Chapter TwoChapter Two

Solar Radiation and the SeasonsSolar Radiation and the Seasons

Energy is defined as “the ability to do work.”The standard unit of energy in the International System (SI)

used in scientific applications 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).

All forms of energy fall into the generalcategories of kinetic energy and potential energy.

Kinetic energy can be viewed as energy in useand is often described as the energy of motion.

Potential energy is energy that has not yet been used, such as a cloud droplet that occupies some position

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.

Energy can be transferred from one place to anotherby 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. Conduction is most effective in solid materials,but it also is an important process in a very thin layer of air

near Earth’s surface.

The transfer of heat by the mixing of a fluid is called convection. Unlike conduction, convection is 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(the tendency for a light fluid to float upward

when surrounded by a heavier fluid).

Of the three energy transfer mechanisms, radiation is the only one that can be propagatedwithout a transfer medium. Unlike conduction orconvection, the transfer of energy by radiation

can occur through empty space.Virtually all the energy available on Earth originates

from the Sun. However, radiation is emitted by all matter.

In the case of radiation, quantity is associated with the height of the wave, or its amplitude. Everything else

being equal, the amount of energy carried is directly proportional to wave amplitude.

The quality, or “type,” of radiation is related to anotherproperty of the wave, the distance between wave crests

or wavelength, which is the distance between any two corresponding points along the wave.

Electromagnetic radiation consists of anelectric wave (E) and a magnetic wave

(M). As radiation travels, the waves migrate in the direction shown by the pinkarrow. The waves in (a) and (b) have thesame amplitude, so the radiation intensity

is the same. However, (a) has ashorter wavelength, so it is qualitatively

different than (b). Depending on theexact wavelengths involved, the radiationin (a) might pass through the atmosphere,

whereas that in (b) might be absorbed.

It is convenient to specify wavelengths usingsmall units called micrometers (or microns).1 micrometer equals one-millionth of a meter.

Perfect emitters of radiation, so-called blackbodies are purely hypothetical bodies that emit the

maximum possible radiation at every wavelength.Earth and the Sun are almost blackbodies.

The single factor that determines how much energy ablackbody 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 powerof its absolute temperature.

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.

Celsius Temperature = (oF - 32) / 1.8

Fahrenheit Temperature = (1.8 x oC) + 32

Kelvin Temperature = oC + 273

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 energyat shorter wavelengths than do cooler bodies.

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 thathaving wavelengths longer than 4 micrometers. This typeof electromagnetic energy is called longwave radiation.

Energy radiated by substances occurs over a wide range of wavelengths. Because of its higher temperature,emission from a unit of area of the

Sun (a) is 160,000 times more intensethan that of the same area on Earth (b).

Solar radiation is also composed ofshorter wavelengths than

that emitted by Earth.

As the distance from the Sun increases, the intensity of the

radiation diminishes in proportionto the distance squared (inverse square law).

The solar constant is the amount of solar energy received by a surfaceperpendicular to the incoming rays at the mean Earth–Sun distance

and is equal to 1367 W/m2.

Earth orbits the Sun once every 365 1/4 days as if it wereriding along a flat plane. We refer to this imaginary surfaceas 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 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).

The Northern Hemisphere has its maximum tilt toward the Sun on or about June 21, (June solstice).Six months later (on or about December 21),

the Northern Hemisphere has its minimum availabilityof solar radiation on the December solstice.

Intermediate between the two solstices are the March equinox on or about March 21, and

the September equinox on or about September 21.On the equinoxes, every place on Earthhas 12 hours of day and night and both

hemispheres receive equal amounts of energy.

The 23.5° tilt of the Northern Hemisphere toward the Sunon the June solstice causes the subsolar point

(where the Sun’s rays meet the surface at a right angle and the Sun appears directly overhead)

to be located at 23.5° N. This is the most northward latitudeat which the subsolar point is located (Tropic of Cancer).

On the December solstice, the sun isdirectly overhead at 23.5° S (Tropic of Capricorn).

On the two equinoxes, the subsolar point is on the equator.

The latitudinal position of the subsolar point is the solar declination, which can be visualized

as the latitude at which the noontime Sunappears directly overhead.

Beam spreading is the increase in the surface area over which

radiation is distributed in response to a decrease of solar

angle. The greater the spreading, the less intense is the radiation.

In (a), the incoming light is received at a 90° angle. In (b), the rays hit the surface more obliquely and the energy is

distributed over a greater area. A beam of light is more effective

if it has a high angle of incidence.

The next chapter examinesThe next chapter examinesenergy balance and temperatureenergy balance and temperature.