Composition and Structure, Energy Balance, Seasons and Climate

Geography 311 – The Atmosphere Summary notes. Part I: Composition and Structure, Energy Balance, Seasons and Climate Composition of atmosphere names and approximate % of the primary gases know which are permanent and which are variable gases carbon dioxide increase since industrialization (burning of fossil fuels), and seasonal cycle (photosynthesis) ozone layer in the stratosphere Vertical structure of atmosphere units of air pressure, approximate air pressure at sea level fall off of pressure with altitude – approx. rate of fall off (halves every 6 km) what causes air pressure (weight of air above) temperature structure of atmosphere names of layers (troposphere thru’ thermosphere) and boundaries separating them approximate altitude of tropopause why does temperature fall off with altitude in troposphere why does temperature increase in stratosphere and thermosphere in which layer does all weather, winds, rain etc. happen no (very little) mixing between troposphere and stratosphere stratosphere is dry, troposphere is wet pollutants get rained out in troposphere Energy, heat and temperature difference between heat (total kinetic energy) and temperature (average speed of molecules) warm air less dense, cold air is more dense air will rise (or sink) until it has the same temperature (and density) as surrounding air temperature scales – Fahrenheit, Centigrade, Kelvin (conversions between these) specific heat – definition – amount of heat required to raise the temperature of unit mass of a substance by 1ºC water – 1 calorie per gm per degree soil – one fifth of this (0.2 cal/g ºC) air – one quarter (0.24 cal/g ºC) means that it takes a lot more energy to change the temperature of water than land so oceans undergo much less daily and seasonal variation in temperature than land difference in specific heat of water and soil is primary reason that ocean air stays cooler in summer and moderates the climate of coastal cities latent heat – heat required to change the phase of a substance without changing its temperature latent heat of melting (heat required to melt water) latent heat of evaporation (heat required to evaporate water into vapor) latent heat of condensation (heat released by condensing water) – provides heat to air when water condenses in clouds. This heat causes air to rise and provides energy to drive thunderstorms. Also important as form of energy transport in moving surplus energy from tropics to high latitudes. latent heat of freezing (heat released by freezing water into ice) sensible heat – heat we feel and can measure with a thermometer mechanisms of heat transfer conduction = transfer of heat through matter via collisions between molecules (requires contact. air is poor conductor. metals are good conductors.) convection = transfer of heat by mass movement (can only take place in fluids, gases) – responsible for rising parcels of warm air, descending parcels of cool air; and for global circulation radiation = heat transfer via electromagnetic waves (light) can travel through a vacuum mechanism by which sun’s energy reaches earth all radiation travels at 300,000 km per sec = 3 x 108 m/s radiation is characterized by its frequency and wavelength Radiation = transfer of energy by electromagnetic waves heating is caused by absorption of radiation speed = frequency x wavelength speed is constant, so frequency varies as 1/wavelength. If wavelength increases frequency decreases and vice versa energy varies with frequency – the higher the frequency, the shorter the wavelength and the higher the energy the lower the frequency, the longer the wavelength and the lower the energy the electromagnetic spectrum – in order of increasing wavelength, λ (decreasing energy): X-rays, gamma rays (λ < 0.2 µm) ultraviolet radiation (UV) (0.2 < λ < 0.4 µm) visible light (0.4 < λ < 0.7 µm) infrared radiation (0.7 < λ < 100 µm) microwaves (mm, cm) radio waves (m) radiation is emitted by everything the wavelengths of radiation emitted by an object depend on the object’s temperature the hotter the object, the shorter the wavelength of radiation emitted hotter objects radiate more total energy per unit area than cold objects Stefan-Boltzman law: E = σT4 , E = energy per sec per unit area Wien’s law: λmax= a/T In equilibrium an object absorbs energy at the same rate it emits it and its temperature remains constant. A “Black Body” is a perfect absorber and emitter of radiation (It absorbs and re-emits all the energy it receives at all wavelengths.) The earth and sun behave as black bodies (and thus obey the Stefan-Boltzman and Wien’s laws) solar spectrum – peaks in visible (about 0.5 µm) (sun’s surface temperature ~ 6000K – use Wien’s Law) with no atmosphere if the earth absorbs and re-emits all radiation it receives from the sun then its temperature would be the “Radiative Equilibrium Temperature” = 255 K. actual average surface temperature = 288 K. Difference is due to “greenhouse effect” of atmosphere – gases in earth’s atmosphere selectively (at specific wavelengths) absorb and emit radiation. Gases in the earth’s atmosphere are selective absorbers (and emitters) – i.e. they absorb radiation at certain very specific wavelengths The earth absorbs nearly all the solar radiation which arrives at its surface and emits the energy at infrared wavelengths (because the temperature of the earth is ~300K, the wavelength where the maximum radiation occurs is ~ 10 µm – from Wien’s Law) Gases that absorb in the solar spectrum block incoming radiation from the sun and heat up the atmosphere – these are primarily oxygen (O2) and ozone (O3). They are important because they prevent the highest energy ultraviolet radiation from reaching the earth. Absorption by ozone heats up the stratosphere, absorption by oxygen heats up the thermosphere. The gases that absorb in the infrared spectrum block outgoing radiation from the earth and re-radiate it back to the earth’s surface, warming the surface. These are greenhouse gases. Energy balance Solar radiation budget – albedo of earth-atmosphere ~ 30% (20% reflected/scattered by clouds, 10% by earth’s surface and by gases) ~19% of incoming solar radiation is absorbed by gases, dust and clouds in the atmosphere ~51% absorbed at the earth’s surface (albedo = % of radiation striking a surface which returns back (gets reflected), scattering = deflection of light by gases and particles) Earth absorbs ~ 51% of solar energy which reaches the top of the atmosphere. It then radiates this energy back into the atmosphere at infrared (IR) wavelengths. (The solar radiation is at short wavelengths because the sun is hot. The earth’s radiation is at long wavelengths because the earth is much cooler.) Some gases in the atmosphere absorb the outgoing IR radiation at certain wavelengths– nitrous oxide, methane, water vapor, carbon dioxide. There is an ‘atmospheric window’ between 8 and 13 µm where most radiation emitted from the earth reaches space without being absorbed. Earth’s energy budget – Solar energy heats the surface of the earth. The earth is in equilibrium (it is not continuously heating up or cooling down) so it gets rid of the same amount of energy as it takes in. (There is a balance of incoming and outgoing energy over the whole earth’s surface over the year, but at any particular location at any particular time there will be a net intake or outflow of energy – eg. during the day the incoming energy exceeds the outgoing energy and vice versa at night.) The earth loses energy by: conduction (contact between surface air and surface of earth) and convection (resulting upward motion of warm air) evaporation (provides latent heat of evaporation) radiation Most of the radiation emitted by the earth is absorbed in the atmosphere by greenhouse gases and clouds. Some of this energy will be reradiated back to the surface of the earth. This raises the temperature of the earth to an equilibrium value of ~ 288K (15 ºC). Seasons The earth’s axis is inclined at 23 ½ º relative to the plane of the earth’s orbit around the sun. It is this tilt that is responsible for the earth’s seasons. (No tilt -> no seasons) The earth spins on its axis once every 24 hrs. The earth revolves in an elliptical orbit around the sun every 365 ¼ days. Earth-sun distance = 150 million km = 1.5 x 1011m. Small difference in distance between earth and sun between Jan and July does NOT produce seasons or play a significant role in producing temperature variations. Seasonal temperature changes are due to: 1. angle of noon sun on any given day only places along a particular latitude will receive overhead (90º) sun. As we move N or S from this location the sun’s rays strike at an ever-decreasing angle. The more oblique the angle, the less intense the light. At summer solstice (June 21) the sun is overhead at 23 ½ ºN (Tropic of Cancer) At winter solstice (Dec 21) sun is overhead at 23 ½ ºS (Tropic of Capricorn) At equinoxes (March 21, Sept 22) sun is overhead at equator. Can calculate angle of noon sun. Secondary effect of angle of sun is that when the angle is more oblique the sunlight has to travel through a thicker layer of the earth’s atmosphere, which reduces its intensity. 2. length of daylight determined by position of earth in its orbit around the sun. Need to compare fraction of line of latitude which is in daylight to that in dark to find # hours daylight vs # hours dark. Equator always has 12 hrs day, 12 hrs night At equinoxes everywhere has 12 hrs day, 12 hrs night At June solstice everywhere in NH has more than 12hrs day, less than 12 hrs night, # daylight hrs increases as you go N, everywhere N of 66 ½ ºN has 24 hrs daylight. Opposite is true in SH. (everywhere has less than 12hrs day, more than 12 hrs night, # daylight hrs decreases as you go S, everywhere S of 66 ½ ºS has 24 hrs night.) At December solstice conditions are reversed for the two hemispheres over June solstice. Heat balance There is a net excess of radiation reaching tropical regions over the year and the earth receives more solar radiation than the earth radiates in the tropical regions. At high latitudes the earth radiates out more energy than it receives as radiation from the sun. But for the earth as a whole the incoming energy = outgoing energy. Therefore energy has to be moved from the tropics towards the poles. 3 mechanisms of heat transfer – by winds by ocean currents (tend to follow same general patterns as winds) by latent heat (water vapor evaporates at tropics and is carried to high latitudes, where it condenses and forms rain) Temperature daily cycle – even though incoming solar energy reaches peak at noon, daily temperature is usually hotter later (earth takes time to heat up) global pattern of air temperatures – all places at same latitude receive same amount of sun, but can have vastly different temperatures. Local temperature is also controlled by: 1. land and water heating 2. ocean currents 3. altitude 4. geographic position 5. cloud cover map of temperature patterns on average, temperature decreases from tropics to pole. Temperature falls off more rapidly with latitude in winter hemisphere than summer hemisphere. 1. effect of land and water isotherms do not run straight across, they bend around continents – in winter temperatures are a lot lower in middle of continent than on W coast in summer temperatures are a lot higher in middle of continent than on W coast Why? Because land heats more rapidly and to higher temperatures than water, and cools more rapidly and to lower temperatures than water, so temperature variations over land are very much greater than over water. Winds are from the west, so west coast has strong ocean influence (maritime climate). The middle of continents and E coast are not influenced much by the ocean (continental climate). Why are temperature changes much smaller over ocean? 1. specific heat of water is high compared to land 2. water is highly mobile (shares its heat over large volume) 3. water is more transparent (shares its heat over large volume) 4. evaporation greater over water (some heat goes into latent heat) Compare annual temperature variations of coastal and inland cities 2. ocean currents: locations on W coast next to warm ocean currents are heated in winter locations on W coast next to cold ocean currents are cooled in summer 3. altitude temperature drops an average of 6.5 ºC per km as you go up in altitude 4. geographic position direction of prevailing wind is important 5. cloud cover clouds keep Earth cool during the day because they reflect incoming sunlight clouds keep Earth warm at night because they trap the outgoing infrared radiation from the Earth