Solar Energy on Earth

The Electromagnetic Spectrum Energy travel as waves and particles called photons Energy is the capacity to do work E=MC2 Where E is energy, M is mass and C is the speed of light (300,000km/sec) Energy has electrical AND magnetic properties ALL objects ABSORB and EMIT EM radiation

EM Radiation Described in terms of Wavelength or Frequency Wavelength distance between successive crests or troughs in km, m, mm, mm Frequency # wave crest/time (1sec) cycles/sec or Hertz (Hz) The longer the Wavelength (l) the lower the frequency Frequency is inversely proportional to l, so the higher the frequency the shorter the l

There is a simple relationship between the wavelength (l), frequency (n), and velocity (c). If you know any 2 of them then you can compute the 3rd using   c = l n or, velocity (c) equals wavelength (l) times frequency (n).

EM Radiation Travel through space In a vacuum travel at the speed of light Pass through liquids, solids, gases EM radiation can be reflected, refracted, transmitted as well as absorbed Solar radiation arrives on Earth primarily as visible (0.4-0.7 mm) and Ultra-violet and is reflected and absorb and re-radiated to space as visible and Infra-red (heat)

Interactions with the Atmosphere Scattering Refraction Absorption

Scattering The redirection of EM energy by particles suspended in the atmosphere or large molecules of atmospheric gases http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/mch/sct.rxml

Refraction The bending of light rays at the contact between two media that transmit light but with different density; when light enters the denser medium, it is defracted toward surface normal

Absorption The atmosphere prevents, or strongly attenuates, transmission of radiation through the atmosphere Three gases: - Ozone (O3): absorbs ultraviolet radiation high in atmosphere - Carbon-dioxide (CO2): absorbs mid and far infrared (13-17.5microm) in lower atmosphere - Water vapor (H2O): absorbs mid-far infrared (5.5-7.0, >27microm) in lower atmosphere

Interactions with Surface  All EM energy reaches earth's surface must be reflected, absorbed, or transmitted Their rate depends on: type of features, wavelength, angle of illumination Reflection Absorption Transmission

What other features impact Solar Energy?

Vegetation Chlorophyll absorbs blue and red, reflects green (visible light spectrum) The spongy mesophyll cells reflect near infrared light that is related to vegetation biomass because the intercellular air space of spongy mesophyll layer is where photosynthesis and respiration occur Vegetation moisture content absorbs mid infrared energy From http://rst.gsfc.nasa.gov/Intro/nicktutor_I-3.html

Soils Soil moisture decreases reflectance Coarse soil (dry) has relatively high reflectance Surface roughness, organic matter, iron oxide affect reflectance

Water Transmission and Absorption Water surface, suspended material, and bottom of water body can affect the spectral response

Solar Altitude Intensity of solar radiation varies with latitude Intensity of solar radiation varies with time of day Intensity of solar radiation varies with path through atmosphere gases

TEMPERATE POLAR TROPIC

Solar Radiation & The Atmosphere Solar radiation interacts with gases and aerosols Scattering & reflection (albedo), absorption (absorptivity) or transmissivity (amount that reaches earth’s surface) of solar radiation must equal 100% In scattering a particle disperses solar radiation in all directions Scattering by molecules is wavelength dependent and preferential scattering of blue-violet light by N2 and O2 is the reason for the daytime sky Water and ice scatter visible light equally at all wavelengths so that clouds appear white

Albedo Fraction of total radiation reflected by an object (surface). Varies according to: Cloud cover. Particles in air. Angle of sun’s rays. Types of surface.

Stratospheric Ozone Ozone is a gas made up of three oxygen atoms (O3). Ozone is destroyed when it reacts with molecules containing nitrogen, hydrogen, chlorine, or bromine. (CFCs) Ozone protects life on Earth from the Sun’s ultraviolet (UV) radiation. Ozone screens all of the most energetic, UV-c, radiation, and most of the UV-b radiation. Ozone only screens about half of the UV-a radiation. Excessive UV-b and UV-a radiation can cause sunburn and can lead to skin cancer and eye damage. In the lower atmosphere (the troposphere) near the Earth’s surface, ozone is created by chemical reactions between air pollutants from vehicle exhaust, gasoline vapors, and other emissions. At ground level, high concentrations of ozone are toxic to people and plants.

Intensity of solar radiation The Earth receives solar radiation unequally over its surface The intensity per unit area of surface is greatest at the equator Intermediate in the middle latitudes The lowest intensity is at the polar regions

Global Solar Radiation Budget Reflected 31% Absorbed by atmosphere 20% Absorbed by Earth’s surface 49% Earth’s climate is controlled by a Greenhouse Effect Gases in the atmosphere control this effect H2Ovapor, CO2, O3, CH4, N2O The percent of IR radiation absorbed varies with l An Atmospheric Window is a range of l over which little or no radiation is absorbed A visible window extends 0.3-0.9mm and major IR 8-13 mm, the peak IR emission of the planet is at 10mm

Absorptivity is very low or near zero in atmospheric window

Greenhouse Gases CO2 is stored in four reservoirs: three that are active and one inactive reservoir including the atmosphere, the oceans, the terrestrial system Earth’s crust Most CO2 is stored in the oceans while the smallest amount is found in the atmosphere. Short-wavelength incoming radiation is not blocked by CO2, but re-radiated long-wavelength energy is, and this warms the atmosphere causing the greenhouse effect