Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 1. Energy and Mass
Chapter 2.
Solar Radiation and the Seasons
Introduction
Solar radiation is transferred to the Earth’s surface where it
is absorbed
This radiation provides energy for atmospheric motion, weather
processes, biologic activity, conversions of state, and many other
activities
Most forms of energy can be classified as either kinetic or
potential energy
Kinetic energy, the energy of motion, can occur from very large to
small scales. At the molecular scale, this energy describes
molecular vibration and determines an object’s temperature.
Potential energy is energy in reserve.
Examples of
kinetic energy
Gas molecules have
no bonds to other
molecules and as such
they move in random
motion
Radiation
An energy transfer mechanism which requires no physical medium
Propagates energy transfer through the vacuum of space
Continually emitted by all substances
Types differ by electrical and magnetic wave properties
In any type of radiation, electrical and magnetic waves, although
closely coupled, are perpendicular to each other
Electromagnetic radiation.
E = electric wave
M = magnetic wave
Radiation quantity
Refers to the amount of energy transferred
Expressed through wave amplitude
Wave quality
Relates to radiation wavelength
Identifies the type of radiant energy
Wavelength is expressed in μm, or micrometers
All electromagnetic energy travels at a constant speed
• 300,000 km/sec (186,000 mi/sec), the speed of light
Electromagnetic energy
Comes in an infinite number of wavelengths
Can simplify by categorizing wavelengths into just a few
individual “bands” along the electromagnetic spectrum
Intensity and Wavelengths of Emitted Radiation
All matter not only radiates energy, but energy is emitted over a
wide range of electromagnetic wavelengths
Physical laws which define the amount and the wavelength of
emitted energy apply only to hypothetical perfect emitters of
radiation known as blackbodies
The Earth and Sun are similar to blackbodies
Stefan-Boltzmann Law
The amount of energy emitted by an object is proportional to the
object’s temperature
Hotter objects emit more energy than cooler ones
Energy emitted is proportional to the fourth power of the emitter’s
absolute temperature
Stefan-Boltzmann Law describes this mathematically as I =T4
• I is the intensity of the radiation in watts/m2,  is the StefanBoltzmann constant (5.67 x 10-8 watts/m2/K4) and T is the
temperature of the body in K
Graybodies denote objects which emit some percentage of the
maximum amount of radiation possible at a given temperature
• Most solids and liquids
True radiation emitted is a percentage relative to a blackbody and
reflects the emissivity of the object
Wein’s Law
Radiation emission is across a wide array of electromagnetic
wavelengths
Useful to determine the wavelength of peak emission, found
through Wein’s Law
Wein’s Law = λmax= 2900/T
• λmax = wavelength of energy radiates with greatest intensity, 2900
equals a constant, T is temperature in K
Wein’s Law tells us that hotter objects radiate at shorter
wavelengths than cooler bodies
Solar radiation peaks at 0.5μm while terrestrial radiation peaks at
10μm
Largest portion of solar radiation emitted in visible spectrum
Largest percentage of terrestrial radiation emitted as longwave
radiation
Solar Constant
The intensity of electromagnetic radiation is not reduced with
distance through the vacuum of space
A reduction of intensity is proportional to increasing distance only
as energy is distributed over a larger area
Due to this, radiation intensity decreases in proportion to the
distance squared
Calculating this inverse square law for Earth’s average distance
from the Sun yields a solar constant of 1367 W/m2
• Solar emission = 3.865 x 1026W divided by distance surrounding the
Sun = 4(1.5 x 1011m)2 = 1367 W/m2
Causes of the Earth’s Seasons
Variations in the relationship between Earth’s orbital alignment and
the Sun are responsible for variations in incoming solar radiation at
Earth’s surface
Revolution
Earth revolves about the Sun along an ecliptic plane
Distance varies
• Perihelion (Jan 3; 147 mil km, 91 mil mi)
• Aphelion (July 3; 152 mil km, 94 mil mi)
Total variation is about 3%
Using the inverse square law, radiation intensity varies by about
7% between perihelion and aphelion
Earth Rotation
Earth rotates on its axis once every 24 hours
Axis of rotation is offset 23.5o from a perpendicular plane through
the ecliptic plane
Because the axis of rotation is never changing, the northern axis
aligns with the star Polaris
Hemispheric orientation changes as the Earth orbits the Sun
A particular hemisphere will either align toward or away from the
Sun, or occupy a position between the extremes
Examining Earth’s axis with a full 90o tilt may help in the
visualization of the changing circle of illumination relative to
hemispheric lines of latitude through revolution
Solstices
Maximum axial tilt in relation to the Sun occurs on only two dates
for each hemisphere, however, the hemispheres are in opposition
relative to the Sun
The June solstice occurs on approximately June 21
The northern (southern) hemisphere axis of rotation is fully
inclined toward (away from) the Sun
This ensures maximum (minimum) solar radiation absorption
through the hemisphere
Exactly opposite conditions occur relative to the December solstice
(on or around Dec 21)
Astronomically, dates designate first days of winter or summer
where appropriate
Northern hemisphere summer solstice
On or about June 21
Subsolar point (solar rays at a right angle to the surface, the
latitudinal position of which equals solar declination) is at 23.5oN,
the Tropic of Cancer
Northern hemisphere winter solstice
On or about Dec 21
Subsolar point is located at 23.5oS, the Tropic of Capricorn
Exactly opposite conditions occur relative to these dates for
the southern hemisphere
Overall, the subsolar point fluctuates 47o between the
Tropics
Equinoxes
Temporally centered between the solstices
March equinox, on or about March 21
September equinox, on or about Sept 21
The subsolar point (solar declination) lies at the equator
Solar Angle
Incident radiation is directly proportional to solar angle
Higher solar angles incorporate reduced radiational beam
spreading which leads to greater heating through energy
concentration
Lower angles induce less intense illumination and heating per unit
area
Demonstration of the effect of
angle of incidence on energy intensity
Period of Daylight
Axial tilt influences day length
During hemispheric alignment either toward or away from the Sun,
lines of latitude are bisected unequally by the circle of illumination
A hemisphere aligned toward (away from) the Sun will have
constant daylight (night) poleward of 66.5o
This line of latitude is designated the Arctic Circle in the northern
hemisphere and the Antarctic Circle in the southern hemisphere
Due to this geometry, day length increases (decreases) from the
equator to the pole of the summer (winter) hemisphere
Lines of latitude are equally split for both hemispheres on the
equinoxes ensuring equal day/night conditions everywhere
Beam depletion
Solar radiation is diminished relative to the amount of atmosphere
the radiation passes through
High solar angles see little reduction in intensity as the path from
the top of the atmosphere to the surface is short
Significant beam reduction occurs where energy is diffused
through larger amounts of atmosphere
• High latitudes - magnified during the winter season
Changes in energy receipt with latitude
Combined effects of solar angle, day length, and beam depletion
cause winter hemisphere to run a deficit of energy, leading to
cooler temperatures
Summer hemispheres have a mean surplus of energy resulting in
warmer temperatures
End of Chapter 2
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt