Geography 103

Weather

Lecture 2: Radiation (Quiz 1)

1. Photon Theory:
    (1).    Definition
             Photon is a basic energy packet which carries a certain amount of energy and travels
             in the wave form of different wavelengths.
             A. Energy: Something that can do work.
             B. Wavelength: The distance between two subsequent wave crests or troughs.
    (2).   Radiation spectra
            A. Cosmic, gamma, X-rays: Wavelength < 0.001 micron (1micron or 1 µ = 10 ^-6 meter = 10 ^-4 cm).
            B. Ultraviolet radiation: Wavelength between 0.001 and 0.4 micron.
            C. Visible light: Wavelength between 0.4 and 0.7 micron.
            D. Infrared Radiation: Wavelength between 0.7 and 100 microns.
            E. Microwave radiation : Wavelength between 100 microns and 0.1 meter.
            F. Radio-wave radiation: Wavelength > 0.1 meter.
    (3).  A shorter-wavelength photon has a higher energy than a longer-wavelength photon.
2. Radiation Laws
    (1).  The Stefan-Boltzmann’s Law
The energy intensity emitted from any substance is in proportion to the 4th power of temperature
            of the substance.
    (2).  The Wien’s Displacement Law
            A. The wavelength having the maximum energy intensity is inversely proportional to the temperature
                of the substance.
            B. Solar radiation: The maximum energy-intensity wavelength is 0.5 micron.
            C. Terrestrial radiation: 10 microns.
            D. The higher the temperature, the more blue and less red is the color of the emitted light.
    (3).  The Planck’s Law
           Describes the energy intensity of individual wavelengths radiated from substances of different temperatures.
3. Black body: A hypothetical body which absorbs all of the electromagnetic radiation striking it
                        (A hypothetical body that obeys the radiation laws).
4. Solar radiation
    (1). Corpuscular radiation: Alpha, beta, gamma, X-rays.
    (2). Electromagnetic radiation at the top of the atmosphere:
          A. Ultraviolet radiation band: 9%
          B. Visible light Band: 41%
          C. Infrared radiation band: 50%.
    (3). Solar constant:
            The amount of solar radiation falling on a surface perpendicular (at a right angle, or 90 ^o) to the solar
            beam at the top of the atmosphere.
     (4). Angot value:
            The amount of solar radiation falling on a horizontal surface tangent to the top of the atmosphere.
            A. The Angot value varies according to latitudes, days, and times.
            B. The solar constant is the Angot value when the sun is directly overhead.
     (5). Processes of the reduction of solar radiation by the atmosphere:
           A. Scattering by molecules and aerosols (particulates, any solid and liquid particles): 32%
                (A). Energy or photon is momentarily captured by molecules or particulates in the air and then is
                       dispersed (changing direction) in all directions without changing its energy intensity and wavelength.
                        a. Forward scattering (to the ground): 25%.
                        b. Backward scattering (reflection or back to the space): 7%.
                (B). Types
                       a. Rayleigh’s scattering: Air molecules scatters most blue light causing the blue sky. .
b. Mie Scattering: Large particulates (fog, haze) scatter all visible light causing blur sky (hazy sky).
                (C). Red sunset and sunrise (differential scattering).
                       When the sun is near the horizon (sunset or sunrise), the blue light is scattered away due to
                       longer atmospheric path and only the red light reaches the observer’s eyes.
            B. Reflection: 35%
The energy is returned to the space.
                (A). Backward scattering: 7%.
                (B). Cloud reflection: 24%
                (C). The earth’s surface reflection: 4%.
            C. Absorption: 17.5%
                (A). Ultraviolet radiation: absorbed by O₃, O₂, and N₂.
                (B). Infrared radiation: absorbed by H₂O (clouds) and CO₂.
                (C). Visible light: Little absorption.
    (6). Global radiation: 47.5%
           A. The short-wave (solar) radiation received on the ground surface per unit area per unit time.
                (A). Diffuse sky radiation (forward scattering): 25%.
                (B). Direct solar beam: 22.5%.
            B. 47.5% + 17.5% + 35% = 100%.
            C. Global radiation may exceed solar constant when the sky is partially cloudy with strong sunshine.
5. Terrestrial radiation (long-wave, 4 to 70 micron, the Planck’s Law))
    (1). Heat transfer processes:
          A. Radiation: Heat transfer without physical contact.
          B. Conduction: Heat transfer with physical contact.
          C. Mixing: Heat transfer involving mass flow.
   (2). Greenhouse effect
         A. The tropospheric atmosphere is heated directly by the terrestrial radiation.
              (The atmosphere absorbs very little solar radiation).
         B. CO₂ and H₂O absorb most terrestrial radiation and make atmosphere warm.
             Air temperature decreases with increasing height normally.
   (3). Atmospheric window
The terrestrial radiation of 8-11 micron can penetrate through the tropospheric atmosphere without
          being absorbed.
6. Counter radiation
    Long-wave or infrared radiation emitted from the atmosphere (mostly from clouds)
7. Net radiation (energy balance) on the earth’s surface
    (1). Input
           Net radiation = (global radiation - reflection) + (counter radiation - terrestrial radiation)
    (2). Net radiation = evaporation + sensible heat flux (air temperature) + soil heat flux (soil temperature)
                                   + photosynthesis.
    (3). On an annual basis
           A. 40^o to Pole: negative net radiation.
           B. 40^o to equator: positive net radiation.
    (4). Global energy balance
           Atmospheric and ocean circulations bring the excess energy from low latitudes to high latitudes.