# Solar Radiation (PowerPoint) by goodbaby

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```									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
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.
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
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

Methane

Nitrous oxide

Ozone, oxygen

Carbon dioxide

Water vapor

Sum of all gases

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

Methane and carbon dioxide absorb much
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
Photon

Energy is redirected by the
molecule.

Internal
energy
constant
T constant

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

Photons that reach the Earth’s surface
without ever being scattered are called
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
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
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

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