Heating Earth’s Surface and Atmosphere p Chapter 2-3 September 6 6, 2009 This chapter discusses: • Temperature and heat transfer – Conduction, Conduction convection convection, radiation • Solar radiation, earth energy balance • The seasons and temperature variations This chapter discusses: • Temperature and heat transfer – Conduction, Conduction convection convection, radiation • Solar radiation, earth energy balance • The seasons and temperature variations Temperature and Heat Transfer f ‘‘Temperature’’ is the quantity that tells us ‘how ‘ hot or cold something is relative to some set standard value’. • Air is made up of billions of atoms and molecules, moving in p g and bumping p g around. They y don’t all all directions,, spinning move at same speed. – Energy associated with this motion is called kinetic energy, the energy of motion motion. – Temperature of air is the measure of its average kinetic energy – Temperature is a measure of the average speed of the atoms and molecules, where higher temperatures correspond to faster average speeds. – Absolute zero – at this temp the atoms and molecules would posses a minimum amount of energy and theoretically no thermal motion Vibration, spinning, moving, bumping around => Kinetic energy ≈ Temperature Temperature Scales • • • • • Kelvin Scale - a temp scale begins at absolute zero (“no motion”) F h Fahrenheit h it Scale S l – assigned i d 32 as the number where water freezes and 212 where water boils. 180 equal divisions called degrees Celsius Scale – Zero on this scale assigned to the temperature at pure water freezes and 100 which p to temp where pure water boils. Divided into 100 equal degrees C=5/9(F-32) K=C+273 Comparison of the Kelvin, Celsius, and Fahrenheit scales Heat • The transfer of energy into or out of an object due to temperature differences between one object to another • Heat flows from a region of higher temp to one of lower temp • • Three types of heat-transfer 1. Conduction 2 Radiation 2. R di ti 3. Convection Additionally latent heat is very important in the atmosphere Additionally, 1)) Conduction • The transfer of heat from the hot end of the metal pin to the cool end by molecular contact is called conduction. d ti • Molecules in the end of the pin absorb energy from the flame and vibrate faster than those farther away from f flame, fl energy is i eventually transferred from molecule to molecule to hand. 2)) Convection • The transfer of heat byy the mass movement of a fluid (such as water and air) is called convection. convection • Convection happens pp naturally y in the atmosphere. (i.e. Thermals – rising bubbles of air.) • Radiator induces convection in a room. Note: In our atmosphere, air that rises expands and Cools air that sinks is compressed and warmed! Incoming solar energy The development of a thermal. A thermal is a rising bubble of air that carries heat energy upward by convection 3)) Radiation • C Carrying radiant energy • Only heat transfer that can travel through the vacuum of space (without medium) • The energy transferred from the sun to your face on a warm day is called radiant energy or radiation. The sunlight travels through the air with little effect on th air the i it itself. lf Th The energy ttravels l iin th the fform off waves that release energy when they are absorbed by an object. j These are called electromagnetic g waves because they have magnetic and electrical properties. This chapter discusses: • Temperature and heat transfer – Conduction, Conduction convection convection, radiation • Solar radiation, earth energy balance • The seasons and temperature variations Electromagnetic Spectrum of Radiation frequ uency Solar radiation Radiation characterized according to wavelength. wavelength As the wavelength decreases, the energy carried per wave increases. Energy carried per wave or photon micrometer is 10-6 *Radiation laws • • • All objects continually emit radiant energy Stefan-Boltzman law – Hotter object emits more energy – The rate of radiation emitted by a body is proportional to the fourth power of the body’s temperature – E=σT4 – Sun: ~6000K, E=~ 73,483,200 W/m2 – Earth: ~300K, E=~ 459 W/m2 (~0.006% of Esun) Wien’s displacement law – Wavelength of maximum emission is inversely proportional to temperature of a radiating body – λmax = C/T – Sun: ~0.483 ɥm – Earth: ~9.66 ɥm • Good absorbers are good emitters Solar radiation and Earth radiation The hotter sun not only radiates more energy than that of the cooler earth (the area under the curve), but it also radiates the majority of its energy at much shorter wavelengths. (The area under (Th d the th curves is equal to the total energy emitted, and the scales for the two curves differ by a factor of 100,000.) Solar radiation is shortwave radiation. Earth radiation is longwave radiation Balancing act – absorption, absorption emission, and equilibrium If the earth and all things on it are continually radiating energy, why doesn't everything get progressively colder? Absorption = Emission (radiative equilibrium) • • • Absorptivity, the rate at which something radiates and absorbs energy depends strongly on its surface characteristics, such as color, texture, moisture i t and d ttemperature. t Blackbody – an object that is a perfect absorber (it absorbs all the radiation that strikes it) and a perfect emitter (emits the maximum radiation di i possible ibl at iits given i temperature). ) D Does not h have to b be bl black k in color. Earths surface is nearly 100% efficient and thus behaves like a blackbody Radiative equilibrium temperature – average temp at which the rate of absorption of solar radiation equals the rate of emission of infrared earth radiation. Going to equilibrium temperature 1. Incoming solar radiation is fixed p warms the Earth 2. Absorption 3. Radiation from Earth increases as Tearth increases 4. Tearth reaches a critical temperature, eventually Absorption = Emission occures Radiative equilibrium temperature of the Earth • A simplest zero-dimensional energy balance model can be formulated as belows (assuming black-body) (1 − α ) S / 4 = σ T e 4 Incoming S energy Earth’s radiation • In case α=0.3 (albedo), S=1370Wm-2 Te≈255K • But observed Ts≈288K, ∆T = 33K => greenhouse effect! Venus, α=0.7, α=0 7 S=2619Wm-2, Te≈242K Æ Ts • In case of the planet Venus ≈730K, ∆T = 488K (!) 27 October 2008 Climate Change Modelling Jee-Hoon Jeong 18 What happens to Incoming Solar Radiation 1. Reflection and Scattering 2. Absorption by Earth’s surface and atmosphere 3. Radiation emitted from Earth surface * Convection and latent heat 1) Reflection and scattering by atmosphere On the average, of all the solar energy that reaches the earth's atmosphere annually, about 30 percent (30/100) is reflected and scattered back to space, giving the earth and its atmosphere an albedo of 30 percent. Of the remaining solar energy, about 19 percent is absorbed by the atmosphere and clouds, and 51 percent is absorbed at the surface. *Albedo Albedo in Earth Earth’s s surface 2) Absorption 2) Absorption - selective absorbers and the atmospheric greenhouse effect Many selective absorbers in the environment. O3, O2, H2O absorb some solar radiation CO2 is i nott a good d absorber of solar radiation, but a good absorber of Earth’s radiation 3) Radiation from Earth Insolation Sunlight warms the earth's earth s surface only during the day, day whereas the surface constantly emits infrared radiation upward. Without water vapor, CO2, and other greenhouse gases, the earth's surface would constantly emit infrared f radiation (IR); ( ) incoming energy from f the sun would be equal to outgoing IR energy from the earth's surface. Without the greenhouse effect, the earth's average surface temperature would be -18°C. 3) Radiation from Earth – IR absorption by atmosphere Greenhouse effect With greenhouse gases, the earth's surface receives energy from the sun and infrared energy from its atmosphere. Incoming energy still equals equa s outgo outgoing ge energy, e gy, but tthe e added IR e energy e gy from o tthe eg greenhouse ee ouse gases raises the earth's average surface temperature about 33°C, to a comfortable 15°C. 3) Radiation from Earth, IR absorption by atmosphere Absorption p of radiation by yg gases in the atmosphere. p The shaded area represents the percent of radiation absorbed Water vapor and carbon dioxide (CO2) are strong absorbers of IR Radiation and poor absorbers of visible solar radiation. Total heat budget *Convection and latent heat release If convection were to suddenly stop - the average Earth temp would rise about 18F (? C?) Air in the lower atmosphere is heated from below. Sunlight warms the ground, and the air above is warmed by conduction, convection, and radiation. Further warming a g occu occurs s du during g co condensation de sat o as latent ate t heat eat is sg given e up to tthe ea air inside s de the cloud. Most absorption takes place near the surface – lower atmosphere is mainly heated from below. Latent Heat Change of State (Phase Change)- changes from gas…liquid…solid gas liquid solid (ice) • The heat/energy required to change a substance (water), from one state to another is called latent heat. (Why?) When a cloud forms from water vapor vapor, it warms atmosphere through releasing latent heat. Namely, additional heat can be transferred as a form of latent heat. Latent heat is an important source of atmospheric energy! Water vapor rising into the air cools and becomes liquid water and ice particles…this process releases heat into the environment Every time a cloud forms, it warms the atmosphere. Inside this developing thunderstorm a vast amount of stored heat energy (latent heat) is given up to the air, as invisible water vapor becomes countless billions of water droplets and ice crystals crystals. In fact fact, for the duration of this storm alone alone, more heat energy is released inside this cloud than is unleashed by a small nuclear bomb. Break?! This chapter discusses: • Temperature and heat transfer – Conduction, Conduction convection convection, radiation • Solar radiation, earth energy balance • The seasons and temperature variations Controls of Temperature • • • • • Latitude (colder near the poles poles, warmer near equator) Geographic position plus land and water distribution Ocean currents (warm/cold currents currents, upwelling) Elevation (air cools with increased elevation) Cl d cover and Cloud d albedo lb d January July Why do we have seasons? • The Earth has an elliptical path around the sun that takes a little over 365 days - revolution • One spin on its own axis in 24 hours - rotation e age distance d sta ce from o ea earth t to su sun is s ~150 50 million o • Average kilometers, varies from 147.3 to 152.1 • Elliptical path takes us closer to sun in January than it does in July – Say what? • Seasons are mostly regulated by sun angle and the number of daylight hours perihelion aphelion The elliptical p p path ((highly g y exaggerated) gg ) of the earth about the sun brings the earth slightly closer to the sun in January than in July. => Minor role in producing seasonal temperature variations Tilted Earth’s rotation axis => Changes g in angle g of sun,, daylight y g length g As the earth revolves about the sun, it is tilted on its axis by an angle of 23.5° (inclination of the axis). The earth's axis always points to the same area in space (as viewed from a distant star). Thus, in June, when the Northern Hemisphere is tipped toward the sun sun, more direct sunlight and long hours of daylight cause warmer weather than in December, when the Northern Hemisphere is tipped away from the sun. (Diagram, or course, is not to scale.) The changing position of the sun, as observed in middle latitudes in the Northern Hemisphere. Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays. D il Temperature Daily T t Variations V i ti • Air temperature is a very important weather element Impacts us every day element. day. If it is warm we don’t always mind the rain, but if its cold cold…..Or Or if it is 40 40°C C outside… outside • A sunny day has its own cycle of warming and cooling. • Temperature lag – Sun directly overhead at noon but noon is not warmest point of a sunny noon, day. Why? What about precipitation cycle? The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth's earth s surface surface. Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (blue shade), the air temperature falls. On a sunny, calm day, the air near the surface can be substantially warmer than the air a meter or so above the surface. surface Sunlight warms ground and ground warms air very near the surface of the earth. [EX: Higher Field temperatures than air temperatures. Higher track temperatures than air temperatures.] Nighttime Cooling • As sun lowers, its energy is spread over a larger area, which reduces the heat available to warm the ground. g • Sometime in the late afternoon or early evening, the earth’s surface and air above begin to lose more energy th they than th receive, i th they start t t to t cool. l • Radiational Cooling – ground and air above cool by radiating infrared energy energy. The ground ground, being a much better radiator than air, is able to cool more quickly. Shortly after sunset the earth’s surface is slightly cooler th the than th air i directly di tl above b it ((what h t iis thi this?). ?) • By late night or early morning the coldest air is next to the ground with slightly warmer air above above. On a clear, calm night, the air near the surface can be much colder than the air above. The increase in air temperature with increasing height above the surface is called a radiation temperature inversion Radiation Inversion Strong radiation inversion occurs when the air near the ground is much colder than the air higher up. Ideal conditions: • Calm conditions – no mixing • Long nights – more time for radiative cooling • Dry air, air cloud cloud-free free – clear skies allow maximum cooling at the surface Cold air On cold, clear nights, the settling of cold air into valleys makes them colder than surrounding di hill hillsides. id Th The region i along l th the side id off th the hill where h th the air i ttemperature t is above freezing is known as a thermal belt Orchard heaters circulate the air by setting up convection currents. Wind machines mix cooler surface air with warmer air above. The daily range of temperature decreases as we climb away from the earth's surface. Hence, there is less day-to-night variation in air temperature near the top of a high-rise apartment complex than at the ground level. Air Temperature and Human Comfort Air ttemperatures Ai t can ffeell diff differentt on different diff t occasions. i 15°C in March can feel warm after a long winter, but can feel cool on a summer afternoon. Sensible temperature – temperature we perceive *Humidity, evaporation – we feel much warmer in humid 30°C 30 C than in dry 30°C 30 C *Wind – windy 0°C vs. windless 0°C Wind Chill factor – how cold the wind makes us feel Instrument Shelter • Why we need this? • To protect p – – – – Wind Direct solar radiation Radiation from sfc Precipitation (water) Dif. of Daily, Dif Daily Monthly and Yearly Temperatures • Daily diurnal range of temperature – difference between the daily maximum and daily minimum • Mean daily temperature – average of the highest and l lowest temperature ffor a 24 h hour period. i d • Annual range of temperature – difference between th average ttemperature the t off the th warmestt and d coldest ld t months • Mean annual temperature – average temperature of a station for the entire year. Temperature data for San Francisco, California (37°N), and Richmond, Virginia (37°N) - two cities with the same mean annual temperature. Thanks! Aurora – caused by charged particles from the sun interacting with the atmosphere atmosphere. Solar wind collides with atmospheric gases. Gases get “excited” and emit light aurora borealis in NH and aurora australis in SH When an excited atom, ion, or molecule de-excites, it can emit visible light. The electron in its normal orbit becomes excited by a charged particle... ...and and jumps into a higher energy level level. When the electron returns to its normal orbit, it emits a photon of light. Aurora Belt The aurora belt (solid red line) represents the region e e you would ou d most os likely ey where observe the aurora on a clear night. (The numbers represent the average number of nights per year on which you might see an aurora if the sky were clear clear.)) The flag MN denotes the magnetic north pole, whereas the flag NP denotes the geographic north pole. *Photons • Think of radiation as streams of particles, or photons,, that are discrete packets p p of energy. gy UV photons carry more energy than a photon of visible light. UV photons produce sunburns, penetrate skin, ki can cause cancer. Light has characteristic of a particle as well as a wave. wave ‘photon’ represents the ‘particle’ side of light. ‘‘short h t wave’’ or ‘l‘long wave’’ representt th the wave side of light.