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Unformatted text preview: Weather Trivia: September 11 1882: Lightning struck and damaged a home in Riverdale, NS. The force threw a mother and her baby into the cellar, seriously injuring them. The lightning bolt then passed from the house to the barn, where the father was milking a cow. It knocked over the pail and killed the cow. Electromagnetic Radiation In vacuum, electromagnetic waves travel at speed of light (300,000 km/s) The shorter the wavelength, the more energy has the wave The continuous electromagnetic spectrum distinguishes different types of waves based on wavelength and frequency Radiation Wavelength is length of one complete wave Frequency is number of wave crests (or troughs) that passes a given point in one second Frequency is measured in Hertz (Hz, or cycle/second) Electromagnetic spectrum Types of Waves Latent & Sensible Heat Latent Figure 2.3 Heat energy, which is a measure of molecular motion, moves Heat between water's vapor, liquid, and ice phases. between As water moves toward vapor it absorbs latent (e.g. not sensed) heat to keep the molecules in rapid motion. sensed) Radiation - Heat Transfer Radiation
Radiation Radiation travels as waves of photons that release energy when absorbed. when All objects above 0° K release radiation, and its heat energy value increases to the 4th power of its temperature. temperature. Figure 2.7 Longwave & Shortwave Radiation Longwave
The hot sun The radiates at shorter wavelengths that carry more energy, and the fraction absorbed by the cooler earth is then re-radiated at longer wavelengths, as predicted by Wein's law. Wein's Figure 2.8 Electromagnetic Spectrum Electromagnetic Figure 2.9 Solar radiation has peak intensities in the shorter wavelengths, Solar dominant in the region we know as visible, but extends at low intensity into longwave regions. intensity Absorption & Emission Absorption Figure 2.10 Solar radiation is selectively absorbed by earth's surface cover. Darker objects absorb shortwave and emit longwave with high efficiency (e.g. Kirchoff's law). efficiency In a forest, this longwave energy melts snow. In Atmospheric Absorption Atmospheric Solar radiation passes rather freely through Solar earth's atmosphere, but earth's re-emitted longwave energy either fits through a narrow window or is absorbed by greenhouse gases and re-radiated toward earth. and Figure 2.11 Greenhouse Effect Greenhouse Figure 2.12B Figure 2.12A Earth's energy balance requires that absorbed solar radiation is Earth's emitted to maintain a constant temperature. emitted Without natural levels of greenhouse gases absorbing and Without emitting, this surface temperature would be 33°C cooler than the observed temperature. observed Warming Earth's Atmosphere Warming Figure 2.13 Solar radiation passes first through the upper atmosphere, but only Solar after absorption by earth's surface does it generate sensible heat to warm the ground and generate longwave energy. warm This heat and energy at the surface then warms the atmosphere This from below. from Scattered Light Scattered Solar radiation Solar passing through earth's atmosphere is scattered by gases, aerosols, and dust. and At the horizon At sunlight passes through more scatterers, leaving longer wavelengths and redder colors revealed. revealed. Figure 2.14 Incoming Solar Radiation Incoming Figure 2.15 Solar radiation is scattered and reflected by the atmosphere, clouds, Solar and earth's surface, creating an average albedo of 30%. and Atmospheric gases and clouds absorb another 19 units, leaving 51 Atmospheric units of shortwave absorbed by the earth's surface. units Earth-Atmosphere Energy Balance Earth-Atmosphere Figure 2.16 Earth's surface absorbs the 51 units of shortwave and 96 more of longwave Earth's energy units from atmospheric gases and clouds. energy These 147 units gained by earth are due to shortwave and longwave greenhouse These gas absorption and emittance. gas Earth's surface loses these 147 units through conduction, evaporation, and radiation. radiation. Radiation Fundamentals All objects emit radiation (unless at 0 K) Energy originates from rapidly vibrating electrons in object Need to understand: amount of radiation emitted and wavelength of this radiation Radiation laws A black body (a theoretical construct) radiates heat at maximum rate given by Planck's law, which applies to objects in thermal equilibrium Sun and atmosphere-Earth systems are not perfect black bodies, but we can still apply black body radiation laws with useful results Planck's law can be described in terms of two simpler laws, Wien's law and Stefan-Boltzmann law Wien’s Law As temperature of an object increases, wavelength of most intense radiation emitted decreases wavelength of maximum intensity (microns) = 3000/T (wavelength in microns, a millionth of a meter; temperature in degrees K) For example, a heated piece of iron glows dull red when it gets hot, and changes to orange and eventually to white as it gets hotter, i.e., visible radiation of decreasing wavelength Blackbody Radiation Stefan-Boltzmann law Rate at which object radiates heat is proportional to fourth power of its temperature intensity of radiation = (constant) x (temperature)4 Thus intensity of radiation emitted from solar photosphere (temperature 5800 K) is many times that from Earth's surface (temperature 15oC or 288 K) [increase is a factor of 160,000 = (5800/288)4 per unit area] Blackbody Radiation Solar radiation solar radiation is emitted from solar photosphere (surface of sun), at a temperature of 5800 K it is most intense at a wavelength of about 0.5 microns (micrometer), which lies in the visible part of the electromagnetic spectrum also known as shortwave radiation Terrestrial radiation terrestrial radiation is emitted from surface of Earth, at a temperature of 15oC or 288 K it is most intense at wavelength of about 10 microns, which lies in the invisible, infrared part of the spectrum also known as longwave radiation Sun and Earth Radiation Earth’s Radiative Equilibrium We can apply our understanding of radiation to examine the temperature of the Earth in response to incoming solar radiation from the Sun … Solar Constant Solar constant (1380 W/m2) is intensity of direct sunlight received at top of atmosphere, at average sun-Earth distance (150 million km) Energy from Sun per unit area (W/m2) is much smaller at top of atmosphere than at Sun’s surface due to falloff of power with distance as Sun’s rays traverse space to reach Earth Albedo Albedo is reflectivity of an object for incident sunlight, i.e., fraction reflected Planetary albedo for Earth is about 30% This albedo has contributions from clouds (20%), Earth's surface (4%), and scattering by air molecules (6%) Examples of Albedo
Fresh snow Clouds Ice Grassy field Water Forest 75-95% 30-90% (large range!) 30-40% 10-30% 10% 3-10% Planetary Energy Balance In the long term, Earth is in state of ‘energy balance’, where … Amount of energy absorbed by Earth = emitted by Earth Slight imbalance exists at a particular time and location as surface temperature is changing Surface Temperature Earth’s surface temperature depends on three factors: Radiation from the sun (known) Earth’s reflectivity (albedo, 30%) Amount of warming provided by atmosphere (greenhouse effect) Estimating the Greenhouse Effect
Assume that Earth is a blackbody with no atmosphere: Q: What would be its surface temperature? A: -18°C But … Q: What is the actual temperature of our Earth? A: 15°C Difference of 33°C is due to naturally occurring greenhouse effect of the Earth’s atmosphere Without Greenhouse Effect Figure 12a: Sunlight warms the earth's surface only during the 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 radiation (IR); incoming energy from 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 (0°F). Greenhouse effect Atmosphere is mainly transparent to solar radiation, but absorbs strongly terrestrial radiation Absorption of terrestrial radiation is mainly by H2O (water vapour) and clouds Other greenhouse gases are CO2, CH4, N2O and O3 With Greenhouse Effect Figure 12b: With greenhouse gases, the earth's surface receives energy from the sun and infrared energy from its atmosphere. Incoming energy still equals outgoing energy, but the added IR energy from the greenhouse gases raises the earth's average surface temperature about 33°C, to a comfortable 15°C (59°F). ‘Greenhouse Effect’ Terminology “Greenhouse effect" is a misnomer; a greenhouse is warm because the glass panels physically prevent heated air from rising and mixing with cooler air outside ...
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This note was uploaded on 03/13/2010 for the course ATOC 245 taught by Professor Aparisa during the Spring '10 term at McGill.
- Spring '10