Insolation and factors affecting it

Insolation is the main energy that heats the Earth's surface. In exams link it with incoming solar energy.

Energy coming from the Sun is called solar radiation. Insolation is related to the received amount of this radiation.

The angle of sun rays shows how concentrated or spread the energy will be. In easy questions first check the angle.

When the Sun is overhead rays fall almost vertically. This concentrates energy over a smaller area.

In the tropical zone sun rays can fall more directly. Therefore insolation is generally higher here.

At high latitudes, the Sun’s rays strike the Earth at an oblique angle. Therefore, the same amount of solar energy is spread over a larger surface area, reducing energy received per unit area. Nearly vertical rays concentrate energy over a smaller area, so option A is incorrect. Exam tip: As latitude increases, the angle of incidence decreases and the spread of energy increases.

When the day is longer, a place receives sunlight and solar energy for a longer period. Therefore, if other conditions remain the same, the total insolation received during the day increases. A change in the Sun’s apparent size is not the reason for a longer duration of insolation. Exam tip: Remember duration of daylight as a factor affecting insolation.

Insolation means incoming solar radiation received by Earth. Total energy received over a day depends not only on the intensity and angle of sunlight but also on the duration for which sunlight is available. If other conditions are broadly comparable, a shorter day provides fewer hours during which the surface can receive solar energy, so the daily total may be lower. Hence option B is correct. Option A is wrong because the Sun does not automatically double its output. Option C is unrelated and says “always,” while option D is incorrect because solar rays are not vertical at every place or time. The word “can” is important because cloud cover, angle and latitude also influence the total.

Clouds reflect part of the incoming solar radiation back towards space and also absorb or scatter some of it. Hence, less direct solar energy reaches the Earth’s surface. The Sun’s altitude angle affects insolation too, but clouds do not change that angle. Exam tip: greater cloud cover generally reduces surface insolation.

Clouds reflect part of the incoming solar radiation and absorb some of it. In a cloudless sky, this obstruction is lower, so more solar energy reaches the Earth's surface and insolation is greater. The Earth does not move closer to the Sun as a normal daily cause of higher insolation. Exam tip: a clear sky allows more direct solar radiation to reach the surface.

Water vapour in a moist atmosphere absorbs part of the incoming solar radiation, especially some longer-wavelength radiation. Therefore, the amount of insolation reaching the Earth’s surface may decrease. Atmospheric moisture does not change the Earth–Sun distance or the actual size of the Sun. Exam tip: Remember water vapour as an important atmospheric absorber affecting insolation.

Dry, clear air allows a larger share of the Sun’s energy to travel through the atmosphere and reach the ground. Water vapour, dust, smoke, and other particles can absorb or scatter radiation. When these obstacles are fewer, less incoming energy is removed from the beam, so surface insolation can be greater. This does not mean that the Sun changes or that the length of the day changes.

Therefore, option A is correct. The important idea is that dry air contains less water vapour and clear air contains fewer suspended particles. Both conditions reduce atmospheric absorption and scattering. Options B, C, and D are incorrect because they describe no physical process that would increase the solar energy reaching the surface.

The direct answer is A: scattering. Atmospheric dust contains small particles that interact with incoming solar radiation. When sunlight strikes these particles, its energy is redirected in many different directions; this process is called scattering. Some radiation may also be absorbed, but among the given options scattering is the correct process. Because the path and amount of radiation change, less direct solar energy may reach the ground, and visibility can decrease. Option A is correct because dust particles disperse light in different directions. Option B, tide, is the regular rise and fall of sea level mainly related to the gravitational effects of the Moon and Sun, not dust. Option C, earthquake, is ground shaking caused by sudden movement in Earth’s crust and is unrelated to atmospheric radiation. Option D, weathering, is the breakdown of rocks at or near Earth’s surface and is not the process by which dust redirects sunlight. Memory cue: tiny particles in air scatter light; a hazy sky signals this effect.

Dark-coloured surfaces absorb a greater proportion of incoming solar radiation and reflect less, so they can become relatively warmer. Light-coloured surfaces reflect more radiation, making option B incorrect. Exam tip: remember: dark colour—more absorption; light colour—more reflection.

Albedo is the fraction of incoming solar radiation that a surface reflects. A high-albedo surface, such as fresh snow, reflects more solar energy back into space and absorbs relatively less energy. Option B describes dark, low-albedo surfaces. Exam tip: higher albedo means higher reflection.

Albedo is the proportion of incoming solar radiation that a surface reflects. A low-albedo surface reflects less energy and absorbs more, so it may become relatively warmer. In contrast, high reflection is a feature of high albedo. Exam tip: dark surfaces and water are generally considered low-albedo surfaces.

Snow is light and bright so it reflects more radiation. Remember it as an example of high albedo.

Black surfaces have low albedo, so they absorb a relatively large part of the incoming insolation. This absorbed energy is converted into heat, making black soil or a black road warmer. In contrast, lighter surfaces reflect more energy and generally absorb less. Exam tip: low albedo means high absorption.

In morning and evening the Sun's altitude is low. Rays spread over a larger area and pass through more atmosphere.

Earth's rotation changes the Sun's position during the day. Therefore insolation differs in morning noon and evening.

Earth's axial tilt and revolution change the angle of rays and day duration. This changes annual insolation.

Because of axial tilt different hemispheres tilt toward the Sun at different times. This changes day length.

In summer the Northern Hemisphere is tilted toward the Sun. Therefore days are longer and rays are more direct.

Around December the Southern Hemisphere is tilted toward the Sun. So days are longer and insolation is higher there.

Perihelion is the point in Earth’s elliptical orbit at which Earth is nearest to the Sun. In contrast, at aphelion, Earth is farthest from the Sun. Seasons are mainly caused by Earth’s axial tilt, not by this change in distance. Exam tip: remember perihelion = near and aphelion = far.