1 calorie per square centimeter (a calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C). Rev. 12 October 2006 The atmosphere and ocean 58 The solar constant, part 2 • The solar constant varies with distance of the Earth from the sun, with the value is at the perihelion 2.04 Langley/min (or 1417 W/m2), and lowest at the aphelion, 1.91 Langley/min (or 1326 W/m2). Rev. 12 October 2006 The atmosphere and ocean 59 Radiation in the atmosphere • Solar radiation reacts with gases and aerosols as it travels through the atmosphere. – Some will be either reflected back into space or scattered. – Some will be absorbed. • Radiation budget: – Incoming solar radiation = percentage of radiation absorbed (absorbtivity) + percentage reflected or scattered (albedo) + percentage transmitted to Earth’s surface (transmissivity) Rev. 12 October 2006 The atmosphere and ocean 60 Reflectivity, part 1 • Reflection is the ability of an object to repel waves without altering either the object or the waves. – Reflectivity: The percentage of incoming solar radiation that is reflected back into space. – Albedo: the fraction of incoming solar radiation reflected by a surface; it is calculated as the ratio of the ratio between relflected radiation and insiden • albedo = (reflected radiation/incident radiation) • Dark surfaces have a low albedo and reflect relatively little solar radiaiton; light surfaces have a high albedo and reflect a lot. Rev. 12 October 2006 The atmosphere and ocean 61 Reflectivity, part 2 • Reflection (continued): – Law of reflection: Angle of incident radiation = the angle of reflected radiation – Clouds are the most important reflector of solar radiation. Rev. 12 October 2006 The atmosphere and ocean 62 Scattering • Scattering is the process by which light waves change in direction, but not in wavelength. Occurs in the atmosphere when particulate matter and gas molecules deflect wavelength and redirect them. – Sometimes when insolation is scattered, the waves are diverted into space; but most continue through atmosphere in altered, random directions. – Amount of scattering depends on wavelength of wave and the size, shape, and composition of the molecule or particulate. Rev. 12 October 2006 The atmosphere and ocean 63 Absorption, part 1 • Absorption is the ability of an object to assimilate energy from the electromagnetic waves that strike it. • The absorbed energy is converted into heat. • Different objects vary in their capabilities to absorb radiant energy (and thus increase in temperature). – Color plays a key role in an object’s absorption ability; dark-colored surfaces more efficiently absorb the visible portion of the electromagnetic spectrum than light-colored surfaces. Rev. 12 October 2006 The atmosphere and ocean 64 Absorption, part 2 • Oxygen, ozone, water vapor, and some aerosols absorb solar heat. – Stratospheric oxygen absorbs ultraviolet radiation at wavelengths shorter than 0.2 micrometers. – Stratospheric ozone absorbs ultraviolet radiation at wavelengths between about 0.2 and 0.3 micrometers. – The absorption by oxygen and ozone reduces the intensity of ultraviolet light at the surface of the Earth and results in a marked warming of the stratosphere. – Water vapor absorbs some wavelengths greater than 0.8 micrometers. Rev. 12 October 2006 The atmosphere and ocean 65 Absorption, part 3 • The atmosphere is more or less transparent to wavelengths between 0.3 and 0.8 micrometers. • Clouds absorb relatively little solar radiation. Rev. 12 October 2006 The atmosphere and ocean 66 Rev. 12 October 2006 The atmosphere and ocean 67 Albedo • Light surfaces have high albedo. – Fresh fallen snow may be as high as 95 percent. • Dark surfaces have low albedo. – Blacktop roads or spruce forests may be as low as 5 percent. • Albedo may vary on some surfaces depending on the angle of the sun above the horizon. – The higher the sun is, the lower the albedo. Rev. 12 October 2006 The atmosphere and ocean 68 Rev. 12 October 2006 The atmosphere and ocean 69 Rev. 12 October 2006 The atmosphere and ocean 70 Rev. 12 October 2006 The atmosphere and ocean 71 Solar radiation and oceans • While the atmosphere is relatively transparent to solar radiation, the oceans are otherwise, absorbing much solar radiation and transporting that heat to greater depths than land surfaces. – The various wavelengths are absorbed at differing depths. • In clear water, reds are completely absorbed within 15 m, wheras greens and blues penetrate to about 250 m. – Content of particles and dissolved substances affect absorption, with the colors that penetrate deepest shifting toward yellow-green in coastal areas to red in very turbid estuarine waters. Rev. 12 October 2006 The atmosphere and ocean 72 Rev. 12 October 2006 The atmosphere and ocean 73 Phase changes • Tremendous amounts of energy are involved in phase changes of water. – Latent heat of melting: 80 cal are required to convert 1 g of frozen water to liquid water at the freezing/melting point • Temperature remains at 0 °C until all ice melts – Latent heat of vaporization: varies, depending on initial temperature of water • 600 cal required to evaporate 1 g of liquid water at 0 °C • 540 cal required to evaporate 1 g of liquid water at 100 °C – Latent heat of sublimation: equals sum of latent heat of melting plus latent heat of vaporization • 680 cal required to evaporate 1 g of frozen water at 0 °C Rev. 12 October 2006 The atmosphere and ocean 74 Rev. 12 October 2006 The atmosphere and ocean 75 Sensible heating, part 1 • Heat transfer by conduction and convection can be measured (sensed) by temperature changes. • Sensible heating incorporates both conduction and convection. – Heating reduces the density of air, causing it to rise above cooler, denser air. • Convection thus transports heat from surface to troposphere • Convection is more important than conduction because air is a poor conductor of heat. Rev. 12 October 2006 The atmosphere and ocean 76 Sensible heating, part 2 • Sensible and latent heating often work together. – As air cools by convection, the water vapor in the air condenses, thus releasing its latent heat as sensible heat – and leading to the formation of cumulus clouds. – The latent heat released by water vapor is converted into sensible heat in the air. This in turn can lead to stronger updrafts, as is seen in cumulonimbus clouds. – By these processes, heat can also be transferred from the atmosphere to the surface, such as on cold nights when radiational cooling causes the surface to have a lower temperature than the air above. Rev. 12 October 2006 The atmosphere and ocean 77 Rev. 12 October 2006 The atmosphere and ocean 78 Rev. 12 October 2006 The atmosphere and ocean 79 Rev. 12 October 2006 The atmosphere and ocean 80 Bowen ratio • The Bowen ratio describes how heat energy received at the Earth is partitioned into sensible and latent heat. – Bowen ration = [(sensible heating)/(latent heating)] • Globally – Bowen ratio = [(7 units)/(23 units)] = 0.3 • The Bowen ratio varies considerably by region and surface type. Rev. 12 October 2006 The atmosphere and ocean 81 Latitudinal differences, part 1 • There is unequal heating of different latitudinal zones for four basic reasons, angle of incidence, day length, atmospheric obstruction, and latitudinal radiation balance: • The angle of incidence is the angle at which rays from the Sun strike Earth’s surface; always changes because Earth is a sphere and Earth rotates on own axis and revolves around the Sun. – Angle of incidence is the primary determinant of the intensity of solar radiation received on Earth. Rev. 12 October 2006 The atmosphere and ocean 82 Latitudinal differences, part 2 • The angle of incidence (continued). – Heating is more effective the closer to 90°, because the more perpendicular the ray, the smaller the surface area being heated by a given amount of insolation. • Angle is 90° if Sun is directly overhead. • Angle is less than 90° if ray is striking surface at a glance. • Angle is 0° for a ray striking Earth at either pole. Rev. 12 October 2006 The atmosphere and ocean 83 Latitudinal differences, part 3 • Day length is important because the longer the day, the more insolation can be received and the more heat can be absorbed. – Middle and high latitudes have pronounced seasonal variations in day length, while tropical areas have little variation. Rev. 12 October 2006 The atmosphere and ocean 84 Latitudinal differences, part 4 • Atmospheric obstructions – such as clouds, particulate matter, and gas molecules – absorb, reflect, or scatter insolation. – How much effect they have depends on path length, the distance a ray must travel. • Because angle of incidence determines path length, atmospheric obstruction reinforces the pattern established by the varying angle of incidence. • Because they must pass through more atmosphere than high-angle rays, low-angle rays are subject to more depletion through reflection, scattering, and absorption. Rev. 12 October 2006 The atmosphere and ocean 85 Latitudinal differences, part 5 • The latitudinal radiation balance occurs because the belt of maximum solar energy swings back and forth through tropics as the direct rays of sun shift northward and southward in course of a year. – Low latitudes (about between 28° N and 33° S) receive an energy surplus, with more incoming than outgoing radiation. – There is an energy deficit in latitudes north and south of these low latitudes. • This simple latitudinal pattern is interrupted principally by atmospheric obstruction. Rev. 12 October 2006 The atmosphere and ocean 86 Rev. 12 October 2006 The atmosphere and ocean 87 Global radiation budget • The Earth’s planetary albedo is 31 percent. – The moon’s albedo is seven percent. • The atmosphere absorbs 20 percent of the total solar radiation. • The Earth’s surface absorbs 49 percent of incoming solar radiation. • Reconciling the budget: – 31 + 30 + 49 = 100 Rev. 12 October 2006 The atmosphere and ocean 88 Radiation budget, part 1 • The global and annual average energy budget (for every 100 units incoming solar radiation): – – – – 31 units scattered and reflected to space 20 units absorbed by the atmosphere 49 units absorbed at the Earth’s surface 100 units total • At the Earth’s surface – 19 units lost due to infrared cooling – 49 units gained by solar heating – 30 units net heating Rev. 12 October 2006 The atmosphere and ocean 89 Radiation budget, part 2 • The atmosphere – 50 units lost due to infrared cooling – 20 units gained by solar heating – 30 units net cooling • Heat transfer from Earth’s surface to atmosphere – 7 units sensible heating (conduction plus convection) – 23 units latent heating – 30 units net transfer Rev. 12 October 2006 The atmosphere and ocean 90 Rev. 12 October 2006 The atmosphere and ocean 91 Rev. 12 October 2006 The atmosphere and ocean 92 Rev. 12 October 2006 The atmosphere and ocean 93 Rev. 12 October 2006 The atmosphere and ocean 94 Heat transfer, part 1 • The tropics would become progressively warmer (and less habitable) until the amount of heat energy absorbed equaled the amount radiated from Earth’s surface if not for two specific mechanisms moving heat poleward in both hemispheres: – Atmospheric circulation is the most important mechanism, accomplishing 75 to 80 percent of all horizontal heat transfer. Rev. 12 October 2006 The atmosphere and ocean 95 Heat transfer, part 2 • Heat transfer mechanisms (continued): – Oceanic circulation (ocean currents) reflect average wind conditions over a period of several years. • Current refers to various kinds of oceanic water movements. • The atmosphere and oceans serve as thermal engines; their currents are driven by the latitudinal imbalance of heat. • There is a direct relationship between these two mechanisms: – Air blowing over ocean is the principal driving force of major surface ocean currents; – Heat energy stored by ocean affects atmospheric circulation. • Waters cooler than the overlying air act as a heat sink. • Waters cooler than the overlying air act as a heat source. Rev. 12 October 2006 The atmosphere and ocean 96 Whither weather? • The sun heats the Earth. • The variations in heating through time and space generate radiation imbalances. • The imbalances generate energy redistribution mechanisms that are among the fundamental causes of weather and climate variations. – Weather systems do not last indefinitely, however, as kinetic energy is dissipated in the form of frictional heat as winds blow across the Earth’s surface. Rev. 12 October 2006 The atmosphere and ocean 97 Air mass advection • Air mass advection refers to the movement of air masses from one region to another. – Cold air advection occurs when wind transports colder air over a warmer land surface. – Warm air advection occurs when wind transports warmer air over a colder land surface. • The significance of air mass advection depends on the initial temperature of the air mass and the degree of modification it undergoes as it is transported. Rev. 12 October 2006 The atmosphere and ocean 98 Air masses, part 1 • An air mass is a large parcel of air that has relatively uniform properties in the horizontal dimension and moves as an entity. Such extensive bodies are distinct from one another and compose the troposphere. • Characteristics – An air mass must meet three requirements: • It must be large (horizontal and vertical). • Its horizontal dimension must have uniform properties (temperature, humidity, and stability). • It must be distinct from surrounding air, and when it moves, it must retain that distinction (not be torn apart. Rev. 12 October 2006 The atmosphere and ocean 99 Air masses, part 2 • Origin – Air mass formation occurs if air remains over a uniform land or sea surface long enough to acquire uniform properties. • Source regions are parts of Earth’s surface that are particularly suited to generate air masses because they are – Extensive – Physically uniform – Associated with air that is stationary or anticyclonic. Rev. 12 October 2006 The atmosphere and ocean 100 Air masses, part 3 • Classification – Because the source region determines properties of air masses, it is the basis for classifying them. – Classifications use a one- or two-letter code. – The following table provides a simplified classification of air masses, along with the properties associated with each. Rev. 12 October 2006 The atmosphere and ocean 101 Rev. 12 October 2006 The atmosphere and ocean 102 Air masses, part 4 • Movement and modification – Some air masses remain in source region indefinitely. – Movement prompts structural change: • Thermal modification – heating or cooling from below; • Dynamic modification – uplift, subsidence, convergence, turbulence; • Moisture modification – addition or subtraction of moisture. – A moving air mass modifies the weather of the region it moves through. Rev. 12 October 2006 The atmosphere and ocean 103 Air masses, part 5 • North American air masses – The physical geography of U.S. landscape plays a critical role in air-mass interaction. • There are no east-west mountains to block polar and tropical air flows, so they affect U.S. weather and climate. • North-south mountain ranges in the west modify the movement, therefore the characteristics, of Pacific air masses. Rev. 12 October 2006 The atmosphere and ocean 104 Air masses, part 6 • North American air masses (continued) – Maritime tropical (mT) air from the Atlantic, Caribbean/Gulf of Mexico strongly influences climate east of the Rockies in the United States, southern Canada, and much of Mexico. • Primary source of precipitation. Also brings periods of uncomfortable humid heat in summer. – Continental tropical (cT) air has insignificant influence on North America, except for bringing occasional heat waves and drought conditions to the southern Great Plains – Equatorial (E) air affects North America only through hurricanes. Rev. 12 October 2006 The atmosphere and ocean 105 Physics • Force: a push or pull that can cause an object at rest to move, or can affect the motion of an object already in motion. – Newton’s first law of motion says that an object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force. – The terms force and acceleration can be used interchangeably, according to Newton’s second law of motion: • force = (mass) x (acceleration) Rev. 12 October 2006 The atmosphere and ocean 106 Forces • Forces that affect the motion of air parcels: – Air pressure gradients – Centripetal forces (actually occurs as a consequence of other forces) – Coriolis effect – Friction – Gravity Rev. 12 October 2006 The atmosphere and ocean 107 Pressure gradient force • The pressure gradient force results from the difference in air pressure between two locations. – The steeper the gradient, the greater the force, and vice versa. – Air flows from where the pressure is greatest to where it is lowest. – Horizontal pressure gradients at the surface can be denoted by the spacing of isobars (lines of equal pressure). – Pressure gradients are measured along lines perpendicular to the isobars. Rev. 12 October 2006 The atmosphere and ocean 108 Centripetal force • Isobars plotted on weather maps are curved, thus the wind blows in curved paths. • The centripetal force is a force that confines an air parcel (or any object to a curved path). – If the force dissipates, the parcel flies off in a straight line in keeping with Newton’s first law of motion (that an object in motion stays in motion and an object at rest stays at rest until being acted upon by some force. Rev. 12 October 2006 The atmosphere and ocean 109 The Coriolis effect, part 1 • The Coriolis effect is the apparent deflection of free moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, in response to the rotation of Earth. – The objects, which are moving in a straight line in the atmosphere, appear to move a long a curved path as the Earth’s surface rotates out from under the object. Rev. 12 October 2006 The atmosphere and ocean 110 The Coriolis effect, part 2 • The Coriolis effect can significantly influence longrange movements. • There are four basic points to remember: – A free moving object appears to deflect to right in Northern Hemisphere and to left in Southern Hemisphere; – The apparent deflection is strongest at the poles and decreases progressively toward the equator where there is zero deflection; Rev. 12 October 2006 The atmosphere and ocean 111 The Coriolis effect, part 3 • Four basic points to remember (continued): – Fast-moving objects seem to be deflected more than slower ones because the Coriolis effect is proportional to the speed of the object; – The Coriolis effect influences direction only, not speed. • The Coriolis effect influences winds and ocean currents, in particular serving as important component of general circulation of oceans. Rev. 12 October 2006 The atmosphere and ocean 112 The Coriolis effect, part 4 • It does not affect the circulation pattern of water draining out of a washbowl – the time involved is too short and water speed so slow; instead draining direction is determined by the characteristics of the plumbing system, shape of washbowl, and pure chance. Rev. 12 October 2006 The atmosphere and ocean 113 Rev. 12 October 2006 The atmosphere and ocean 114 Rev. 12 October 2006 The atmosphere and ocean 115 Friction, part 1 • Friction is the resistance an object encounters as it moves against another object. • Viscosity: Friction of fluid flow – Molecular viscosity: results from the random motion of molecules in a liquid or gas – Eddy viscosity: results from the large, irregular motions that develop within fluids • Example: Effect of rocks in a fast-moving stream • Snow fences demonstrate effects of frictional slowing of wind. – Eddy velocity most important in meteorological processes Rev. 12 October 2006 The atmosphere and ocean 116 Friction, part 2 • The rougher the surface of the Earth, the greater is the eddy viscosity of the wind. – Forest-covered landscapes have more eddy viscosity than grass-covered ones. • Horizontal wind speed increases with altitude, up to about 1,000 m above the surface. – The portion of the atmosphere below 1,000 m is called the atmospheric boundary layer (ABL). • Turbulence is fluid flow caused by eddy motion. – Turbulence is often demonstrated as gusts of wind. Rev. 12 October 2006 The atmosphere and ocean 117 Gravity • All air parcels are subject to gravity. • Gravity results from the interaction of two forces, the centripetal force, and gravitation. – Gravitation is the force of attraction between two objects. • The magnitude of gravitation is the directly proportional to the masses of the two objects and inversely proportional to the distance between their centers of mass. – The force (acceleration) of gravity is about 9.8 m/sec2 – Gravity acts directly downward (toward the heaviest object’s center of mass. – Gravity does not modify the horizontal wind. Rev. 12 October 2006 The atmosphere and ocean 118 Joining forces, part 1 • The five forces (pressure gradient, centripetal, Coriolis effect, friction, and gravity) interact to control the horizontal and vertical motions of the atmosphere. – – – – Hydrostatic equilibrium Geostrophic wind Gradient wind Surface winds Rev. 12 October 2006 The atmosphere and ocean 119 Joining forces, part 2 • The hydrostatic equilibrium is point at which the gravitational force is equals the vertical pressure gradient force, such that the net vertical acceleration of a parcel of air is zero. • The geostrophic wind is a wind above the ABL that moves parallel to isobars as a result of balance between the pressure gradient force and the Coriolis effect. – Air parcels move in an oscillatory pattern which dampens as they approach geostrophic equilibrium, this is called an inertial oscillation. Rev. 12 October 2006 The atmosphere and ocean 120 Rev. 12 October 2006 The atmosphere and ocean 121 Joining forces, part 3 • Gradient wind is similar to geostrophic wind except that it blows in a curved path as a result of interactions among the pressure gradient force, Coriolis effect, and centripetal forces. – Centripetal forces prevent equilibrium, however. – Gradient winds blow around anticyclones and cyclones – Idealized anticyclone (Northern Hemisphere) • Pressure gradient force: away from center of cyclone • Coriolis effect: inward, slightly greater than pressure gradient force, leading to inward centripetal force • Clockwise flow (above ABL, parallel to isobars) Rev. 12 October 2006 The atmosphere and ocean 122 Rev. 12 October 2006 The atmosphere and ocean 123 Joining forces, part 3 • Gradient wind (continued): – Idealized cyclone (Northern Hemisphere) • Pressure gradient force: in toward center of cyclone, slightly greater than Coriolis effect, leading to inward centripetal force • Coriolis effect: outward • Counterclockwise flow (above ABL, parallel to isobars) Rev. 12 October 2006 The atmosphere and ocean 124 Rev. 12 October 2006 The atmosphere and ocean 125 Surface winds, part 1 • Geostrophic and gradient winds are frictionless (they occur at altitudes above the ABL). • In the ABL, friction combines with the Coriolis effect to balance the horizontal pressure gradient force. – Friction works in a direction opposite to the wind direction. – Coriolis effect operates at an angle perpendicular to the wind direction. – Friction slows wind velocity, thus weakening the Coriolis effect, with the result that wind direction shifts toward low pressure. Rev. 12 October 2006 The atmosphere and ocean 126 Rev. 12 October 2006 The atmosphere and ocean 127 Surface winds, part 2 • The effect of friction decreases with altitude to the point where it is nil at the top of the ABL. • Friction also affects winds flowing around anticyclones and cyclones, shifting wind directions toward low pressure. – Anticyclone (Northern Hemisphere): clockwise circulation that spirals outward – Cyclone (Southern Hemisphere): counterclockwise circulation that spirals inward Rev. 12 October 2006 The atmosphere and ocean 128 Rev. 12 October 2006 The atmosphere and ocean 129 Rev. 12 October 2006 The atmosphere and ocean 130 Rev. 12 October 2006 The atmosphere and ocean 131 Rev. 12 October 2006 The atmosphere and ocean 132 Rev. 12 October 2006 The atmosphere and ocean 133 Rev. 12 October 2006 The atmosphere and ocean 134 General circulation, part 1 • According to an idealized circulation pattern (on a non-rotating earth with a uniform surface), unequal heating would create a two-cell circulation pattern. – The cells, one in each hemisphere, would have rising air at the equator and descending air at the poles. – Winds at the surface would blow from the poles toward the equator, and winds aloft would blow from the equator toward the poles. • The rotation of the Earth and its variable surfaces create a complex atmospheric circulation pattern. Rev. 12 October 2006 The atmosphere and ocean 135 General circulation, part 2 • Only the tropical regions have complete vertical circulation cells, called Hadley cells. – In a Hadley cell, heating at the equator warms the air above, causing it to rise to elevations of about 15 km, where it cools, moves poleward, then subsides. The air descends at roughly 30° north or south latitude. – There are two Hadley cells. • Outside the tropical and subtropical latitudes, vertical cells do not exist or are weakly and sporadically developed. Rev. 12 October 2006 The atmosphere and ocean 136 Rev. 12 October 2006 The atmosphere and ocean 137 General circulation, part 3 • Global pressure patterns drive global wind patterns. • The patterns observed migrate north and south with the patterns of solar heating produced as the Earth orbits the sun. – The north-south migration of climate patterns is enhanced over the continents and reduced over the oceans. Rev. 12 October 2006 The atmosphere and ocean 138 Rev. 12 October 2006 The atmosphere and ocean 139 General circulation, part 4 • The general circulation of the atmosphere has seven surface components: – – – – – – – Subtropical highs Intertropical convergence zone Polar highs Subpolar lows Trade winds Midlatitide westerlies Polar easterlies Rev. 12 October 2006 The atmosphere and ocean 140 Rev. 12 October 2006 The atmosphere and ocean 141 General circulation, part 5 • Subtropical highs – The subtropical latitudes serve as the “source” of the major surface winds of the planet. – The subtropical highs (STHs) are large semipermanent anticyclones centered at about 30° latitude over the oceans – Their average diameter is about 3,200 kilometers. – They develop from the descending limbs of Hadley cells. – The location of the subtropical highs are coincident with most of the world’s major desert belts. – Migration of the anticyclones affects weather of midlatitudes. Rev. 12 October 2006 The atmosphere and ocean 142 General circulation, part 6 • Subtropical highs (continued) – The Horse latitudes are areas in the subtropical highs characterized by warm, tropical sunshine and an absence of wind. • They exist because the weather within a subtropical high is nearly always clear, warm, and calm. – The subtropical highs serve as source for two of the world’s three major surface systems: • Trade winds • Westerlies Rev. 12 October 2006 The atmosphere and ocean 143 General circulation, part 7 • Intertropical convergence zone – The intertropical convergence zone (ITCZ) is a belt of calm air where the northeast trades and southeast trades converge, generally in the vicinity of the equator (or at least the heat equator). • The zone is also called the equatorial front, the intertropical front, and the doldrums. • Intertropical convergence zone thunderstorms provide the updrafts where all the rising air in of the tropics ascends. • The zone often appears as a narrow band of clouds over oceans, but it is less distinct over continents. Rev. 12 October 2006 The atmosphere and ocean 144 General circulation, part 8 • Polar highs – The polar highs are anticyclones centered over the polar regions. • The Antarctic high is quite different from the Arctic high because it forms over an extensive, high-elevation, and very cold continent, while the Artic high forms primarily over sea ice. • The Antarctic high is strong, persistent, and almost permanent, while the Arctic high is much less pronounced and more ephemeral. • The polar highs are the source of the polar easterlies, which blow toward the equator and toward (not from) the west. Rev. 12 October 2006 The atmosphere and ocean 145 General circulation, part 9 • Subpolar lows – The subpolar lows are a zone of low pressure situated at about 50° to 60° of latitude in both the Northern and Southern hemispheres. • They often contain the polar front. • The characteristics vary in either hemisphere because the continents modify circulation in the Northern hemisphere, while circulation in the Southern hemisphere is over a virtually continuous expanse of ocean, the Southern Ocean. • The polar front is the meeting ground of the cold polar easterlies and the warm midlatitude westerlies, and is the site of genesis of many midlatitude weatehr systems. Rev. 12 October 2006 The atmosphere and ocean 146 General circulation, part 10 • Trade winds – The trade winds are the major wind system of the tropics, originating from the equatorward sides of the subtropical highs and blowing toward the west as well toward the equator. – The trades are the most reliable of all winds in terms of both direction and speed. – They are named for the direction they blow from. • In the Northern Hemisphere, they blow from the northeast, so are called the northeast trades. • In the Southern Hemisphere, they blow from the southeast, so are called the southeast trades. Rev. 12 October 2006 The atmosphere and ocean 147 General circulation, part 11 • Trade winds (continued) – The trades are warming, drying winds, but are capable of holding enormous amounts of moisture. • They generally do not release moisture unless forced by a topographic barrier or a pressure disturbance. – The winds typically pass over low-lying islands, drying them of moisture and turning them into desert islands. – On the other hand, windward slopes exposed to the trades, as in the mountains of Hawai’i, are some of the wettest places on Earth. Rev. 12 October 2006 The atmosphere and ocean 148 General circulation, part 12 • Midlatitude westerlies – The westerlies are the great wind system of the midlatitudes, flowing from west to east around the world in a latitudinal zone between about 30° and 60˚ both north and south of the equator. – They originate from the poleward side of the subtropical highs, blowing toward the poles and toward the east. – There are two cores of high-speed winds at high altitudes in the westerlies: • Polar front jet stream • Subtropical front jet stream Rev. 12 October 2006 The atmosphere and ocean 149 General circulation, part 13 • Midlatitude westerlies (continued) – A major feature of the midlatitude westerlies are the Rossby waves, sweeping north-south undulations that frequently develop aloft. • The undulating motion of the Rossby waves, coupled with the migratory pressure systems and storms associated with the westerlies, give the middle latitudes more short-run weather variability than any other place on Earth. • Anticyclonic circulation at the surface is associated with ridges in the waves, while cyclonic circulation is associated with troughs in the waves. Rev. 12 October 2006 The atmosphere and ocean 150 Rev. 12 October 2006 The atmosphere and ocean 151 General circulation, part 14 • Polar easterlies – The polar easterlies are a global wind system that occupies most of the area between the polar highs and about 60° of latitude. • The winds move generally from east to west and are typically cold and dry. Rev. 12 October 2006 The atmosphere and ocean 152 General circulation, part 15 • Vertical patterns of the general circulation – Winds in the upper elevations of troposphere differ from surface winds. • The most dramatic difference occurs between surface trade winds and the upper-elevation antitrade winds. • Antitrade winds are tropical upper air winds that blow toward the northeast in the Northern Hemisphere and toward the southeast in the Southern Hemisphere. Rev. 12 October 2006 The atmosphere and ocean 153 General circulation, part 16 • Vertical patterns (continued): – Trade wind inversion • Forms over tropical and subtropical oceans • The air just over the surface forms the marine boundary layer. • Air subsiding in the descending limb of the Hadley circulation undergoes compressional warming. • Where the descending air meets the marine boundary layer, a temperature inversion, in which the upper air is warmer than the lower air; this is the trade wind inversion. • The air in the trade wind inversion is very stable, as the inversion acts as a cap over the vertical movement of air, thus the convective development of clouds and precipitation. – It also limits orographic precipitation. Rev. 12 October 2006 The atmosphere and ocean 154 Rev. 12 October 2006 The atmosphere and ocean 155 Midlatitude cyclones, part 1 • A midlatitude cyclone is a large migratory lowpressure system that occurs within the middle latitudes and moves generally with the westerlies; midlatitude cyclones are also called lows or wave cyclones, depressions. – Midlatitude cyclones are probably most significant of all atmospheric disturbances. – They are basically responsible for most day-to-day weather changes. – They bring precipitation to much of the world’s populated regions. Rev. 12 October 2006 The atmosphere and ocean 156 Midlatitude cyclones, part 2 • Characteristics – Typical mature midlatitude cyclone is 1,600 kilometers (1,000 miles) in diameter; it has an oval shape. – Patterns of isobars, fronts, and wind flow in the Southern Hemisphere are mirror images of those in the Northern Hemisphere. – Northern Hemisphere patterns: • Circulation pattern converges counterclockwise; • Wind-flow pattern attracts cool air from north and warm air from south; creates two fronts. • These two fronts divide the cyclone into a cool sector north and west of center and a warm sector south and east. Rev. 12 October 2006 The atmosphere and ocean 157 Midlatitude cyclones, part 3 • Characteristics (continued) – Northern Hemisphere patterns (continued) • Circulation pattern converges counterclockwise; • Wind-flow pattern attracts cool air from north and warm air from south; creates two fronts. • These two fronts divide the cyclone into a cool sector north and west of center and a warm sector south and east. • Size of sectors varies with location: on ground, cool sector is larger, but in atmosphere, warm sector is more extensive. • Warm air rises along both fronts, causing cloudiness and precipitation, which follows patterns of cold and warm fronts. Rev. 12 October 2006 The atmosphere and ocean 158 Midlatitude cyclones, part 4 • Characteristics (continued) – Northern Hemisphere patterns (continued) • Much of cool sector is typified by clear, cold, stable air, while air of warm sector is often moist and tending toward instability, so may have sporadic thunderstorms. May have squall fronts of intense thunderstorms. • Movements – Midlatitude cyclones move throughout their existence. • Movement is typically from West to East. • Cold front moves faster than warm front. Rev. 12 October 2006 The atmosphere and ocean 159 Rev. 12 October 2006 The atmosphere and ocean 160 Rev. 12 October 2006 The atmosphere and ocean 161 Midlatitude cyclones, part 5 • Lifecycle – Origin to maturity typically takes 3 to 6 days, then another 3 to 6 days to dissipate. – Cyclogenesis is the birth of cyclones. – Most common cause believed to be upper-air conditions in the vicinity of the polar-front jet stream. – Most begin as waves along the polar front. – Cyclogenesis can also occur on the leeward side of mountains. – Often bring heavy rain or snowstorms to the northeastern United States and southeastern Canada. Rev. 12 October 2006 The atmosphere and ocean 162 Midlatitude cyclones, part 6 • Lifecycle (continued) – A comma cloud may be apparent at the peak of the storm. – After cyclonic circulation is well developed, occlusion begins. – After occluded front is fully developed, cyclone dissipates. • Occurrence and Distribution – Occur at scattered but irregular intervals throughout the zone of the westerlies. – Route of cyclone is likely to be undulating and erratic, but it generally moves west to east. Rev. 12 October 2006 The atmosphere and ocean 163 Rev. 12 October 2006 The atmosphere and ocean 164 Rev. 12 October 2006 The atmosphere and ocean 165 Rev. 12 October 2006 The atmosphere and ocean 166 Midlatitude cyclones, part 7 • Bomb cyclogenesis is the rapid development of a cyclone. – Bombs are defined as storms whose central pressure drops by 24 mb in 24 hours. – Most form in winter over warm water, such as over the Kuroshio Current off Japan and the Gulf Stream off the United States. • The “Perfect Storm” of 1991 is a classic example of a bomb. Rev. 12 October 2006 The atmosphere and ocean 167 Rev. 12 October 2006 The atmosphere and ocean 168 Rev. 12 October 2006 The atmosphere and ocean 169 Rev. 12 October 2006 The atmosphere and ocean 170 Rev. 12 October 2006 The atmosphere and ocean 171 The Fitz, part 1 • In 1976, Gordon Lightfoot, a Canadian singer/songwriter, released a song, The Wreck of the Edmund Fitzgerald, that recounts the November 10, 1975, sinking of the largest ore carrier on the Great Lakes. The song, which became a huge hit, has a haunting melody which seared the wreck of the ship, with the loss of 29 lives, into the consciousness of many who have heard Lightfoot’s recording. Rev. 12 October 2006 The atmosphere and ocean 172 Rev. 12 October 2006 The atmosphere and ocean 173 Rev. 12 October 2006 The atmosphere and ocean 174 The Fitz, part 2 • The 222-meter-long ship, carrying 26,000 tons of iron ore, departed the Duluth-Superior harbor on the afternoon of November 9. The Fitz’s destination was a plant at Zug Island on the Detroit River. • At 0600 CST on November 9, a low-pressure system began developing over central Kansas. It rapidly intensified as it tracking toward the northeast. • The storm passed near La Crosse, Wis., at 0600 on November 10. Rev. 12 October 2006 The atmosphere and ocean 175 The Fitz, part 3 • The storm was centered just west of Marquette, Mich., at noon, with a central pressure of 982 mb. – Gale-force northeast winds swept the eastern end of Lake Superior, gusting to 115 km (71 mph) at Sault Ste Marie, Mich. • At 0100 on November 10, the Fitz reported northeast winds of 97 km with winds to 3 m. • At 0700, with the ship about 73 km north of Copper Harbor, Mich., the ship reported northeast winds of 67 km. Rev. 12 October 2006 The atmosphere and ocean 176 The Fitz, part 4 • The Fitz’s captain, Ernest McSorley, then chose a course that took the ship through waters that sheltered it from the strong northeast winds. • The storm passed over the ship during the afternoon. • The storm’s center approached Moosonee, Ontario, a town on the shore of James Bay, that evening. • By that time, winds over Lake Superior had shifted from the northeast to the north, then to the northwest and west. Rev. 12 October 2006 The atmosphere and ocean 177 The Fitz, part 5 • The longer fetch (length of open lake surface exposed to the wind) of the northwest and west wind fueled the development of higher waves. • A nearby ship, the Arthur M. Anderson, reported winds of 95 km, gusting to 137 km, with waves of 3.5 m to 5 m. • Sometime between 0615 and 0625, the Fitz disappeared from the Anderson’s view as well as radar, sinking in 163 m of water within 27 km of Whitefish Point, Mich., and shelter from the wind. Rev. 12 October 2006 The atmosphere and ocean 178 Rev. 12 October 2006 The atmosphere and ocean 179 Midlatitude anticyclones, part 1 • A midlatitude anticyclone is an extensive migratory high-pressure cell of the midlatitudes that moves generally with the westerlies. – They are typically larger than a midlatitude cyclone, but also moves west to east. – They travel at the same rate, or little slower, than midlatitude cyclone. • Anticyclones are prone to stagnate or remain over same region (while cyclones do not). • They can cause concentration of air pollutants. Rev. 12 October 2006 The atmosphere and ocean 180 Midlatitude anticyclones, part 2 • Cold- and warm-core systems – A cold-core anticyclone is a dome of cold air. • In the Northern Hemisphere, continental polar (cP) or Arctic (A) air masses are cold-core systems. – cP air masses form from the polar high. – A air masses form from the arctic high. • Cold-core anticyclones are the most intense (with highest pressures) in the winter. – Powerful systems can bring freezing weather to South Florida. Rev. 12 October 2006 The atmosphere and ocean 181 Midlatitude anticyclones, part 3 • Cold- and warm-core systems (continued): – A warm-core anticyclone forms south of the polar front and consists of subsiding warm, dry air. • Warm-core anticyclones bring intense heat waves and droughts. • The subtropical highs, such as the Bermuda High, are warm-core anticyclones, but others may form over the interior of continents. • Cyclones and anticyclones alternate with one another in an irregular sequence. – There is often a functional relationship between the two. • An anticyclone can be visualized as a polar air mass with the cold front of cyclone as its leading edge. Rev. 12 October 2006 The atmosphere and ocean 182 Sea and land breezes • Sea and land breezes are a common local wind system along coastlines. – They are essentially a convectional circulation caused by differential heating of land and water surfaces. – A land breeze is a local wind blowing from land to water, usually at night (and normally considerably weaker flow than that of sea breeze). – A sea breeze is a local wind blowing from sea toward the land, usually during the day. Rev. 12 October 2006 The atmosphere and ocean 183 Rev. 12 October 2006 The atmosphere and ocean 184 Greenhouse effect, part 1 • In the atmosphere, atmospheric gases, known as greenhouse gases, transmit the incoming solar shortwave radiation, which are absorbed by Earth’s surface. • Atmospheric gases do not transmit the outgoing longwave terrestrial radiation, but instead absorb it, then reradiate the terrestrial radiation back toward the surface. • Heat is then trapped in the lower troposphere. • This is known as the greenhouse effect. Rev. 12 October 2006 The atmosphere and ocean 185 Greenhouse effect, part 2 • It is more appropriately called the atmospheric effect, because the warming of the atmosphere is not the same as what happens in actual greenhouses, as originally thought. – Greenhouses stay warm because warm air is trapped inside and does not mix with the cooler air outside, so the heat does not dissipate. Rev. 12 October 2006 The atmosphere and ocean 186 Greenhouse effect, part 3 • The warming up of the atmosphere is more similar to what occurs in a closed automobile parked in the sunlight. – The window glass transmits shortwave radiation, which is then absorbed by the upholstery. – The car emits longwave radiation, which is not readily transmitted through the glass. Rev. 12 October 2006 The atmosphere and ocean 187 Greenhouse effect, part 4 • A natural greenhouse effect maintains a livable temperature at the surface of the Earth. – From space, the Earth radiates at -18° C. – Average temperatures at the surface are about 15° C. – The 33° C difference is a result of the natural greenhouse effect. Rev. 12 October 2006 The atmosphere and ocean 188 Greenhouse effect, part 5 • The gases in the atmosphere that trap and reradiate longwave energy are called greenhouse gases. – Major greenhouse gases include water vapor, carbon dioxide, ozone, methane, and nitrous oxide. • Water vapor is the principal greenhouse gas. – These gases absorb relatively little radiation in wavelengths near the Earth’s peak infrared intensity – These windows of radiation transmission – in which little or no radiation is absorbed – are called atmospheric windows. Rev. 12 October 2006 The atmosphere and ocean 189 Greenhouse effect, part 6 • Clouds are composed of water vapor, but their role in the greenhouse effect are unclear. – They can absorb and radiate infrared radiation, thus warming the atmosphere. – They can reflect solar radiation, thus cooling the atmosphere. Rev. 12 October 2006 The atmosphere and ocean 190 Global warming, part 2 • There are strong indications that this effect has been intensified by human actions. • According to climate data, the average global temperature has increased about 0.6 degree C during the 20th century, with the warmest records occurring since 1990s. • Measurements of this temperature increase, both direct and proxy, have pointed toward a clear warming trend on the Earth in recent decades. Rev. 12 October 2006 The atmosphere and ocean 191 Global warming, part 3 • Although the climate changes do occur naturally, the evidence is increasingly pointing to these changes being caused by anthropogenic sources. – The International Panes on Climate Change (IPCC) released a report in 2001 discussing climatic changes on both global and local scales and the strong evidence pointing to this change being a result of human activities. Rev. 12 October 2006 The atmosphere and ocean 192 Global warming, part 4 • Carbon dioxide and other “greenhouse gasses” appear to be the principal offenders. – Carbon dioxide is believed to be responsible for about 75 percent of the human-enhanced greenhouse effect. – Since 1750 carbon dioxide levels have increased by more than 30 percent. – This increase in carbon dioxide is attributed to the increased burning of fossil fuels in recent decades. – The warming is further exacerbated by the burning of the tropical rainforests which serve to absorb carbon from the atmosphere. Rev. 12 October 2006 The atmosphere and ocean 193 Rev. 12 October 2006 The atmosphere and ocean 194 Global warming, part 5 • Principal offenders (continued): – Carbon dioxide concentrations are the highest in 420,000 years and the rate of increase is greater than any in the past 20,000 years. – Carbon dioxide concentrations may be double preindustrial levels by the end of the century. – A 50 percent decrease in fossil fuel consumption would be required to eliminate the warming trend. – The increased use of other gases (methane, chloroflurocarbons, and nitrous oxides) have also contributed to the increase in global temperatures. Rev. 12 October 2006 The atmosphere and ocean 195 Global warming, part 6 • According to the IPCC Synthesis report, it is estimated that: – There will be an average global temperature increase of 1.4 to 5.8 degrees C between 1990 and 2100 – a climate change greater in magnitude than anything in the past 10,000 years. – Climate zones may shift poleward – by as much as 550 km in North America. – Sea levels may rise because of melting icecaps and thermal expansion of seawater. • Sea level rise will accelerate coastal erosion and flooding. Rev. 12 October 2006 The atmosphere and ocean 196 Global warming, part 7 • Warming effects (continued): – There will be an increase in climate variability. – There will be increasing socioeconomic costs related to weather damage. • It’s not all bad news: – Growing seasons may lengthen in higher latitudes. – Savings in heating costs may offset cost of air conditioning in middle and low latitudes. – Seasonally dependent navigation may benefit from longer ice- and snow-free periods. Rev. 12 October 2006 The atmosphere and ocean 197 Global warming, part 8 • The warming has not been globally uniform, but rather widespread. – Warming so far has been greatest over North American and Eurasia. – Seven of the warmest years on record have occurred since 1990. – On regional scales, there is clear evidence of changes in variability or extremes, with a general trend in the reduction of diurnal temperature range over more than 40 percent of the global landmass. Rev. 12 October 2006 The atmosphere and ocean 198 Global warming, part 8 • Widespread warming (continued): – There has been an increase in the number of frost free days, and an increase in the number of days in the growing season. – Average sea level rise has been increasing at a rate of 1 to 2 mm per year. – Arctic sea ice thickness had decreased between 10 percent and 15 percent. – Montane glaciers are in widespread retreat. Rev. 12 October 2006 The atmosphere and ocean 199 Rev. 12 October 2006 The atmosphere and ocean 200 Global warming, part 9 • The view from the crystal ball is obscure. – Isolated events may affect climate in unforeseen ways. • The eruption of Mount Pinatubo in the Philippines in 1991 emitted massive amounts of sulfur dioxide (SO2) into the atmosphere. • The sulfur dioxide mixed with water droplets to form sulfuric acid; some formed sulfate aerosols. • The sulfate aerosols, by absorbing, scattering, and reflecting solar radiation back into space, reduced the amount reaching the Earth’s surface, contributing to a reduction in global temperatures that lasted until 1993. Rev. 12 October 2006 The atmosphere and ocean 201 Global warming, part 10 • Some scientists do not believe global warming is occurring. – Some claim the observed warming is an artefact caused by changes in instrumentation, urbanization, and gaps in the global network of weather stations. – Some argue that global climate models used to predict the effects of a carbon-dioxide enriched atmosphere are flawed. Rev. 12 October 2006 The atmosphere and ocean 202 Global warming, part 11 • Most scientists argue that the risks of doing nothing outweigh the risks of taking steps to head off global warming. • Suggested solutions include: – – – – Reducing fossil fuel consumption; Increasing reliance on alternative energy sources; Increasing energy efficiency; Halting deforestation and reforesting areas previously cleared. • These suggested solutions have added benefits. Rev. 12 October 2006 The atmosphere and ocean 203