Insolation and factors affecting it

At aphelion Earth is farthest from the Sun. Distance affects insolation slightly but the main cause of seasons is tilt.

In the tropics, the Sun’s rays generally strike at a higher angle. Thus, the same energy is concentrated over a smaller area and travels through a relatively shorter path in the atmosphere, producing higher insolation. Day length may vary, but it is not the main reason for high tropical insolation. Exam tip: More direct rays produce greater heating.

At the poles rays are oblique and spread over a larger area. High snow albedo can also reduce heating.

At equinox vertical rays fall on the Equator. At this time day and night are nearly equal in both hemispheres.

Around the June solstice vertical rays fall on the Tropic of Cancer. It is linked with summer in the Northern Hemisphere.

Around the December solstice vertical rays fall on the Tropic of Capricorn. It is linked with summer in the Southern Hemisphere.

At the Equator the Sun's altitude remains quite high throughout the year. Therefore insolation is relatively high.

On a sunny slope, solar rays strike at a relatively more direct angle. It therefore receives more insolation per unit area and can become warmer. Higher altitude generally lowers temperature, so option B is not the correct reason. Exam tip: the direction a slope faces is called its aspect, and it affects insolation.

Different exposure of slopes to the Sun causes different insolation. Therefore local temperature can differ.

Water has high heat capacity and heats and cools slowly. Therefore marine temperature changes slowly.

Land heats faster than water. So even with similar insolation land temperature can rise faster.

When tree shade is absent, there is no local obstruction to prevent the Sun’s rays from reaching the ground. Therefore, the surface of an open field receives more direct sunlight and relatively greater insolation. The length of the day is determined by Earth’s rotation and axial tilt, not by tree shade. Exam tip: Less shade means greater direct solar radiation at the surface.

The direct answer is A: the tree canopy reduces insolation. A dense forest has many leaves and branches forming an upper layer called the canopy. Step by step: sunlight reaches the forest top; leaves absorb some radiation for photosynthesis and reflect or scatter some; branches and foliage shade the lower layers; less direct sunlight reaches the ground; therefore the ground may receive less energy and remain relatively cool, especially compared with exposed open land. Option A is correct because reduced incoming radiation can reduce surface heating. B is wrong because the Sun does not fear forests. C is wrong because Earth continues rotating inside forests. D is wrong because forests still experience day and night. The word “can” is important: humidity, wind, soil, and decomposition also affect temperature. Memory cue: thick canopy means shade and less ground heating.

Black roads and dark buildings have low albedo, so they absorb a larger share of incoming solar radiation. This absorbed energy warms the surface and nearby air, increasing urban heating. In contrast, light-coloured surfaces reflect more energy and generally absorb less. Exam tip: remember that dark surfaces mean greater absorption, while light surfaces mean greater reflection.

A grassy surface does not absorb energy like a black road. Evaporation also helps reduce temperature.

Insolation means the incoming solar energy received at the Earth’s surface; therefore, option A is correct. Terrestrial radiation is heat radiated outward by the Earth, not solar energy received from the Sun. Exam tip: Remember “in” in insolation—it refers to incoming solar radiation.

Higher insolation means that the surface receives more solar energy. By absorbing this energy, the surface warms, so its temperature is generally likely to rise. The actual rise can be modified by clouds, wind, humidity, and the nature of the surface; therefore, it is not necessarily an absolute increase in every situation. Exam tip: Insolation and surface temperature usually have a direct relationship.

Insolation is the solar energy received at the Earth’s surface. When insolation is low, the surface absorbs less energy, so its heating is low. “Unchanged” is incorrect because surface heating depends on the amount of solar energy received. Exam tip: Remember that insolation and surface heating generally have a direct relationship.

In reflection, incoming radiation strikes a surface and returns from it, so option A is correct. When a surface takes in radiation, the process is called absorption, not reflection. The proportion of radiation reflected by a surface is expressed by its albedo. Exam tip: the phrase “returns from a surface” indicates reflection.

Absorption of radiation occurs when matter, such as land, water, or the atmosphere, takes in incoming radiant energy. The absorbed energy may raise the temperature of that matter. Option B describes reflection, in which energy returns from a surface, not absorption. Exam tip: absorption means taking in energy, while reflection means sending it back.

In a longer atmospheric path rays interact with more particles. Therefore less energy may reach the surface.

When the Sun's altitude is high rays fall more directly. Therefore the surface receives more insolation.

Direct answer: Option A, dense fog and clouds. Solar radiation must pass through the atmosphere before reaching the ground. Thick clouds and fog can reflect, absorb and scatter part of this radiation. Consequently, less direct solar energy reaches the surface, although some diffuse light may still arrive. The reasoning is: dense cloud or fog increases obstruction; obstruction reduces transmitted radiation; reduced radiation means lower surface insolation. Option A is correct. Option B is wrong in this comparison because a clear sky usually allows more incoming sunlight to reach the ground, even though atmospheric gases still scatter a small part. Option C is wrong because vertical rays are concentrated on a smaller area and usually give stronger insolation. Option D is wrong because a long day normally allows more time for energy receipt, all else being equal. Do not confuse reduced sunlight with total darkness: fog and clouds reduce or redistribute radiation rather than necessarily blocking every ray.

The angle of rays and duration of day greatly affect insolation. This is the most basic exam clue.

Albedo is the fraction of incoming solar radiation that a surface reflects. Therefore, a high-albedo surface reflects more energy and absorbs relatively less. Option B describes a low-albedo condition, while the length of the day is not determined by albedo. Exam tip: bright surfaces such as snow and clouds generally have high albedo.