Solar Radiation
Driving
the
Earth’s Weather
Energy and Power
• Energy is defined as “the ability to do work.”
• The standard unit of energy in the International
System (SI) is the joule (J).
• Power is the rate at which energy is released,
transferred, or received.
• The unit of power is the watt (W), which
corresponds to 1 joule per second
– (1 joule = 0.239 calories).
Forms of Energy
• Energy can be grouped into two general categories.
• Kinetic energy can be viewed as energy in use.
– Often described as the energy of motion.
– Wind
– Most important type is radiant energy
• Potential energy is energy that has not yet been
used.
– A cloud droplet, for instance, has potential energy due to
the position it occupies above Earth’s surface. Like all
other objects, the droplet is subject to the effect of gravity.
– The higher the droplet’s elevation, the greater its potential
energy.
– Food
– How about a lake behind a dam? – Destructive work
Forms of Energy
Transfer of Energy
• Heat (Energy) can be transferred from one place to
another by three processes: conduction,
convection, and radiation.
• Conduction is the movement of heat through a
substance without the movement of molecules in
the direction of heat transfer. Transfer from
molecule to molecule. Heat travels from warmer
object to colder object.
– Most effective in solid materials
– Also an important process in a very thin layer of air
near Earth’s surface.
– Ex. Transfer of heat from hot stove to hand
Transfer of Energy
• Convection is the transfer of heat
by the mixing of a fluid.
– Accomplished by
displacement (movement) of
the medium.
• During the daytime, heating of
Earth’s surface warms a very thin
layer of air in contact with the
surface. Above this thin laminar
layer, air heated from below
expands and rises upward because
of the inherent buoyancy of warm
air (thermals).
– Buoyancy is the tendency for a
light fluid to float upward
when surrounded by a heavier
fluid.
Transfer of Energy
• Of the three energy transfer mechanisms,
radiation is the only one that can be propagated
without a transfer medium.
• The transfer of energy by radiation can occur
through empty space.
• Virtually all the energy available on Earth
originates from the Sun.
• Radiation, however, is emitted by all matter.
• Face absorbs radiation on a sunny, summer day
and is converted to thermal energy.
Electromagnetic Waves
• Electromagnetic Radiation (short wave) travels in the form
of waves
• Energy transferred from the sun that releases energy when
absorbed by an object (ozone)(face on a hot summer day)
• Radiation can vary.
• Measured in micrometers.
•Travel at speed of light.
Wavelength
Amplitude
EM Wave Characteristics
• The quantity (intensity) of radiation or
energy transported is associated with the
height of the wave, or its amplitude.
• The distance between wave crests, or
wavelength, determines the quality, or
“type” of radiation
Wavelength
Amplitude
The Electromagnetic Spectrum
Visible – Light that we
can see. One hundredth
of the diameter of a
human hair. Eyes are
sensitive to this
wavelength and thus we
get color.
Some forms carry more
energy (photons) than
others. (sunburn)
Sun emits at all
wavelengths
Wavelength Units
It is convenient to specify wavelengths using
small units called micrometers (or microns).
1 micrometer equals one-millionth of a meter.
Blackbody Properties
• Blackbodies emit the maximum possible radiation
at every wavelength.
• As long as the temperature is above absolute zero,
the object emits radiation.
• Earth and the Sun are almost blackbodies.
• The single factor that determines how much
energy a blackbody radiates is its temperature.
• Hotter bodies emit more energy than do cooler
ones.
• The intensity of energy radiated by a blackbody
increases according to the fourth power of its
absolute temperature.
Stefan-Boltzmann Law
This relationship is represented by the
Stefan-Boltzmann law, expressed as:
I = σT4
Where :
I is the intensity of radiation in
watts per square meter,
σ is a constant (5.67 x 10-8 watts per square meter)
and T is the temperature of the body in kelvins.
(Surface of Sun – 10,500° F, Earth - 59° F)
Temperature Relationships
Celsius Temperature = (oF - 32) / 1.8
Fahrenheit Temperature = (1.8 x oC) + 32
Kelvin Temperature = oC + 273
Wien’s Law
For any radiating body, the wavelength of peak emission
(in micrometers) is given by Wien’s law:
max = constant (2900)/T
where max refers to the wavelength of
energy radiated with greatest intensity.
Wien’s law tells us that hotter objects
radiate energy at shorter wavelengths
than do cooler bodies.
Solar vs Terrestrial Radiation
• Solar radiation is most intense in the visible portion of
the spectrum. Most of the radiation has wavelengths less
than 4 micrometers which we generically refer to as
shortwave radiation.
• Radiation emanating from Earth’s surface and
atmosphere consists mainly of that having wavelengths
longer than 4 micrometers. This type of electromagnetic
energy is called longwave radiation.
Earth’s Orbit
Earth orbits the Sun once every 365 1/4 days as if it were
riding along a flat plane. We refer to this imaginary surface
as the ecliptic plane and to Earth’s annual trip about the
plane as its revolution. Earth is nearest the Sun (perihelion)
on or about January 3 (147,000,000 km). Earth is farthest
from the Sun (aphelion) on or about July 3 (152,000,000 km).
Earth’s Tilt and Rotation
• Earth also undergoes a spinning motion called
rotation.
– Rotation occurs every 24 hours around an imaginary
line called Earth’s axis, connecting the North and South
Poles.
– The axis is not perpendicular to the plane of the orbit of
Earth around the Sun but is tilted 23.5° from it.
– The axis is always tilted in the same direction and
always points to a distant star called Polaris (the North
Star).
Sun Directly
Overhead
(90° Angle)
Earth’s axis points in the same direction all year long.
Equinox and Solstice
• Summer Solstice (On or about June 21st)
– The Northern Hemisphere has its maximum tilt toward
the Sun
• Winter Solstice (On or about December 21st)
– The Northern Hemisphere has its minimum availability
of solar radiation
• Spring and Autumnal Equinox (On or about
March 21st and September 21st)
– On the equinoxes, every place on Earth has 12 hours of
day and night
– Both hemispheres receive equal amounts of energy.
Why is it colder in the winter if
the earth is closer to the sun?
Tilt and Solar Altitude
• Less Daylight – decrease in amount of solar energy
Absorption of Radiation
• Process where radiation is
captured by atmospheric gases,
particulates, and droplets.
• Radiation striking the surface of
the particle is converted to heat
energy.
• This has two effects:
– the absorber gains energy and
warms (Ozone, face)
– The amount of energy
delivered to the surface is
reduced.
Atmospheric Absorption
Reflection of Radiation
• Radiation making contact with some
material is simply redirected away
from the surface without being
absorbed.
• Angle of incident radiation = angle
of reflected radiation.
• Albedo = reflected radiation /
incident radiation. (Snow)
• When light strikes a mirror, it is
reflected back as a beam of equal
intensity, in a manner known as
specular reflection.
Solar radiation and the earth’s
surface
• The fraction of solar radiation that does make it to
the earth’s surface is either reflected or absorbed
(increasing the surface’s temperature)
– Common Albedos
• Urban area: 14-18 – daytime highs warmer during
sunny days
• Cirrus clouds: 40-50 – keeps nighttime temperatures
warmer – daytime cooler
• Fresh snow: 75-95 – daytime temps cooler
Atmospheric Scattering
• Radiation is reflected from an object
as a large number of weaker rays
traveling in different directions.
• Scattering can be from large solid
surfaces, gas molecules, particulates,
and small droplets.
• Scattering is wavelength dependent.
• Radiation is scattered both back to
space and toward the surface.
• Scattered energy reaching Earth’s
surface is thus diffuse radiation,
which is in contrast to unscattered
direct radiation.
The sky appears blue because gases and particles in the atmosphere
scatter some of the incoming solar radiation in all directions. Air molecules
scatter shorter wavelengths most effectively. Thus, we perceive blue light,
the shortest wavelength of the visible portion of the spectrum.
Sunrises and sunsets appear red because sunlight travels a longer path
through the atmosphere. This causes a high amount of scattering to remove
shorter wavelengths from the incoming beam radiation. The result is sunlight
consisting almost entirely of longer (e.g., red) wavelengths.
Sensible and Latent Heat
• When energy is added to a substance, an increase
in temperature occurs that we physically sense.
• This is called (sensible heat).
• Magnitude of temperature increase is related to:
– Specific heat, which is defined as the amount of energy
needed to produce a given temperature change per unit
mass of the substance.
• Latent heat is the energy required to change the
phase of a substance (solid, liquid, or gas).
• In meteorology we are concerned with the heat
involved in the phase changes of water.