Radiation

• Radiation: travels out in all directions from its source
• Only mechanism of energy that can travel through _______ of space
• Thus, all solar energy transferred to Earth by solar ________

Solar Radiation

• All radiation travels through space at the speed of light (300,000 km/s), slightly slower in air, in the form of electromagnetic waves
• The waves come in various sizes, i.e. different wavelengths
• Shorter waves are associated with greater energy
• UV rays have more energy than Infrared (IR)

Radiation--Fig. 2-12

Solar Radiation

• Sun emits all forms (wavelengths) of radiation, but some more than others
• Over 95% emitted between wavelengths of 0.1 and 2.5 µm (micrometers--10^-6 m)
• 43% _______ (0.4 to 0.7 µm)
• 49% ____ near visible (0.7 to 2.5 µm)
• 7% ____ (< 0.4 µm)

Laws of Radiation

• 1. All objects above -273ºC (absolute zero) ____________ emit radiant energy over a range of wavelengths
• That means YOU!

 Laws of Radiation

• 2. Hotter objects radiate more total energy per unit area than colder objects
• This is the Stefan-Boltzman Law (Box 2-2)
• Radiation emitted is proportional to T⁴
• The Sun’s average surface temperature is 20 times that of Earth’s (using Kelvin scale), so it emits 20⁴ (or 160,000) times more radiation per unit area

Laws of Radiation

• 3. Hotter objects radiate maximum energy from shorter wavelengths than colder objects
• This is Wien’s Displacement Law (Box 2-2)
• Since the Sun's average surface temperature is 20 times hotter than Earth's, it’s wavelength of maximum emission is 20 times smaller than Earth’s (~0.5 µm compared to ~10 µm)
• Thus, Sun emits max from visible; Earth emits max from IR

Laws of Radiation—Fig. 2-14

Laws of Radiation--See Fig. 2-14

• Since sun’s max wavelength is so much shorter, we typically call its radiation "shortwave radiation" or SW
• Earth’s is then called "longwave radiation" or LW

Laws of Radiation

• 4. Objects that are good absorbers of radiation are also good emitters
• _______ in = _______ out; if you absorb a lot of radiation, you also emit a lot so you can stay the same ___________

Incoming Solar Radiation

• Radiation may be absorbed, transmitted, or redirected (reflected or ________)

Incoming Solar Radiation—Fig. 2-15

Incoming Solar Radiation--See Fig. 2-15

• 50% absorbed at surface
• 30% reflected and scattered back to space, mostly by clouds, but also by atmosphere and surface
• 20% absorbed in atmosphere by atmosphere's gases and clouds

Reflection and Scattering

• Reflection: light comes off an object at the ____ _____ and ________ at which it struck the object
• Scattering: light is ________, producing a ______ number of _______ rays traveling in different directions (more forward than backward)

Reflection and Scattering--Fig. 2-16

Albedo

• Albedo: fraction of incoming solar radiation that is ________ by a surface
Mult. Choice Question:

Albedo

• Mult. Choice Answer:
• Different surfaces _______ different amounts, however

Albedo—Fig. 2-17

 Scattering and Diffused Light

• Much of the light reaching our eyes is not from a direct beam, but is from diffused light, light that has been scattered
• Explains how we can see under shade trees, or in a room not receiving direct sunlight
• One half of solar radiation absorbed at Earth’s surface arrives as scattered light

Blue Skies and Red Sunsets

• ____________ accounts for brightness and blue color of daytime sky
• Degree of scattering determined by _____ of particle doing the scattering
• Smaller particles, like gas molecules, more effectively scatter _______ wavelengths of visible light than _______ wavelengths

Blue Skies and Red Sunsets

• Blue and violet are _______ wavelengths
• Small gas molecules, plentiful in the atmosphere, scatter the blue and violet
• Thus, the sky appears blue

Red Sunsets—Fig. 2-18

• When Sun is rising or setting, its angle is low, so rays must go through more atmosphere
• Most of the short wavelength visible light gets scattered out of the direct beam
• So what reaches us is what is left, the red and orange

Blue Skies and Red Sunsets—Fig. 2-19

Earth Radiation

• Over 95% of Earth’s radiation has a wavelength of 2.5 to 30.0 µm, ____ range

Heating the Atmosphere

• Gases are selective absorbers: absorb radiation from some wavelengths, but not others
• Gases that absorb SW and/or LW radiation play major role in heating atmosphere

Heating the Atmosphere—Fig. 2-21

 Heating the Atmosphere--See Fig. 2-21

• Oxygen and Ozone absorb short UV rays from Sun
• Water vapor absorbs longer, near-IR, rays from Sun

 Heating the Atmosphere--See Fig. 2-21

• Notice the visible wavelengths, where no gases absorb
• Since Sun emits maximum in visible, we say atmosphere is nearly transparent to incoming solar radiation
• Direct solar energy absorption is NOT an effective "heater" of Earth’s atmosphere since most of it is not absorbed by the atmosphere's gases
• Instead, the Earth's surface warms by absorbing the solar energy, then the atmosphere warms by absorbing the IR energy emitted by the Earth's surface

Heating the Atmosphere--See Fig. 2-21

• Water vapor and carbon dioxide absorb much of the IR wavelengths at which Earth emits
• Water vapor absorbs about 60% of radiation emitted by Earth
• Accounts for warm temperatures of lower troposphere, where it’s most highly concentrated

Heating the Atmosphere--See Fig. 2-21

• Atmospheric Window: IR radiation between 8 and 12 µm not readily absorbed by atmosphere
• Passes through to outer space
• Clouds (made of tiny liquid water droplets) may absorb some radiation of these wavelengths

The Greenhouse Effect--Fig. 2-22 (b)

The Greenhouse Effect

• Gases like H₂O and CO₂ absorb LW radiation from surface, heat atmosphere
• They also emit LW radiation back to surface
• This further heats surface
• Surface emits more LW radiation
• Gases absorb more, atmosphere becomes even warmer, and so on, until a potential balance is reached

The Greenhouse Effect

• _______ of atmosphere through _________ of LW by certain gases is what keeps it _____ enough to allow humans to live
• Thus, it is a natural process
• However, if humans add more _________ gases, like ____, into the atmosphere, it may ________ Earth’s temperature

Role of Clouds

• Clouds are good absorbers/re-emitters of LW
• Keep surface ___________ at night
• Low, thick clouds reflect SW from Sun, hold _______ temps during day
• Cooling/ warming effect almost balanced
• However, averaging influence of all clouds around globe shows net __________ effect