AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/4/4-7)
Unit 4 · Topic 4.7
4.7 Solar Radiation and Earth's Seasons
Incoming solar radiation, or insolation, is strongest where sunlight hits the ground most directly, near the equator, and weaker toward the poles. Seasons happen because Earth's axis is tilted, which changes the angle of sunlight and the length of daylight through the year, not because Earth's distance from the sun changes.
Key terms
- insolation
- latitude
- axial tilt
- angle of sunlight
- solstice
- equinox
Insolation and the angle of sunlight
Insolation is the amount of solar energy reaching a given area of Earth's surface, usually measured in watts per square meter (W/m²). It's the main driver of temperature, winds, ocean currents and climate.
The angle of sunlight is what matters most. When the sun is directly overhead, a beam of sunlight hits a small area, concentrating its energy. When the sun is low in the sky, the same beam spreads over a much larger area, so each square meter gets less energy. Low-angle light also passes through more atmosphere, which absorbs and scatters some of it.
Near the equator, the sun is high in the sky all year, so insolation is high and fairly steady. Toward the poles, the sun is lower, so insolation is lower and varies more with the seasons. This difference in heating by latitude drives global winds and ocean currents.
Why we have seasons
Earth's axis is tilted about 23.5° from vertical, and it stays pointed the same direction in space as Earth orbits the sun. So for part of the year, the Northern Hemisphere is tilted toward the sun, and for the other part, it's tilted away.
When a hemisphere is tilted toward the sun, it gets more direct sunlight and longer days, so it has summer. When it's tilted away, sunlight is at a lower angle and days are shorter, so it has winter. The two hemispheres always have opposite seasons.
| Date (approximate) | Event | What happens |
|---|---|---|
| June 21 | June solstice | Northern Hemisphere tilted most toward the sun; sun directly overhead at the Tropic of Cancer (23.5° N); longest day in the north |
| December 21 | December solstice | Southern Hemisphere tilted most toward the sun; sun overhead at the Tropic of Capricorn (23.5° S); shortest day in the north |
| March 20 and September 22 | Equinoxes | Neither hemisphere tilted toward the sun; sun overhead at the equator; day and night about 12 hours everywhere |
Distance is not the reason
Earth's orbit is nearly circular. Earth is actually closest to the sun in early January, during Northern Hemisphere winter, and farthest in early July. The difference in distance is only about 3%, far too small to cause seasons. If distance caused seasons, both hemispheres would have summer at the same time, and they don't.
Albedo
Not all insolation that reaches the surface is absorbed. Albedo is the fraction of sunlight a surface reflects. Fresh snow and ice have a high albedo and reflect most sunlight, while dark surfaces like oceans, forests and asphalt have a low albedo and absorb most of it. Surfaces that absorb more sunlight warm up more, which is why melting ice speeds warming in the Arctic and why cities with dark pavement are warmer.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Summer and winter sun at 40° N
At noon on the June solstice, the sun at 40° N is about 73° above the horizon; on the December solstice it's about 27° above the horizon. The energy hitting each square meter of flat ground is proportional to the sine of the sun's angle above the horizon. About how many times more energy per square meter arrives at noon in June than in December?
Show the solutionHide the solution
- Step 1: June: sin(73°) ≈ 0.96.
- Step 2: December: sin(27°) ≈ 0.45.
- Step 3: Ratio = 0.96 ÷ 0.45 ≈ 2.1.
- Step 4: Interpretation: the same sunlight is spread over about twice as much ground in December. Add the shorter December days, and the total daily energy is even lower in winter.
Answer: About 2 times as much energy per square meter at noon in June.
- Example 2
The distance trap
A student claims that summer happens because Earth is closer to the sun. Use two pieces of evidence to refute the claim.
Show the solutionHide the solution
- Step 1: Evidence 1: Earth is closest to the sun in early January, which is winter in the Northern Hemisphere.
- Step 2: Evidence 2: When it's summer in the Northern Hemisphere, it's winter in the Southern Hemisphere. If distance caused seasons, both would have summer at the same time.
- Step 3: Give the correct cause: the 23.5° tilt of Earth's axis changes the angle of sunlight and the length of days.
Answer: The claim is wrong: Earth is closest to the sun in January, and the hemispheres have opposite seasons. Seasons are caused by the tilt of Earth's axis.
Common mistakes
- Saying seasons are caused by distance from the sun. They're caused by axial tilt.
- Thinking the equator gets more total hours of sunlight in a year. It gets more direct, intense sunlight, not more hours.
- Forgetting that the hemispheres have opposite seasons.
On the exam
- Be ready to explain why insolation is greater at the equator using the angle of sunlight and the area it's spread over.
- If asked to explain seasons, mention the tilted axis, the angle of sunlight and the length of daylight.
Connected topics
Videos
Check yourself
4 questions on 4.7 Solar Radiation and Earth's Seasons. Pick an answer to see if you got it, and why.
Earth is closest to the sun in early January, yet that is winter in the Northern Hemisphere. Which statement best explains this?
Why does a square meter of ground at 60° N receive less solar energy on average than a square meter at the equator?
On the June solstice, which location receives the most hours of daylight?
On the March and September equinoxes, the sun at noon is directly overhead at which location?
0 of 4 answered