GES 2613: Solar Energy and Seasons

Announcements

• Class Participation: Details will be provided regarding how class participation will be managed moving forward.
• Canvas Usage: Instructions will be given on how to effectively use Canvas.
• Upcoming Test: A plan for the next three classes will be outlined in preparation for an upcoming test.
• Assignment: An assignment will be announced on Wednesday and is due on the $24^{th}$ of the month.

Career Conversations Series: Thomas Richardson

• Event Details:
  • Topic: "Solidly Unacceptable: The San Antonio River's Environmental Challenge".
  • Speaker: Thomas Richardson.
  • Date: Tuesday, September 9, 2025.
  • Location: MS 4.02.64, UTSA.
  • Time: 4:30 PM.
  • Attendees: All majors and faculty are welcome.
  • Refreshments: Refreshments will be served.
• Speaker Background:
  • Thomas Richardson is a San Antonio native.
  • He graduated in 2023 with a B.A. in UTSA's Geography and Environmental Sustainability program.
  • He is currently an M.A. candidate in the same program.
  • For the past two years, he has focused his studies on freshwater salinization in the San Antonio River.
  • His thesis, which is slated for presentation this fall, investigates the San Antonio River's ecological health and the impacts stemming from urban wastewater.
  • His interest in this critical topic, which affects San Antonio's future, was sparked by an Environmental Exchange study abroad program in Costa Rica.

Chapter 2 Mini Review: Latitude/Longitude

• Latitude (Parallels):
  • Imaginary lines that circle the Earth parallel to the Equator.
  • Measured in degrees north or south from the Equator.
  • Equator: $0°$
  • Tropic of Cancer: $23°26' N$
  • Tropic of Capricorn: $23°26' S$
  • Arctic Circle: $66°34' N$
  • Antarctic Circle: $66°34' S$
  • North Pole: $90° N$
  • South Pole: $90° S$
  • Divides the Earth into the Northern Hemisphere and Southern Hemisphere.
• Longitude (Meridians):
  • Imaginary lines that run from the North Pole to the South Pole.
  • Measured in degrees east or west from the Prime Meridian.

Solar Energy and Seasons (Week 2 - Topic 1)

Questions for Today

• What are the characteristics of our solar system?
• How does energy reach Earth from the Sun?
• Which orbital parameter contributes most to seasons?

Solar System

• Location: Our solar system is situated within the Milky Way Galaxy.
• Composition: It comprises eight planets and over $100$ moons.
• Formation: The solar system originated from a nebula, which is defined as a large, slowly rotating, collapsing cloud of dust and gas.
• Earth's Orbit:
  • Earth follows an elliptical orbit around the Sun.
  • On average, Earth is $150$ million kilometers away from the Sun.
  • It takes approximately $8.5$ minutes for light emitted by the Sun to reach Earth.
  • Perihelion: The point in Earth's orbit where it is closest to the Sun, occurring around January 3rd.
  • Aphelion: The point in Earth's orbit where it is furthest from the Sun, occurring around July 4th.
  • Note: The varying distance between Earth and the Sun due to its elliptical orbit is not responsible for seasonality.

The Sun

• Mass: The Sun captured $99.9\%$ of the matter from the original solar nebula.
• Energy Source: It is the primary energy source fueling most processes within Earth's atmosphere and biosphere.
• Energy Generation: The Sun generates energy through a fusion reaction.
  • This process involves extremely high temperatures and pressure forcing hydrogen atoms together.
  • This fusion forms helium and simultaneously releases a tremendous amount of energy.
• Solar Cycle:
  • The solar cycle monitors the variation in the Sun's energy output.
  • Sunspots: These are magnetic storms that appear as dark areas on the Sun's surface.
  • Solar Radiation: A greater number of sunspots correlates with increased solar radiation.
  • Cycle Duration: There is an $11$-year cycle between a solar minimum (fewer sunspots, lower output) and a solar maximum (more sunspots, higher output).

Solar Winds

• Emission: The Sun continuously emits clouds of electrically charged particles, known as solar winds.
• Interaction with Earth: Solar winds interact with Earth’s magnetosphere.
  • The magnetosphere effectively deflects these charged particles, directing them towards the Earth's poles.
• Auroras: This interaction between solar wind particles and the upper layers of Earth's atmosphere produces auroras (e.g., Aurora Borealis, Aurora Australis) in high-latitude regions.

Electromagnetic Radiation

• Nature: The Sun emits electromagnetic waves, which possess both electrical and magnetic properties.
  • These properties allow them to travel through the vacuum of space.
• Electromagnetic Spectrum: This spectrum encompasses all the different wavelengths of electromagnetic energy.
• Energy and Wavelength Relationship:
  • High Energy: Corresponds to shorter wavelengths and higher frequencies.
  • Low Energy: Corresponds to longer wavelengths and lower frequencies.
• Solar Emission Breakdown: The Sun predominantly emits:
  • $8\%$ Ultraviolet (UV), X-Ray, & Gamma radiation.
  • $47\%$ Visible light.
  • $45\%$ Infrared radiation.
• Visible Light Spectrum: Ranges from $0.40$ micrometers ($\mu m$) (violet) to $0.70$ micrometers ($\mu m$) (red).
• Temperature (Kinetic Energy):
  • Defined as the average kinetic energy, or the average speed of molecular movement, of individual molecules within a substance.
• Heat (Energy Transfer):
  • Refers to the transfer of kinetic energy between objects.
  • This transfer occurs due to temperature differences, always moving from a hotter object to a colder object.

Shortwave and Longwave Radiation

• Shortwave Radiation:
  • Emitted primarily by the Sun.
  • Its energy is concentrated around $0.5$ $\mu m$ (micrometers) in the visible portion of the electromagnetic spectrum.
• Longwave Radiation:
  • Emitted by the Earth.
  • Its energy is concentrated around $10$ $\mu m$ in the infrared portion of the electromagnetic spectrum.

Insolation at the Top of the Atmosphere

• Insolation: This term refers to the incoming solar radiation intercepted by Earth.
• Measurement: Insolation is measured in watts per square meter (W/m$^2$).
• Solar Constant:
  • Defined as the average insolation received at the very top of Earth's atmosphere.
  • Its established value is approximately $1372$ W/m$^2$.
  • Note: Not all of this radiation successfully reaches the Earth's surface.
• Solar Radiation Spectrum (Example Data from Sea Level vs. Top of Atmosphere):
  • At the distance we are from the Sun, total solar irradiance is approximately $1360$ W/m$^2$.
  • At sea level, solar irradiance is roughly $1000$ W/m$^2$.
  • Energy is absorbed by water vapor and atmospheric gases at various absorption bands.
• Insolation Distribution:
  • Due to Earth's curved surface, insolation is unevenly distributed across different latitudes.
  • Subsolar Point: This is the only location where insolation arrives perpendicular (at a $90°$ angle) to the Earth's surface.
    • It occurs between $23.5°$ N and $23.5°$ S latitude.
    • At this point, solar energy is most concentrated.

Net Radiation

• Definition: Net radiation is the overall balance between incoming shortwave (solar) radiation and outgoing longwave (Earth) radiation.
• Distribution:
  • Lower Latitudes: Typically experience positive net radiation values (more incoming energy than outgoing).
  • Higher Latitudes: Typically experience negative net radiation values (less incoming energy than outgoing).
• Significance: This imbalance in net radiation is the fundamental driving force behind both atmospheric and oceanic circulations, distributing heat around the globe.

Seasonality

• Definition: Seasonality describes the predictable changes in insolation that occur throughout a given year.
• Influencing Factors: Seasonality is primarily influenced by two key factors:
  • Sun Altitude: The angle measured between the horizon and the position of the Sun ($0°$ at the horizon, $90°$ directly overhead).
  • Daylength: The duration of insolation, measured from sunrise to sunset.
• Produced by Earth's Orbital Characteristics: Seasonal and diurnal (daily) variations are a direct result of Earth's orbital characteristics:
  • Revolution
  • Rotation
  • Tilt

Revolution

• Definition: Revolution refers to Earth's elliptical orbit around the Sun.
• Duration: This orbit takes approximately $365.2$ days to complete, defining the length of an Earth year, within which seasons occur.
• Distance Variation: The distance between Earth and the Sun varies by only about $3\%$ over the course of the year.
• Seasonal Impact: This small variation in distance produces only a minor change in insolation; therefore, revolution is not a significant factor in causing Earth's seasons.

Rotation

• Definition: Rotation is the turning of Earth on its own axis.
• Duration: This rotation takes approximately $24$ hours to complete.
• Impacts:
  • Daylength: Determines the duration of daylight and nighttime.
  • Apparent Deflection: Creates the apparent deflection of winds and ocean currents (Coriolis Effect).
• Circle of Illumination: This is an imaginary line that divides the Earth into the areas experiencing day and night.
• Equator: At the Equator, daylight is always evenly split for $12$ hours of day and $12$ hours of night, regardless of the season.

Tilt

• Definition: The axis upon which Earth rotates is tilted at an angle of $23.5°$ relative to the plane of its orbit (the plane of the ecliptic).
• Seasonal Importance: This axial tilt is considered the most important orbital parameter for producing Earth's seasons.
• Impact: When the Northern Hemisphere is tilted towards the Sun during its summer, it receives more direct and concentrated insolation, leading to warmer temperatures and longer days, and vice-versa for the Southern Hemisphere.

Annual March of the Seasons

• December Solstice (approx. December 21st or 22nd):
  • Subsolar Point: The Sun is directly overhead at $23.5°$ S latitude (Tropic of Capricorn).
  • Northern Hemisphere: Tilted away from the Sun, resulting in its winter.
    • Areas above the Arctic Circle (approx. $66.5°$ N) experience $24$ hours of darkness.
    • Receives less intense insolation and shorter daylengths.
  • Southern Hemisphere: Tilted towards the Sun, resulting in its summer.
• Vernal (Spring) Equinox (approx. March 20th or 21st):
  • Subsolar Point: The Sun is directly overhead at $0°$ latitude (Equator).
  • Global Day/Night: The circle of illumination passes through both the North and South Poles.
  • All locations on Earth experience approximately $12$ hours of daylight and $12$ hours of nighttime.
• June Solstice (approx. June 20th or 21st):
  • Subsolar Point: The Sun is directly overhead at $23.5°$ N latitude (Tropic of Cancer).
  • Northern Hemisphere: Tilted towards the Sun, resulting in its summer.
    • Areas above the Arctic Circle experience $24$ hours of daylight.
    • Receives more intense insolation and longer daylengths.
  • Southern Hemisphere: Tilted away from the Sun, resulting in its winter.
• Autumnal (Fall) Equinox (approx. September 22nd or 23rd):
  • Subsolar Point: The Sun is directly overhead at $0°$ latitude (Equator).
  • Global Day/Night: The circle of illumination passes through both the North and South Poles.
  • All locations on Earth experience approximately $12$ hours of daylight and $12$ hours of nighttime.
• Declination of Sun: The latitude where the Sun is directly overhead varies throughout the year, cycling between the Tropic of Cancer ($23.5°$ N) and the Tropic of Capricorn ($23.5°$ S). This annual movement is responsible for the progression of seasons.