GEG1301 Lecture 3: Sun-Earth Energy System (PDF)

GEG1301 The physical environment Lecture 3: Sun-Earth Energy System Textbook reading: Chapter 3 Fall 2024 Prof: Roxanne Frappier Outline Review: The solar system and Earth-Sun relation Energy Transfers and Transformation The Principles of Radiation and t...

GEG1301 The physical environment Lecture 3: Sun-Earth Energy System Textbook reading: Chapter 3 Fall 2024 Prof: Roxanne Frappier Outline Review: The solar system and Earth-Sun relation Energy Transfers and Transformation The Principles of Radiation and the Electromagnetic Spectrum Seasons and distribution of solar radiation Review: The solar system and Earth- Sun relation The Solar System Our Sun and the 8 planets appeared around 4.6 billion years ago. Milky Way Galaxy 100,000 light-years across Our Solar System 11 light-hours across Earth is 8 minutes 20 seconds from the Sun at the speed of light Our Moon is 1.28 light-seconds away Earth-Sun relations Rotation and revolution of Earth are anti-clockwise. Plane of ecliptic: flat surface that intersects all points in Earth’s orbit. Perihelion: closest to the Sun (January 3) Aphelion: farthest from the Sun (July 4) Source: manoa.hawaii.edu (Byron Inouye) Energy Transfers and Transformation Energy Transfers and Transformations On Earth, Exogenic energy (>99.9%) Endogenic energy ( 95km Green – O ~ 240km Radiation: Wavelength and Frequency Radiant energy from the Sun travels in the form of waves Two ways of describing electromagnetic radiation: Wavelength: distance between 2 crests Frequency: # of waves passing a fixed point in 1 second Copyright © 2013 Pearson Canada Inc. Electromagnetic Spectrum of Radiant Energy Radiation Balance The radiation balance is determined by different principles (physical laws): 1. How much energy is emitted from a body: Stefan-Boltzmann Law 2. The characteristics of the energy emitted (max wavelength): Wien’s Displacement Law 3. Reduction in intensity of the energy with distance: Inverse Square Law 4. Solar constant How much energy is emitted Stefan-Boltzmann Law E =  T4 where: E is the intensity of radiation emitted by a black body  is the Stefan-Boltzmann Constant (5.6 x 10-8 W m-2 K-4) T is the absolute temperature (K) Example: A black body at 300K (27ºC) emits 453 W m-2 A black body at 6000K (5727ºC) emits 72,000,000 W m-2 Characteristics of the energy emitted Wien’s Displacement Law 2898 μm ⋅ K lmax = T where: lmax is the wavelength of maximum emission (m) T is the absolute temperature (K) Example: The energy emitted by black-body at 5500K will peak around 475 nm. The energy emitted by black-body at 4000K will peak around 650 nm. Characteristics of the energy emitted Wiens' displacement law Characteristics of the energy emitted Wiens' displacement law Sun lmax= 0.48 m Earth lmax= 10.2 m Source: Briggs et al. (1989) Wiens' displacement law lmax= 0.48 m lmax= 10.2 m (Sun) (Earth) Visible part Infrared portion Wavelength of radiated energy depends on the temperature of the radiating body. Sun is much hotter, so radiates shorter wave energy, mainly visible and near infrared. 8% ultraviolet, X-ray, gamma-ray; 47% visible; 45% infrared wavelenghts Earth is cooler, so radiates longer wave energy, mainly thermal infrared. Reduction in intensity of the energy with distance Inverse square Law I I/4 I/9 Intensity reduces according to the square of the distance from the energy source (Inverse square Law). Intensity ≈ 1 / distance2 Earth is 150 x 106 km away from the Sun. Earth only receives ~ 0.5 x 10 -9 of the Sun’s energy output (half of one-billionth) Solar Constant Earth’s distance from the Sun results in the interception of a small fraction of the Sun’s total energy output (half of 1 billionth of Sun’s total energy) The average amount of radiation received at the top of the atmosphere (thermopause; ~ 480km above the earth surface) = 1370 W/m2 Thermopause Atmosphere Summary: The Radiation Balance Radiation from the Sun Stefan-boltzman law (determines how much energy is emitted) E = σT4 Wien’s Law (determines max. wavelength of emission) 2898 μm⋅K lmax = T Inverse square-law (determines how much energy reaches the top of the atmosphere; thermopause). Solar constant is 1370Wm2 (average insolation received at the top of the atmosphere) Simplified Earth’s energy budget  Seasons and distribution of solar radiation Seasonal variations in insolation Why do we have seasons? Variations in insolation The perihelion (closest point to the Sun) and aphelion (farthest position) does not explain the cause for the seasons. Currently, Earth is at perihelion on January 3rd. Why do we have seasons? Tilt of planet relative to Sun Why do we have seasons? Tilt of Earth’s axis (Obliquity) Axis is tilted 23.5º from plane of ecliptic Axial parallelism Axis maintains alignment during orbit around the Sun Fall Winter Summer Spring Annual March of the Seasons: solstices and equinoxes Effect of Earth’s curved surface on solar angle and insolation Continuously varying angle to the incoming parallel rays of insolation o Results in uneven distribution of insolation and heating Reduces the intensity at higher latitudes. o Distance traveled though the atmosphere is greater (subject to more scattering and reflection).  Latitudinal variation in sun angle At larger angles of incidence, solar radiation is distributed over a large surface area. Effect the angle of incidence has on insolation intensity follows this simple equation (not corrected for Earth's tilt): Solar radiation intensity = sin(90 - Lat): Lat 10: sin(90-10) = 0.98 or 98% Lat 20: sin(90-20) = 0.94 or 94% Lat 30: sin(60) = 0.87 or 87% Lat 40: sin(50) = 0.77 or 77% Lat 50: sin(40) = 0.64 or 64% Lat 60: sin(30) = 0.50 or 50% Lat 70: sin(20) = 0.34 or 34% Lat 80: sin(10) = 0.17 or 17% Effect of Earth’s tilt on solar angle and insolation 1. Insolation is perpendicular to the surface only at lower latitudes (between 23.5ºN and 23.5ºS) – known as the subsolar point 2. Latitude of subsolar point changes throughout the year 1. It is above the equator twice a year (equinoxes) 2. It is above the Tropic of Cancer (June 20-21) and Tropic of Capricorn (December 21-22) once a year Uneven Distribution of Insolation Average annual incoming solar radiation reaching top of atmosphere Annual August February Uneven Distribution of Insolation Seasonal Variations Uneven Distribution of Insolation Daily mean insolation Insolation factors Combines solar insolation and duration of daily sunlight Isoline represent daily mean insolation in Wm-2 Shaded areas represent zero insolation Source:https://www.sciencedirect.com/topics/earth-and-planetary-sciences/insolation Uneven Distribution of Insolation: Summary Earth’s tilt causes imbalance in the intensity of solar radiation received; tropics receive more concentrated insolation (energy per unit area) Annually, the tropics receive 2.5 times the amount received at the poles Earth’s revolution causes seasonal variations in amount of insolation received Earth’s rotation causes daily variations in amount of insolation received  Next... Today: Prof available at GSG information center (library) from 1pm to 3pm to answer questions about the lab for Group A02. Friday: Lab #1 for Group A01 with TA Location: Morrisset library, level 3, Room 309 (Geographic, Statistic and Government Information Center) Bring: Pencil Spare sheets of paper Ruler Calculator Prof will also be available next tuesday to answer questions for group A01. Lecture for everyone: Earth's Energy Balance 9/16/2024