GEO 130 - Solar Radiation and Energy Balance

Energy flow path (Sun to Earth)

Space → atmosphere → surface → back to space

Electromagnetic radiation (EMR)

Emitted by matter above absolute zero; has wavelength & frequency

Wavelength

Distance between two successive waves

Frequency

Number of waves passing a point per second

Wavelength × frequency

Equals the speed of light (constant)

Blackbody

Radiates all absorbed energy; no transmission or reflection

Blackbody radiation depends on

Absolute temperature (K) only

Temperature-wavelength relationship

Higher temp → more energy emitted, shorter wavelength

The Sun's temperature

~6000 K; makes high-power, short-wavelength energy via fusion

Solar wind

Charged particles from the Sun; deflected to poles by magnetosphere, causing auroras

Sun's energy output

Essentially constant; varies slightly with sunspot activity

Visible light

0.4–0.7 μm peak of solar radiation; visible to humans; drives photosynthesis

Insolation

Incoming solar radiation; energy received per area per time (W/m²)

Solar constant

1372 W/m²; average insolation at thermopause, 1 AU

Flashlight effect

Insolation decreases away from subsolar point due to Earth's curvature

Atmospheric interactions with solar energy

Reflection, scattering, absorption

Insolation returned to space unaltered

~30% (reflection + scattering)

Insolation entering Earth system

~70% (absorbed, scattered to ground, or transmitted)

Refraction

Light bending through media of differing densities; causes rainbows/mirages

Reflection (atmospheric)

Energy bounced directionally back to space (clouds/ground)

Scattering

Redirects radiation's direction only, omnidirectionally

Selective scattering

By gas molecules; causes blue sky

Non-selective scattering

By clouds; causes white clouds

Diffuse radiation

Scattered solar energy that reaches the ground

Atmospheric absorption

Selective; creates atmospheric windows

Ozone layer's radiative role

Absorbs UV; re-radiates equal long-wave energy

Radiation blocked vs. passed by atmosphere

X-rays/gamma rays blocked; visible light passes through

Five greenhouse gases

H2O, CO2, CH4, N2O, O3

Greenhouse gas function

Delay escape of infrared (long-wave) radiation from the ground

EMR at Earth's surface

Reflected, absorbed, transmitted, or used to evaporate water

Albedo

% of insolation reflected from a surface

Heat transfer mechanisms

Radiation, conduction, convection/advection, evapotranspiration

Conduction

Molecule-to-molecule heat transfer across a temperature gradient

Convection

Vertical gas/liquid heat movement

Advection

Horizontal gas/liquid heat movement

Latent heat

Heat added to atmosphere via evapotranspiration

Energy balance

Total insolation input = total energy output (bucket analogy)

Long-wave radiation released

~70% of received energy, matching input

Greenhouse effect (importance)

Maintains Earth's energy equilibrium & biosphere-friendly temp

Increased greenhouse effect

Causes global warming