Lecture on Seasonal Radiation Controls

Introduction to Solar Radiation

• The Sun emits radiation across all wavelengths.
• Average radiation received at the top of the atmosphere: 1317 watts/m².
• Termed the solar constant; an average over all latitudes and seasons.
• Powers weather and climate systems.

Insulation and Radiative Receipt

• Insulation: Incoming or intercepted solar radiation.
• Radiative receipt is influenced by:
  • Incident Angle of the Sun: The height of the Sun in the sky at a given location, highest at solar noon.
  • Day Length: Time the Sun is above the horizon.

Understanding Solar Angles

• Noon Solar Altitude Angle: Angle between Earth and Sun at noon.
• Maximum at 90° when the Sun is directly overhead, leading to concentrated radiation.
• Lower angles spread radiation over larger areas, reducing intensity.

Earth's Rotation and Revolution

• Rotation: Earth spins on its axis (~24 hours per rotation).
• Revolution: Earth's orbit around the Sun; takes slightly longer than a year, leading to leap years.
• Orbit is elliptical, causing perihelion (closest to Sun) in January and aphelion (farthest from Sun) around July 4th.

Seasonality and Earth's Tilt

• Tilt of Earth's Axis: 23.5° relative to orbital path; remains constant.
• Controls which latitudes receive more direct solar radiation.
• Seasons: Results from tilt; different hemispheres experience opposite seasons.

Solar Declination

• Latitude receiving the most direct solar radiation (90° solar angle).
• Changes daily within the tropics.
• Analemma: Figure 8 shape showing solar declination throughout the year.

Solstices and Equinoxes

• Equinox: Sun's direct rays at the equator; equal day length globally.
• Solstice: Sun's direct rays at farthest position into a hemisphere.
  • June Solstice: Sun over Tropic of Cancer.
  • December Solstice: Sun over Tropic of Capricorn.
• Changes in day length are most significant at polar circles.

Circle of Illumination

• The dividing line between day and night across Earth.
• Controls day length, affecting radiation received.
• On equinoxes, cuts across the poles; 12-hour day globally.
• During solstices, can result in 24-hour daylight or darkness at poles.

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