Radiation, Heat, and Temperature

Aphelion Perihelion Page: 60 FIGURE 3.1 The elliptical path (highly exaggerated) of Earth about the sun brings Earth slightly closer to the sun in January than in July. Aphelion Perihelion

Figure 3.2 Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays.

Page: 60 FIGURE 3.2 Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays.

Figure 3.2 Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays.

FIGURE 3.6 During the Northern Hemisphere summer, sunlight that reaches the earth’s surface in far northern latitudes has passed through a thicker layer of absorbing, scattering, and reflecting atmosphere than sunlight that reaches the earth’s surface farther south. Sunlight is lost through both the thickness of the pure atmosphere and by impurities in the atmosphere. As the sun’s rays become more oblique, these effects become more pronounced.

Great Circle Largest possible circle on a sphere Formed by passing a plane through the center of a sphere Great circles bisect sphere (cut it in half) Great circle routes are the closest distance between two points on surface Example: Equator

Any two great circles must bisect each other Blue circle cuts red circle in half (and vice-versa)

Latitude “lines” identify north-south location on a sphere All latitudes are “small circles” except for the equator

LONGITUDE Longitude “lines” (meridians) identify east-west location on a sphere Each longitude is half of a great circle (example: 90°west and 90°east)

CIRCLE OF ILLUMINATION A great circle dividing sunlit and dark hemispheres of Earth (day vs. night) The circle of illumination is not usually oriented along Earth’s polar axis.

TIME DEPENDS ONLY ON LONGITUDE Looking down on North Pole

SEASONS Page: 61 FIGURE 3.3 As Earth revolves about the sun, it is tilted on its axis by an angle of 23 1⁄2°. Earth’s axis always points to the same area in space (as viewed from a distant star). Thus, in June, when the Northern Hemisphere is tipped toward the sun, more direct sunlight and long hours of daylight cause warmer weather than in December, when the Northern Hemisphere is tipped away from the sun. (Diagram, of course, is not to scale. The timing of each solstice and equinox varies slightly from year to year; the exact date may be a day earlier or later than shown here, depending on your time zone.)

APPARENT SOLAR PATH AT DIFFERENT LATITUDES Page: 65 FIGURE 3.8 The apparent path of the sun across the sky as observed at different latitudes on the June solstice (June 21), the December solstice (December 21), and the equinox (March 20 and September 22). Chapter 3, pp. 60–66

Cross-Quarter Days Oct 31 Aug 1 Feb 2 May 1

Page: 39 FIGURE 2.8 Radiation characterized according to wavelength. As the wavelength decreases, the energy carried per wave increases.

Solar Radiation Spectrum Page: 41 FIGURE 2.10 The sun’s electromagnetic spectrum and some of the descriptive names of each region. The numbers underneath the curve approximate the percent of energy the sun radiates in various regions.

Sun vs. Earth Infrared Page: 40 FIGURE 2.9 The hotter sun not only radiates more energy than that of the cooler Earth (the area under the curve), but it also radiates the majority of its energy at much shorter wavelengths. (The area under the curves is equal to the total energy emitted, and the scales for the two curves differ by a factor of roughly 1 million.)

Rayleigh Scattering longer waves are not scattered shorter waves are scattered Page: 49 FIGURE 2.15 The scattering of light by air molecules. Air molecules tend to selectively scatter the shorter (violet, green, and blue) wavelengths of visible white light more effectively than the longer (orange, yellow, and red) wavelengths.

Rayleigh Scattering Page: 567 FIGURE 20.2 The sky appears blue because billions of air molecules selectively scatter the shorter wavelengths of visible light more effectively than the longer ones. This causes us to see blue light coming from all directions.

Rayleigh Scattering Cyan sky is mix of visible intensities coupled with eye sensitivity

Rayleigh Scattering Page: 48 FIGURE 6 At noon, the sun usually appears a bright white. At sunrise and at sunset, sunlight must pass through a thick portion of the atmosphere. Much of the blue light is scattered out of the beam (as illustrated by arrows), causing the sun to appear more red.

Setting sun appears red because shorter visible wavelengths (blue through yellow/orange) are scattered Page: 570 FIGURE 20.11 Red sunset near the coast of Iceland. The reflection of sunlight off the slightly rough water is producing a glitter path.

Mie Scattering all wavelengths are scattered Page: 568 FIGURE 20.5 Cloud droplets scatter all wavelengths of visible white light about equally in a process called geometric (nonselective) scattering. The different colors represent different wavelengths of visible light. all wavelengths are scattered

Mie Scattering Page: 568 FIGURE 20.6 Since tiny cloud droplets scatter visible light in all directions, light from many billions of droplets turns a cloud white.

20.1 TABLE 20.1 The Various Types of Scattering of Visible Light (Mie)

Page: 570 FIGURE 20.10 Because of the selective scattering of radiant energy by a thick section of atmosphere, the sun at sunrise and sunset appears either yellow, orange, or red. The more particles in the atmosphere, the more scattering of sunlight, and the redder the sun appears. Low sun = low transmissivity (longer path of radiation through atmosphere)

Page: 44 FIGURE 2.11 The melting of snow outward from the trees causes small depressions to form. The melting is caused mainly by the snow’s absorption of the infrared energy being emitted from the warmer tree and its branches. The trees are warmer because they are better absorbers of sunlight than is the snow.

Solar “constant” decreases with increasing distance from sun Page: 42 FIGURE 3 The intensity, or amount, of radiant energy transported by electromagnetic waves decreases as we move away from a radiating object because the same amount of energy is spread over a larger area. Solar “constant” decreases with increasing distance from sun

Page: 44 FIGURE 2.12 Absorption of radiation by gases in the atmosphere. The dark purple shaded area represents the percent of radiation absorbed by each gas. The strongest absorbers of infrared radiation are water vapor and carbon dioxide. The bottom figure represents the percent of radiation absorbed by all of the atmospheric gases.

Figure 2.11 Absorption of radiation by gases in the atmosphere. The shaded area for each gas represents the percent of radiation each gas can absorb. The strongest absorbers of infrared radiation are water vapor and carbon dioxide. The bottom figure represents the percent of radiation absorbed by all of the atmospheric gases.

Figure 2.11 Absorption of radiation by gases in the atmosphere. The shaded area for each gas represents the percent of radiation each gas can absorb. The strongest absorbers of infrared radiation are water vapor and carbon dioxide. The bottom figure represents the percent of radiation absorbed by all of the atmospheric gases.

Figure 2.11 Absorption of radiation by gases in the atmosphere. The shaded area for each gas represents the percent of radiation each gas can absorb. The strongest absorbers of infrared radiation are water vapor and carbon dioxide. The bottom figure represents the percent of radiation absorbed by all of the atmospheric gases.

Figure 2.12 Sunlight warms the earth’s surface only during the day, whereas the surface constantly emits infrared radiation upward during the day and at night. (a) Near the surface without water vapor, CO2, and other greenhouse gases, the earth’s surface would constantly emit infrared radiation (IR) energy; incoming energy from the sun would be equal to outgoing IR energy from the earth’s surface. Since the earth would receive no IR energy from its lower atmosphere (no atmospheric greenhouse effect), the earth’s average surface temperature would be a frigid –18°C (0°F).

Figure 2.12 Sunlight warms the earth’s surface only during the day, whereas the surface constantly emits infrared radiation upward during the day and at night. (b) With greenhouse gases, the earth’s surface receives energy from the sun and infrared energy from its atmosphere. Incoming energy still equals outgoing energy, but the added IR energy from the greenhouse gases raises the earth’s average surface temperature about 33°C, to a comfortable 15°C (59°F).

Page: 45 FIGURE 2.13 (a) Near the surface in an atmosphere with little or no greenhouse gases, Earth’s surface would constantly emit infrared (IR) radiation upward, both during the day and at night. Incoming energy from the sun would equal outgoing energy from the surface, but the surface would receive virtually no IR radiation from its lower atmosphere (i.e., there would be no atmospheric greenhouse effect). Earth’s surface air temperature would be quite low, and small amounts of water found on the planet would be in the form of ice. (b) In an atmosphere with greenhouse gases, Earth’s surface not only receives energy from the sun but also infrared energy from the atmosphere. Incoming energy still equals outgoing energy, but the added IR energy from the greenhouse gases raises Earth’s average surface temperature to a more habitable level.

Page: 49 FIGURE 2.16 On the average, of all the solar energy that reaches Earth’s atmosphere annually, about 30 percent (30/100) is reflected and scattered back to space, giving Earth and its atmosphere an albedo of 30 percent. Of the remaining solar energy, about 19 percent is absorbed by the atmosphere and clouds, and about 51 percent is absorbed at the surface.

Page: 50 FIGURE 2.17 The Earth-atmosphere energy balance. Numbers represent approximations based on surface observations and satellite data. While the actual value of each process may vary by several percent, it is the relative size of the numbers that is important.

Page: 47 FIGURE 2.14 Air in the lower atmosphere is heated from the ground upward. Sunlight warms the ground, and the air above is warmed by conduction, convection, and infrared radiation. Further warming occurs during condensation as latent heat is given up to the air inside the cloud.

Clouds and Solar Radiation Page: 569 FIGURE 20.7 Average percent of radiation reflected, absorbed, and transmitted by clouds of various thickness.

Net Radiation by Latitude Page: 51 FIGURE 2.18 The average annual incoming solar radiation (yellow lines) absorbed by Earth and the atmosphere along with the average annual infrared radiation (red lines) emitted by Earth and the atmosphere.

AVERAGE JANUARY TEMPERATURE (°F) Page: 78 FIGURE 3.27 Average air temperature near sea level in January (°F). Temperatures in Central Antarctica are not visible on this map.

AVERAGE JULY TEMPERATURE (°F) Page: 78 FIGURE 3.28 Average air temperature near sea level in July (°F). Temperatures in Central Antarctica are not visible on this map.