1. Chapter Two Solar Radiation and the Seasons

2. 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). Energy and power

3. All forms of energy fall into the general categories of kinetic energy and potential energy. Kinetic energy can be viewed as energy in use and 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. Kinetic and potential energy

5. Energy can be transferred from one place to another by three processes: conduction, convection, and radiation. Energy transfer: Conduction, convection, radiation

6. 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. Conduction

7. 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). Convection

8. Of the three energy transfer mechanisms, radiation is the only one that can be propagated without a transfer medium. Unlike conduction or convection, 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. Radiation

9. 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 another property of the wave, the distance between wave crests or wavelength, which is the distance between any two corresponding points along the wave. Quantity: amplitude or #fotons Quality: wavelength/frequency or energy/#fotons

10. Electromagnetic radiation consists of an electric wave (E) and a magnetic wave (M). As radiation travels, the waves migrate in the direction shown by the pink arrow. The waves in (a) and (b) have the same amplitude, so the radiation intensity is the same. However, (a) has a shorter wavelength, so it is qualitatively different than (b). Depending on the exact wavelengths involved, the radiation in (a) might pass through the atmosphere, whereas that in (b) might be absorbed.

11. It is convenient to specify wavelengths using small units called micrometers (or microns). 1 micrometer equals one-millionth of a meter. Types of radiation energy

12. 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 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. Blackbodies: Perfect emission and absorption

13. This relationship is represented by the Stefan-Boltzmann law, expressed as I = σT 4 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. Stefan-Bolzmann’s law

14. Celsius Temperature = ( o F - 32) / 1.8 Fahrenheit Temperature = (1.8 x o C) + 32 Kelvin Temperature = o C + 273

15. 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. Wien’s law

16. 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. Shortwave: Solar, visible/UV Longwave: Earth and atmosphere, IR

17. 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 intense than that of the same area on Earth (b). Solar radiation is also composed of shorter wavelengths than that emitted by Earth.

18. As the distance from the Sun increases, the intensity of the radiation diminishes in proportion to the distance squared (inverse square law). The solar constant is the amount of solar energy received by a surface perpendicular to the incoming rays at the mean Earth–Sun distance and is equal to 1367 W/m 2. Inverse square law

19. 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 orbit

20. 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). Rotation around a tilted axis

21. 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 availability of 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 Earth has 12 hours of day and night and both hemispheres receive equal amounts of energy. Solstice - equinox

22. The 23.5° tilt of the Northern Hemisphere toward the Sun on 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 latitude at which the subsolar point is located (Tropic of Cancer). On the December solstice, the sun is directly overhead at 23.5° S (Tropic of Capricorn). On the two equinoxes, the subsolar point is on the equator. Overhead sun: zenith Subsolar point

23. NH summer solstice

25. The latitudinal position of the subsolar point is the solar declination, which can be visualized as the latitude at which the noontime Sun appears directly overhead. Solar declination

26. 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. Beam spreading