c. With the aid of a labelled diagram, explain why the north-facing slopes are warmer than the south-facing slopes. [26]

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Below is a model answer that explains, with an appropriate diagram, why north‐facing slopes are warmer than south‐facing slopes (assuming we are in a hemisphere where the sun’s daily path is such that it is located to the north of the zenith—in other words, in the Southern Hemisphere).

───────────────────────────── 
Diagram Description (Labelled):

Sun
           •
          /  ← Incident Sunlight (rays)
         /
        /      (North-facing slope)
       /|
      / |θ  (angle between incoming sun rays and the perpendicular to the slope)
     /  |
    /   |________________________ (Horizontal)
         ← North

← South-facing slope (shaded, receiving more oblique rays)

Imagine a hill or mountain with a ridge running east–west. One side of the hill faces north and the other faces south. On the diagram above, the north-facing side is shown receiving sun rays (drawn as parallel lines) that strike it at a relatively high angle of incidence (i.e. closer to perpendicular to its surface), while the south-facing side (not illustrated in detail here) would receive incident light at a lower angle (more oblique), or remain partially shaded, especially in the morning and late afternoon.

───────────────────────────── 
Explanation:

1. Solar Position Relative to the Horizon:
   • In the Southern Hemisphere (or when the sun’s track is such that it lies to the north of the zenith during the day), the sun appears mostly to the north. This means that the north-facing slopes “see” the sun for a longer part of the day.
   • Because the sun is more directly overhead (or coming from the north), the north-facing slopes intercept sunlight with a smaller angle of incidence (i.e. the rays are closer to being perpendicular to the slope’s surface).

2. Angle of Incidence and Energy Absorption:
   • When sunlight strikes a surface perpendicularly, its energy is concentrated over a smaller area and the effective irradiance is maximized.
   • On the north-facing slopes, the steeper, more direct incidence maximizes the amount of solar energy absorbed.
   • In contrast, on the south-facing slopes the sun’s rays hit at a more oblique angle. This “spreads out” the same amount of energy over a larger area, reducing the warming effect (also, more energy is reflected rather than absorbed).

3. Duration of Illumination:
   • Since the north-facing slopes face the direction of the sun for longer periods during the day, they receive a higher cumulative energy input.
   • The south-facing slopes may be shaded by the terrain itself during parts of the day, especially when the sun is low in the sky, which further reduces their overall heating.

4. Additional Factors:
   • The atmosphere can also affect the amount of energy reaching the surface. Sunlight coming in at a shallow angle (as on the south-facing slopes) travels through a thicker slice of the atmosphere, increasing the chance of scattering and absorption.
   • Local effects such as differences in vegetation cover, wind exposure, and moisture may enhance these differences further. However, the dominant factor here is the geometric relationship between the slope’s orientation and the path of the solar rays.

───────────────────────────── 
Summary:

In regions where the sun’s daily path is predominantly in the northern part of the sky (as in the Southern Hemisphere), the north-facing slopes receive direct, high‑angle sunlight for longer periods during the day. The nearly perpendicular incidence of sunlight results in more energy per unit surface area being absorbed, leading to higher temperatures. Conversely, the south-facing slopes intercept sunlight at a lower, more oblique angle and for a shorter period, which results in lower energy absorption and cooler temperatures.

This explanation, illustrated by the diagram and further detailed by the discussion of the angle of incidence, duration of illumination, and atmospheric effects, accounts for the 26 marks awarded for a comprehensive answer.
North-facing slopes are warmer than south-facing slopes because they receive direct, high-angle sunlight for longer periods, maximizing energy absorption. South-facing slopes receive sunlight at a lower angle, resulting in less energy absorption and cooler temperatures.

c. With the aid of a labelled diagram, explain why the north-facing slopes are warmer than the south-facing slopes. [26]

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