Transcript PPT
AOS 101-304 Energy Transfer February 19/21 Four mechanisms of transfer • • • • Conduction Convection Advection Radiation • Conduction: transfer of heat in a substance, molecule by molecule. – Conductivity varies by substance – Air is very poor (0.023 W/m K), – Metals are very good (silver: 427 W/m K) • Convection: transfer of heat by the mass movement of a fluid away from the heat source. – Warm air is less dense than cool air meaning it wants to float on the cool air. Cool air more dense sinks Warm air less dense rises HEAT SOURCE • In the atmosphere, the heat source is the ground – Warm bubbles heat up near the ground and rise – During ascent, the warm bubbles cool and clouds form • Advection: transfer of heat in the horizontal direction (in the atmosphere by winds). WAA = warm air advection, temp increase CAA = cold air advection, temp decrease • Radiation: transfer of heat via electromagnetic (EM) waves. – No substance needed – Objects emit energy based on temperature • (higher temperatire = more energy). – All EM waves travel at the same speed • speed of light, c = 3 x 108 m s-1. – E = hν, Energy is proportional to frequency (ν) • higher frequency = higher energy. – Wave also related to its wavelength (λ = c / ν). • shorter wavelength = higher energy, E ~ 1/ λ. Electromagnetic Spectrum • A variety of EM waves exist around us at any given time. INCREASING ENERGY • Visible light = λ of 400-700 nm. Radiation (EM waves) can be… • 1. Absorbed: wave energy is converted into the internal energy of the absorbing substance, wave ceases to exist. – Absorbtivity (a): fraction of radiation absorbed • 2. Reflected: wave bounces off substance and is sent in the opposite direction – Reflectivity (r) or Albedo is the fraction of radiation reflected by a surface. – Snow, thick clouds: albedo = 70-80% – dark soil = 10% • 3. Scattered: similar to reflection but radiation is sent in all directions (diffuse light) – For example: overcast skies • 4. Transmitted: radiation passes through a substance unchanged without being absorbed, reflected or scattered. (direct light) – Transmittivity (t): fraction of radiation transmitted • Ignoring scattering, the fractions of absorbtivity, reflectivity and transmittivity will sum to 1. OR: a + r + t = 1. ATMOSPHERE REFLECTED ABSORBED CLOUD TRANSMITTED SCATTERED GROUND Solar (shortwave) Radiation • On average, for solar (shortwave) radiation: the atmosphere (ignoring scattering) – absorbs 20%, – reflects 30%, – transmits 50% to the ground. • In other words, a = 0.2, r = 0.3 and t = 0.5 – 0.2 + 0.3 + 0.5 = 1.0 At the earth’s surface… • No radiation is transmitted through the ground (i.e. t = 0) • So a + r = 1 meaning the surface absorbs whatever radiation is not reflected. – Surfaces with high albedos (like snow) will not absorb as much energy as surfaces with low albedos (like asphalt). For terrestrial (longwave) radiation… • Atmosphere is nearly opaque to the earth’s longwave radiation (t = 0.1) • Thus, the atmosphere absorbs 90% of the earth’s longwave radiation. • According to Kirchoff’s law – atmosphere is a good absorber of LW, must be a good emitter of LW. • Thus the atmosphere emits back LW to the earth’s surface (greenhouse effect). 100 BIG PICTURE heat budget of earth and atmosphere 30 SPACE 12 58 ATMOSPHERE 20 Shortwave 50 Longwave 102 7 94 GROUND Conduction Convection 23 Latent Heat Why is the sky blue? • The atmosphere scatters visible light, so we receive direct light (from the direction of the sun) and diffuse light (from all directions). • The atmosphere more effectively scatters shorter wavelengths (blue) than longer wavelengths (red). • Thus when the sun is high in the sky, away from the sun the sky appears blue. • At sunset, sunlight must pass through a larger amount of atmosphere, enough so that red light is also scattered resulting in a red sky/clouds near the horizon. AIR Molecules GROUND