GPH 212, Week 7 Notes
GPH 212, Week 7 Notes GPH 212
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This 6 page Class Notes was uploaded by Sheridan Smede on Thursday October 6, 2016. The Class Notes belongs to GPH 212 at Arizona State University taught by Matei Georgescu in Fall 2016. Since its upload, it has received 3 views. For similar materials see Introduction to Meteorology in Physical Geography at Arizona State University.
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Date Created: 10/06/16
Atmospheric Influence on Insolation ● 3 processes: ○ Absorption ■ Particular gases, liquids, and solids in the atmosphere reduce the intensity of insolation by absorption ● This represents an energy transfer to the absorber ■ Therefore, less energy is transferred to the earth’s surface ■ Atmospheric gases are generally poor absorbers of solar energy (ex: visible radiation) ■ The fact that we can see means some visible radiation has reached our eyes ● If it didn’t reach our eyes, we’d see only black ■ O3 absorbs UV radiation in stratosphere ■ Water vapor and CO2 absorb near-infrared radiation ○ Reflection ■ Energy can be reflected without being absorbed ■ Albedo - the percentage of energy reflected by an object (different substances reflect wavelengths differently) ■ Specular reflection - reflection of energy as an intense beam (ex: light striking a mirror) ○ Scattering ■ Incoming energy is dispersed into less intense beams that diffuse reflection (example: snow) ■ Gases in the atmosphere scatter radiation back to space and toward the surface ( iffuse radiation) ● Example: being in a desert when it’s overcast ■ Energy that reaches the surface is scattered and different in intensity from direct radiation ■ Rayleigh scattering - agents smaller than 1/10 the wavelength of incoming radiation disperses radiation ● This type of scattering results in our blue skies, it’s why they look blue to us (blue colors have the shortest wavelengths) ● Sunsets appear reddish because the other colors have already been scattered ● Scattering occurs equally in all directions ■ Mie scattering - mostly forward scattering, diverting relatively little energy backward to space ● Forward scattering dominates ● Too much (all) colors of visible radiation - we would see white ■ Particle size (agent) relative to wavelength of incoming solar radiation ■ Causes sunrises and sunsets to be more red when pollution is present ○ Nonselective scattering - scattering by clouds; scattered equally ○ Why isn’t the sky violet? ■ Sun radiates more blue than violet ■ Clouds appear white because all radiation is scattered equally ○ Water selectively absorbs differently than the atmosphere ■ Absorbs all red and orange wavelengths ■ This leaves blue and violet, which makes the ocean look blue ○ Transmission ■ The percentage of energy transmitted through the atmosphere to the surface ■ Dependent upon atmosphere’s ability to absorb, scatter, and reflect ■ Energy varies from place to place ■ Cloudy conditions - reduction from direct radiation ● Amount of insolation reaching surface depends on: ○ Insolation available at top of atmosphere ○ Reduction in atmospheric insolation due to absorption, reflection, and scattering ● 66.5 degrees north = Arctic circle ● Insolation can be affected by cloud coverage ● The atmosphere is generally transparent to incoming solar radiation ● Insolation varies by 7% ● Measure atmosphere by range of 100 units ○ Atmospheric absorption averages 23 units ○ 6 units absorbed in stratosphere, 17 units absorbed in troposphere ● Reflection by clouds averages 17 units, reflection/scattering by atmospheric gases/aerosols averages 6 units ● Atmospheric albedo - 23% ● Remaining 54 units are available for surface absorption ○ Surface is not a perfect absorber, so 7 units are reflected back to space ○ These 7 plus 23 scattered to space from atmosphere equates to a planetary albedo of 30% ● Remaining 47 units at surface are absorbed and used to heat the surface ● Energy balance: the earth system must lose as much energy as it gains Surface-Atmosphere Radiation Exchange ● Earth’s surface and atmosphere radiate longwave energy Extra Credit Opportunity ● Oral presentation (single or teams) ● Discussion of meteorologically relevant phenomena ○ Include socioeconomic implications and outcome of event ● Presentation date: last day of classes ● Email intent to present by November 2nd ○ Include topic and title ● Duration: 5 minutes per individual The fate of solar radiation ● Atmosphere absorbs 23% ● Surface absorbs 47% ● The earth system must lose as much energy as it gains Surface-atmosphere radiation exchange ● Earth’s surface and atmosphere radiate longwave energy ● Longwave energy transfer is more complex than solar energy because longwave energy has no obvious beginning or end point ○ Most emission occurs in the thermal infrared portion of the EM spectrum ● The atmosphere absorbs most longwave radiation emitted from the surface ○ This increases temperature of atmosphere which causes it to radiate energy in all directions (including toward the surface) ● This causes additional surface heating (remember the Stefan-Boltzmann Law) ○ And then the cycle repeats ● Water vapor and CO2 are the primary absorbers Energy transfer between surface and atmosphere ● Water vapor and CO2 are greenhouse gases ● Range of wavelengths (8-12 microns) matches those radiated with greatest intensity by the earth’s surface ● This range of wavelengths not absorbed is called the atmospheric window ● If clouds are present, radiation between these microns is absorbed in our atmosphere ● Cloud coverage makes it warmer ○ Because radiation is trapped in atmosphere by clouds; concentration of radiation creates heat ● Net radiation = shortwave + longwave (in units) Conduction ● As the surface warms, a temperature gradient develops in the upper few centimeters of the ground ● Temperature become greater at the surface than below ● Laminar boundary layer ○ Only a few centimeters thick, a thin sliver of adjacent air ○ Energy conducted through the laminar boundary layer is redistributed throughout the rest of the atmosphere vi onvection Convection ● Heat is transferred by movement of fluid ● Happens whenever surface temperature exceeds air temperature ● At night the surface cools more rapidly than air and energy is transferred downward ● “Riding a thermal” - when birds fly without flapping their wings ● Transfers two types of energy: ○ Sensible heat ○ Latent heat ● Sensible heat ○ Energy is added to a substance, an increase in temperature can occur ○ Higher mass requires more energy for heating ○ 5 units of energy are transferred from the surface to the atmosphere as sensible heat ○ Direct energy that is transferred to a surface ● Latent heat ○ Energy required to change the phase of a substance ○ Ex: turn glass of water into evaporated water vapor ○ In the atmosphere, we relate this to phase changes of water ○ Energy must be supplied in order to melt an ice cube (latent heat of fusion) or evaporate water (latent heat of evaporation) ○ Turning vapor into ice: deposition ○ Turning ice into vapor: ublimation ● These two modes of energy transfer account for 29 units of energy loss ● Albedo - the reflectivity of a surface ● From the sun: downwelling shortwave radiation ● From the earth: upwelling longwave radiation