G109 4 Solar and Terrestrial Radiation 1 4 S OLAR T ERRESTRIAL R ADIATION P ART I R ADIATION Reading Assignment A B Ch 2 p 43 53 LM Lab 5 1 Introduction Radiation Mode of Energy transfer by electromagnetic waves only mode to transfer energy without the presence of a substance fluid or solid works best in a vacuum empty space Radiation the only way for Earth to receive energy from the Sun Weather systems are powered by radiation From Earth Sun geometry we know spatial and temporal variations of receipt of radiation at the top of the atmosphere From Atmospheric Composition important for radiation at the surface O3 UV radiation shortwave H2O CO2 IR radiation greenhouse longwave need to consider different types of radiation Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 2 2 Electromagnetic Radiation radiation waves exhibit characteristics of both electric fields and magnetic fields from A B Figure 2 5 a Electromagnetic radiation moves at speed of light radiation spreads in all directions and moves in straight lines from A B Figure 2 9 Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 3 Electromagnetic radiation is described by three interdependent variables wavelength lambda m m frequency nu s 1 Hz velocity c m s 1 c speed of light 3 108 m s 1 c 3 Radiation Spectrum Definition The Radiation Spectrum is the distribution of radiative energy over different wavelengths or frequencies In meteorology only small part of EM spectrum of interest three important ranges ultraviolet radiation UV visible radiation infrared radiation IR Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 4 Radiation in the Earth Atmosphere System Wavelength Effect Ultraviolet Radiation UV 10 2 0 4 m Sunburn Class sun output Earth output shortwave radiation longwave radiation Radiation doc Visible Radiation 0 4 0 7 m sunlight 0 4 m violet blue 0 5 m green yellow 0 6 m orange 0 7 m red Infrared Radiation IR 0 7 100 m heat radiation near IR far IR 0 7 1 5 1 5 100 m m Shortwave radiation 7 0 43 0 37 0 only solar radiation IR radiation emitted by the E A system 9 12 03 longwave radiation 11 100 G109 4 Solar and Terrestrial Radiation 5 4 Radiation Laws Read A B Chapter 2 p 35 39 i General Principles all things emit radiation o the amount and wavelengths depend primarily on the emission temperature o higher the T faster the electrons vibrate shorter wavelength more total radiation emitted when any radiation is absorbed by an object increase in molecular motion increase in temperature ii Black Bodies and Gray Bodies an object or body that absorbs all radiation incident on it is termed a black body idealization perfect black bodies do not exist often a good approximation for absorption in a given range of wavelengths many natural substances behave nearly like black bodies Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 6 a black body is also an ideal emitter emission spectrum follows a general law Planck s curve describing the maximum possible emission for a given temperature is often used as comparison standard for emission spectrum a black body has an ideal emission efficiency termed emissivity 1 an object or body with a less than ideal emission efficiency same at all wavelengths is termed a gray body a gray body has a non ideal emission efficiency emissivity is often a good approximation for emission spectra of real objects or bodies 1 iii Reflection Absorption Transmission only three things can happen when radiation with a wavelength hits an object or substance 1 part or all can be reflected fraction reflected reflectivity this part does not interact with the object it is rejected 2 part or all can be absorbed fraction absorbed absorptivity a this part raises the temperature of the object Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 7 radiative energy is converted to heat 3 part or all can be transmitted t fraction transmitted transmissivity this part does not interact with the object it just goes through it Since these are the only possibilities it follows from the principle of conservation a t 1 iv Stefan Boltzmann Law the total emitted energy flux All objects or substances emit radiation at a rate proportional to the 4th power of their absolute temperature Total energy flux emitted Ftot W m 2 Ftot T 4 emissivity 0 1 depends on quality of material see Lab Manual 5 for list of values Stefan Boltzmann constant 5 67 10 8 W m 2 K 4 T absolute temperature of emitting object K T4 fourth power faster than linear increase with temperature Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 8 18000 4th power 16000 Ftot W m2 14000 12000 4 16 x 2 10000 8000 6000 linear 4000 2x 2000 0 200 300 400 500 600 700 800 T K Example Problem see web under this topic for more exercise problems If a cloud bottom has a temperature of 10 C how much energy would it be emitting if the emmissivity were 1 0 Solution convert temperature to SI unit C K T 10 C 273 15 263 15 K use Stefan Boltzmann law for 1 black body Fcloud T4 1 x 5 67 10 8 x 263 15 4 271 9 W m 2 Check units units okay physics okay T4 1 x W m 2 K 4 x K4 W m 2 9 Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation v Wien s Displacement Law 9 the wavelength of maximum emittance A rise of temperature in an object not only increases the total radiant output but also shifts this energy output to shorter wavelengths in inverse proportion to the absolute temperature Wavelength of maximum emmittance max m max a a T 1 T 1040 1038 5800 K 4000 K 2000 K 1000 K 500 K 255 K Te m pe 1036 ra tu 1034 re Blackbody Irradiance x W m 2 max wavelength m a constant 2898 m K T absolute temperature K 1032 10 7 10 6 10 5 10 4 10 3 Wavelength m Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 10 Example Problem see web under this topic for more exercise problems If a cloud bottom has a temperature of 10 C what is the wavelength of the peak energy emission What part of the electromagnetic spectrum is this in Solution convert temperature to SI unit C K T 10 C 273 15 263 15 K use Wien s law max a T 1 2898 263 15 11 0 m Check units units okay physics okay a T 1 m K x K 1 m 9 Radiation doc 9 12 03 G109 4 Solar and Terrestrial Radiation 11 P ART II A TMOSPHERIC I NFLUENCES ON R ADIATION Reading Assignment A B Ch 3 p 68 76 LM Lab 5 1 Introduction Global Shortwave Radiation Balance overview 30 of solar radiation is reflected by clouds atmospheric gases and the surface 25 …