It is warmer in summer than in winter because

Question

BlackTom AI · Accepted Answer

Earth's axial tilt of about 23.5° causes the Sun to appear higher in the sky and the daylight period to be longer in the hemisphere experiencing summer.

1. Relevant knowledge points and concepts
- Earth's axis is tilted relative to its orbital plane (the ecliptic) by ~23.5°, producing seasons.
- Solar irradiance on a horizontal surface depends on the Sun’s elevation: incident energy per unit area scales approximately with cos(zenith angle).
- Day length (hours of sunlight) determines the total daily energy input.
- Atmospheric path length (air mass) increases for low Sun angles, reducing surface heating by scattering and absorption.
- Earth’s orbital distance changes little and is not the primary cause of seasons.

2. Step-by-step reasoning
- When a hemisphere is tilted toward the Sun, the Sun’s elevation at midday is higher (smaller zenith angle). This decreases the area over which the Sun’s rays are spread and increases the energy per unit area.
- Example using cosine law: if the Sun is at zenith angle θ, surface irradiance ∝ cos θ. For θ = 30°, cos30° ≈ 0.866; for θ = 60°, cos60° = 0.5. Thus the higher Sun gives roughly 73% more irradiance (0.866/0.5 = 1.732).
- Higher Sun angles also reduce atmospheric path length (air mass ≈ 1/cos θ), so less scattering/absorption and more energy reaches the ground.
- Longer days multiply the hourly irradiance by more hours of sunlight, increasing total daily energy compared with shorter winter days.

3. Calculations / intermediate steps
- Cosine example above: irradiance ratio between θ = 30° and θ = 60° is cos30°/cos60° = 0.866/0.5 ≈ 1.732.
- Air mass example: at θ = 60° air mass ≈ 1/cos60° = 2 (twice the path), at θ = 30° air mass ≈ 1.155.

4. Why the correct answer is correct
Combining higher solar elevation (greater instantaneous irradiance) with longer daylight (greater integration time) explains why summers are warmer in the tilted hemisphere.

5. Why the other options are incorrect
- "Sun is higher in the sky and the days are shorter.": contradicts reality—summer days are longer, not shorter.
- "Sun is lower in the sky and sunlight passes through more atmosphere, thereby warming it more during summer.": incorrect direction; lower Sun and longer atmospheric path reduce surface heating, and this describes winter conditions, not summer.
- "Earth is closer to the Sun in summer.": seasonal temperature differences are dominated by axial tilt; Earth is actually closest (perihelion) in January, so distance does not explain summer in either hemisphere.

6. Conclusion
Therefore the correct explanation is that summer is warmer because the Sun is higher in the sky and the days are longer, which matches the provided answer.

It is warmer in summer than in winter because单项选择题

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