Download (EM) Radiation

Part 1. Energy and Mass
Chapter 2.
Solar Radiation and the Seasons
Introduction
Solar Radiation
Initiates atmospheric motions and weather
processes
Energy Classified as
Kinetic or potential
Kinetic energy examples
Energy Transfer Mechanisms
1) Conduction
Energy transfer through solids
2) Convection
Energy transfer through fluids (gases, liquids)
Gas molecules move
at random
3) Electromagnetic (EM) Radiation
Energy transferred by electromagnetic EM
waves (made up of electric and magnetic fields)
Electromagnetic (EM) radiation
E = electric field wave
M = magnetic field wave
EM Energy
EM spectrum classifies
types by wavelength
(usually measured in
micrometers, mm)
The amount of energy
transferred is
proportional to the wave
amplitude
Wavelength
Amplitude
Stefan-Boltzmann Law
EM Energy emitted is proportional to temperature
• Hotter objects emit more EM energy
Blackbodies are hypothetical perfect emitters
Graybodies emit a percentage of the maximum
possible for a temperature
Wein’s Law
Hotter objects have their spectrum peak at shorter
wavelengths
• Solar (Sun) radiation = 0.5 mm peak
• Terrestrial (Earth) radiation = 10 mm peak
The Sun’s
radiation
spectrum peak
is a .5 mm
(shortwave
radiation).
Energy
The Earth’s
radiation
spectrum peak
is a 10 mm
(longwave
radiation).
Wavelength
The Solar Constant is the amount of energy
emitted from the Sun’s surface every second
(1367 W/m2)
The energy intensity of solar radiation decreases with
distance from the Sun.
Mars receives less
intense solar radiation
than the Earth because
it is farther from the
Sun.
Climate and the Earth’s Seasons
Causes of the Earth’s Seasons
The Earth revolves around the Sun in the ecliptic plane
• Perihelion (Earth closest to the Sun) is January 3; 147 million
km or 91 million miles
• Aphelion (Earth farthest from the Sun) is July 3; 152 million
km or 94 million miles
Seasonal radiation variation in solar radiation is ~7%
Earth Rotation
Once every 24 hours
Rotational axis offset by about 23.5o from the
ecliptic plane
The rotation axis appears to point to a fixed
point in the heavens (Polaris, also called the
North Star)
The part of the Earth that points directly toward
the Sun receives the greatest solar intensity.
This changes throughout the year, causing the
seasons on the Northern and Southern
Hemisphere.
Solstices
and
Equinoxes
Tropic of
Cancer
Tropic of
Capricorn
This
figures
shows the
variation in
solar
radiation
intensity at
the time of
the June
solstice.
Differences
in solar
radiation
intensity at
the June
and
December
Solstices.
Equinoxes
Temporally centered between solstices
• ~ March 21 and ~ Sept 21
The subsolar point = 0o
The length of
day is
controlled by
the position of
the Earth in its
orbit.
Beam Spreading and Radiation Intensity
Radiation intensity is proportional to solar angle
Larger angles of incidence give reduced beam
spreading and greater heating
Less beam
90° angle of incidence
(solar angle)
45° angle of incidence
(solar angle)
spreading =
greater
heating
More beam
spreading =
less heating
The sun at
sunrise and
sunset has a
lower solar
angle and
the radiation
travels
farther
through the
atmosphere,
reducing the
heating
effects.