Topic_2_Ch_5_Radiation

Topic 2: Energy and Radiation Balances

Radiation Fundamentals
- Speed of light (c) is constant for all forms of radiation:
  - Relationship: c = λ ν (where λ = wavelength and ν = frequency)
- Shorter wavelengths correlate with more energy, while longer wavelengths correlate with less energy.
- Energy quantification: Q = h ν or Q = h c/λ
The Electromagnetic Spectrum
- All radiation types travel at the speed of light and in a vacuum.
- All objects emit radiation based on their temperature.
- Energy transfer occurs via all forms of radiation.

Planck's Law
- Relates temperature of a substance to its emitted radiation.
Stefan–Boltzmann Law
- Emission rate (E) proportional to the fourth power of temperature (T):
  E = ε σ T^4
- E = flux density (W·m^–2),
- ε = emissivity (1 for black bodies),
- σ = Stefan-Boltzmann constant (5.67 × 10^–8 W·m^–2·K^–4).
Wien's Law
- Maximum emission wavelength inversely related to temperature.
Planck's Law (Detailed)
- Radiation comprises energy bundles called photons.
- Shorter wavelengths = higher energy per photon.

Black body approximation for both Earth and Sun:
- Sun (T = 5,800 K): emission mainly 0.15 - 3.0 µm (Shortwave)
- Earth (T = 288 K): emission mainly 3.0 - 100 µm (Longwave)

Incident radiation can:
1. Be absorbed (a)
2. Be reflected (α)
3. Be transmitted (t)
Equation: a + α + t = 1 (or 100%)

Radiation Interaction
- Absorbed radiation heats the atmosphere.
- Scattered/reflected radiation can reach Earth's surface as diffuse radiation.
- Direct radiation reaches the surface without interaction.
Scattering Types
1. Rayleigh scattering
2. Mie scattering
Absorption Characteristics
- Longwave radiation absorbs more strongly than shortwave due to selective gases.
- Key absorbent gases:
  - Oxygen, ozone, water vapor (strong in UV)
  - Water vapor, CO2, methane, nitrous oxide (greenhouse gases for longwave)

Solar radiation transmits well through the atmosphere; Earth's longwave radiation is absorbed by greenhouse gases.
Without the atmosphere, Earth's temperature would drop by 33 K.

Three maximum temperatures occur due to absorption at specific wavelengths:
1. Upper thermosphere (λ = 0.1–0.2 μm)
2. Upper stratosphere (λ = 0.2–0.3 μm)
3. Lowest troposphere (heat from Earth's surface)

Solar Constant: Average solar radiation at top of the atmosphere - approx. 1361 W·m^–2.
Sphericity Effect: Causes variation in Sun angles based on curvature.
- Low latitudes: higher Sun angles; High latitudes: lower Sun angles.

Illustrated by two definitions:
1. Altitude angle
2. Zenith angle
Influenced by latitude, longitude, time of year, and time of day.
Lower altitude means lower radiation intensity due to beam spreading and depletion.

Seasons defined by:
- Altitude of the Sun above horizon
- Intensity of Sun energy
- Duration of day length.
Earth’s Orbit Details:
- 23.5° tilt from the perpendicular ecliptic plane.
- Constant angle and orientation during orbit.

Influential factors impacting climate variability:
1. Eccentricity: 100,000-year cycle affecting distance from the Sun.
2. Tilt variation: 40,000-year cycle affecting seasonality.
3. Direction of tilt: 26,000-year cycle dictating seasonal intensity at perihelion and aphelion.

Sphericity affects solar intensity across latitudes and leads to varying seasons.
- Seasonal changes impact duration and intensity of Sun received by Earth, influenced by multiple physical characteristics of Earth and its orbit.