Earth is closest to the Sun in January — dead of Northern winter. So the seasons aren't about distance; they're about tilt. Our planet leans 23.5°, and as it orbits, that lean points the Northern Hemisphere toward or away from the Sun, changing how directly sunlight strikes. The same "how directly does energy arrive" logic controls climate by latitude and even drives the ocean-atmosphere seesaw called El Niño that can flood Peru and parch Australia in the same year. This lesson closes Unit 4 by connecting incoming solar energy to seasons, and then follows water across the land (watersheds) and across the Pacific (ENSO).
The Sun drives nearly all Earth's systems. Insolation (incoming solar radiation) is not evenly distributed: - Near the equator, sunlight strikes directly (high angle), concentrating energy on a small area → warm. - Near the poles, sunlight strikes at a low angle, spreading the same energy over a larger area and passing through more atmosphere → cold.
This latitudinal difference in insolation drives temperature patterns, atmospheric circulation (Lesson 12), and biome distribution (Lesson 3).
Some incoming solar energy is reflected. Albedo is the fraction of solar radiation a surface reflects: bright surfaces (ice, snow, clouds) have high albedo (reflect much); dark surfaces (forests, oceans, asphalt) have low albedo (absorb much). Albedo matters for climate feedbacks (melting ice lowers albedo, absorbing more heat — Unit 9).
Earth's axis is tilted 23.5° relative to its orbital plane. As Earth orbits the Sun, the tilt causes the hemispheres to receive different amounts of direct sunlight through the year:
[DIAGRAM: Earth at four orbital positions around the Sun. June solstice: Northern Hemisphere tilted toward Sun (NH summer, direct sunlight, long days). December solstice: NH tilted away (NH winter). March and September equinoxes: neither hemisphere tilted toward Sun (equal day/night). Sun's direct rays hit the Tropic of Cancer (23.5°N) in June, the Tropic of Capricorn (23.5°S) in December.]
The seasons are opposite in the two hemispheres. This is why the tropics (between 23.5°N and 23.5°S) stay warm year-round and the poles have extreme seasonal daylight variation.
A watershed (drainage basin) is all the land area that drains water into a common body of water (a river, lake, or the ocean). Ridges (divides) separate one watershed from another. Everything that happens on the land — farming, paving, logging, dumping — affects the water that drains through the watershed.
[DIAGRAM: Watershed cross-section — rain falls across a sloped landscape bounded by ridge lines (divides); water flows downhill via tributaries into a main river and out to a lake/ocean. Show a farm field, a paved town, and a riparian buffer strip along the stream.]
ENSO is a periodic shift in Pacific Ocean temperatures and winds:
[DIAGRAM: Two panels of the equatorial Pacific. NORMAL/La Niña: strong east→west trade winds, warm pool in west, upwelling of cold nutrient water off South America. EL NIÑO: weak/reversed winds, warm water shifted east, suppressed upwelling off South America.]
ENSO affects global weather, fisheries (Lesson 3's upwelling link), and agriculture, on a roughly 2–7 year irregular cycle (an episodic/periodic disruption).
Insolation and tilt explain the seasons and latitude climate (common conceptual MC). Watersheds tie land use to water pollution (Units 5, 8). ENSO connects ocean circulation to productivity and weather — a favorite systems-thinking topic.
Explain why the equator is warmer than the poles even though both receive sunlight.
Solution: At the equator, sunlight arrives nearly vertically (direct), concentrating energy on a small area. At the poles, sunlight arrives at a low, slanted angle, spreading the same energy over a larger area (and through more atmosphere), so the poles receive far less energy per unit area.
Interpretation: It's the angle (concentration) of insolation, not distance to the Sun.
It is June. Describe the tilt situation and season for the Northern and Southern Hemispheres.
Solution: In June, the Northern Hemisphere is tilted toward the Sun → direct rays, long days → NH summer. The Southern Hemisphere is tilted away → indirect rays, short days → SH winter. Seasons are opposite between hemispheres.
Interpretation: Toward Sun = summer; away = winter; hemispheres are always opposite.
A town paves large areas within a watershed. Predict two effects on the local river.
Solution: (1) Increased runoff and flooding — impervious surfaces prevent infiltration, so more water rushes to the river quickly. (2) More nonpoint-source pollution — runoff carries oil, sediment, and nutrients from streets into the river, degrading water quality.
Interpretation: Pavement → more/faster runoff → flooding + pollutant delivery.
Describe how El Niño affects upwelling and fisheries off the coast of Peru.
Solution: During El Niño, weakened trade winds let warm surface water shift east toward South America, which suppresses upwelling of cold, nutrient-rich deep water. Without those nutrients, phytoplankton decline, the food web collapses, and fisheries crash off Peru.
Interpretation: El Niño = warm water east = no upwelling = fishery collapse.
(B) Filters runoff and stabilizes banks.
Earth is actually closest to the Sun in January, so distance can't explain seasons; the 23.5° tilt does. At the June solstice, the NH is tilted toward the Sun → direct rays, longest day → summer. At the December solstice, the NH is tilted away → indirect rays, shortest day → winter.
(a) El Niño. (b) Warm surface water suppresses upwelling of cold, nutrient-rich water, so phytoplankton and the fish food web collapse. (c) El Niño typically brings drought to Australia/Indonesia (western Pacific).
FRQ rubric (10 pts): - (a) 1 pt impervious surfaces reduce infiltration → more/faster runoff → flooding; 1 pt runoff carries pollutants (oil, sediment, nutrients) into the river. (2) - (b) 1 pt loss of shade lets sunlight warm the water (raising temperature/lowering dissolved oxygen); 1 pt loss of root systems increases bank erosion and sediment input. (2) - (c) 1 pt nonpoint source; 1 pt because it comes from diffuse runoff across many farms/lawns rather than a single identifiable pipe. (2) - (d) For each of two practices: 1 pt name + 1 pt justification. Acceptable: restore riparian buffers (filter runoff, shade, stabilize banks); build detention/retention ponds or rain gardens (slow runoff, allow infiltration); use permeable pavement; limit impervious cover/zoning; cover crops and reduced fertilizer on farms. (4)
A river's watershed has been heavily developed with suburbs, roads, and farms. The river now floods more often, carries high sediment and nutrient loads, and has warmer water.
(a) Explain how impervious surfaces increase both flooding and pollution in the watershed. (2 pts) (b) Explain how removing streamside (riparian) vegetation raises water temperature and sediment. (2 pts) (c) Identify whether the nutrient pollution is point or nonpoint source and justify. (2 pts) (d) Propose two watershed-management practices to reduce these impacts, and justify each. (4 pts)
MC: 1. (B) Earth's 23.5° axial tilt. 2. (B) More direct/concentrated sunlight. 3. (B) Land draining to a common water body. 4. (B) Suppressed during El Niño. 5. (B) Reflectivity of solar radiation. 6. (B) Summer. 7. (B) Increase runoff and flooding. 8. (C) Fresh snow (high albedo). 9. (B) East to west (normal trade winds). 10. (B) Filters runoff and stabilizes banks.
Earth is actually closest to the Sun in January, so distance can't explain seasons; the 23.5° tilt does. At the June solstice, the NH is tilted toward the Sun → direct rays, longest day → summer. At the December solstice, the NH is tilted away → indirect rays, shortest day → winter.
(a) El Niño. (b) Warm surface water suppresses upwelling of cold, nutrient-rich water, so phytoplankton and the fish food web collapse. (c) El Niño typically brings drought to Australia/Indonesia (western Pacific).
FRQ rubric (10 pts): - (a) 1 pt impervious surfaces reduce infiltration → more/faster runoff → flooding; 1 pt runoff carries pollutants (oil, sediment, nutrients) into the river. (2) - (b) 1 pt loss of shade lets sunlight warm the water (raising temperature/lowering dissolved oxygen); 1 pt loss of root systems increases bank erosion and sediment input. (2) - (c) 1 pt nonpoint source; 1 pt because it comes from diffuse runoff across many farms/lawns rather than a single identifiable pipe. (2) - (d) For each of two practices: 1 pt name + 1 pt justification. Acceptable: restore riparian buffers (filter runoff, shade, stabilize banks); build detention/retention ponds or rain gardens (slow runoff, allow infiltration); use permeable pavement; limit impervious cover/zoning; cover crops and reduced fertilizer on farms. (4)
⭐ Exam strategy: "Seasons = tilt, not distance" is a guaranteed conceptual point — never say "closer to the Sun." For ENSO, anchor on El Niño = warm water east + no upwelling + fishery collapse; La Niña = the normal pattern on steroids.
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