Solar Radiation (PowerPoint)

Document Sample
Solar Radiation (PowerPoint) Powered By Docstoc
					Solar Radiation
  Solar Radiation at the Top of the
        Earth’s Atmosphere
Three factors determine the amount of solar
   radiation that enters the top of the
   atmosphere.
1. Time of year
2. Latitude
3. Time of day

These factors determine the Sun’s altitude.
  Solar Radiation at the Top of the
    Earth’s Atmosphere (Cont.)
Sun’s altitude is the angle between the
  Sun, a point on the surface of the Earth
  and the nearest horizon.
Sun’s altitude
 Solar Radiation at the Top of the
   Earth’s Atmosphere (Cont.)
Sun’s noon altitude (Sna) is the angle
  between the Sun, a point on the surface
  of the Earth and the nearest horizon at
  noon Local Solar Time. It is the highest
  point in the sky that the Sun reaches on
  any given day.
  Solar Radiation at the Top of the
    Earth’s Atmosphere (Cont.)
Sna = 90° - |(latitude – declination)|

where
declination – is the latitude that the sun
   appears directly over head at noon Local
   Solar Time during the day. Latitudes in
   the northern hemisphere are positive and
   latitudes in the southern hemisphere are
   negative.
  Solar Radiation at the Top of the
    Earth’s Atmosphere (Cont.)
For example, what is the Sna in Columbus,
   Ohio if the declination is 4°?

Sna = 90° - |(40° - 4°)|

Sna = 90° - |36°|

Sna = 54°
            Solar Constant
The solar constant is the rate at which
 solar radiation (i.e. the irradiance) passes
 through an area perpendicular to the
 direction that the electromagnetic waves
 are traveling at the top of the Earth’s
 atmosphere.

The solar constant is 1367 W m-2.
  Solar Radiation at the Top of the
    Earth’s Atmosphere (Cont.)
As the Sun’s altitude decreases from 90°,
   the solar energy is spread over a larger
   area due to the spherical shape of the
   Earth and its surrounding atmosphere.
Lower Sun’s altitude spreads
energy over larger area        Solar Constant




     Top of Atmosphere
9.1 hours of
daylight




               14.9 hours of
               darkness
12 hours of
daylight




              12 hours of
              darkness
14.9 hours of
daylight




                9.1 hours of
                darkness
 Solar Radiation at the Top of the
          Atmosphere
Columbus, OH       40 °N,    83 °W

Miami, FL          25.5°N,   80 °W

Point Barrow, AK   71 °,     157°W
Solar Radiation (X 10,000,000 J/ day/ meter squared)
                                                               Daily Total of Solar Radiation at the Top of the Atmosphere
                                                       5
                                                                                                                  Legend
                                                                                                                  Columbus, OH
                                                                                                                  Miami, FL
                                                                                                                  Point Barrow, AK0
                                                       4



                                                       3



                                                       2



                                                       1



                                                       0
                                                           0         2          4           6          8          10                  12
                                                                                         Month
Solar Radiation in the Atmosphere
What happens to solar radiation after it
  enters the atmosphere?

(1) Clear sky processes – occur all the
    whether or not there are clouds because
    they are caused by the gases and
    aerosols in the atmosphere.
        Clear Sky Processes
(a) absorption by gases (primarily ozone
    and water vapor) – the photon of radiant
    energy strikes a gas molecule and is
    absorbed and stored as internal energy
Photon




         Energy is absorbed by the
         molecule.


           Internal
           energy
           increases
           T increases
                   Solar Radiation




    Methane


Nitrous oxide

Ozone, oxygen

Carbon dioxide

Water vapor


Sum of all gases


                   UV   V     IR
Solar Radiation in the Atmosphere
              (Cont.)
Water vapor and ozone are the two gases
 that absorb the most solar radiation.

Methane and carbon dioxide absorb much
 smaller amounts of solar radiation.
  Clear Sky Processes (Cont.)
(b) Scattering – occurs when a photon of
    solar radiation hits a gas molecule and
    is redirected without being absorbed.

The scattered radiation may travel in any
   direction. It still has the same
   wavelength and is still considered to be
   solar radiation.
Photon




              Energy is redirected by the
              molecule.


                 Internal
                 energy
                 constant
                 T constant




Scattered photon still has same wavelength.
   Direct and Diffuse Radiation
Photons of solar radiation that are scattered
 at least once are called diffuse solar
 radiation.

Photons that reach the Earth’s surface
 without ever being scattered are called
 direct solar radiation.
               Sky Color
The gases in the Earth’s atmosphere scatter
 blue wavelengths of visible light more than
 any other. Thus, the sky appears blue on
 clear sunny days.
Scattered blue light reaches the surface from all directions making
the sky blue.
          Sky Color (Cont.)
Dust particles and small water droplets
  (haze) are larger than individual gas
  molecules and they tend to scatter more of
  the longer wavelengths of visible light.
Since the color red has the longest
  wavelengths, these aerosols give the sky
  a reddish color when there are a lot of
  them in the atmosphere.
   Effects of the Sun’s Altitude
(1) When the Sun’s altitude is lower, the
    radiant energy is spread over a larger
    area due to the spherical shape of the
    Earth’s surface.
Effects of the Sun’s Altitude (Cont.)
(2) When the Sun’s altitude is lower, the
    photons of solar radiation must travel
    a longer path length (i.e. distance
    through the atmosphere in order to reach
    the Earth’s surface.
A longer path length increases the potential
  for a gas molecule to absorb or scatter the
  photons before they reach the surface.
                         Higher Sun’s
Lower Sun’s altitude =   altitude =
Longer path length       Shorter path
                         length and more
                         solar radiation
                         reaches the
                         surface.
Solar Radiation (x 10,000,000 J/day/square meter)             Solar Radiation at the Surface on a Clear Day
                                                    3.0
                                                                                                         Legend
                                                                                                         Columbus, OH
                                                                                                         Miami, FL
                                                    2.5                                                  Point Barrow, AK




                                                    2.0


                                                    1.5


                                                    1.0


                                                    0.5


                                                    0.0
                                                          0   2          4          6          8         10                 12
                                                                                 Month

				
DOCUMENT INFO