What occurs when sunlight strikes Earth at a shallower angle

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The Sun provides the Earth with most of its energy. Today, about 71% of the sunlight that reaches the Earth is absorbed by its surface and atmosphere. Absorption of sunlight causes the molecules of the object or surface it strikes to vibrate faster, increasing its temperature. This energy is then re-radiated by the Earth as longwave, infrared radiation, also known as heat. The more sunlight a surface absorbs, the warmer it gets, and the more energy it re-radiates as heat. This re-radiated heat is then absorbed and re-radiated by greenhouse gases and clouds, and warm the atmosphere through the greenhouse effect.

Earth’s surfaces are better at absorbing solar radiation than air, especially surfaces that are dark in color. You can feel this on a cold winter day when the sunshine warms your face and the air around you remains cold. Your skin and your clothes also absorb solar radiation and convert it to heat. If you wear a black jacket, it will absorb more radiation and make you feel warmer than if you wear a white or light-colored jacket. Similarly, Earth’s different surfaces and parts of the atmosphere absorb solar radiation at different rates.

The Earth is unevenly heated because it is a sphere.

Because Earth is a sphere, not all part of the Earth receives the same amount of solar radiation.  More solar radiation is received and absorbed near the equator than at the poles. Near the equator, the Sun’s rays strike the Earth most directly, while at the poles the rays strike at a steep angle. This means that less solar radiation is absorbed per square cm (or inch) of surface area at higher latitudes than at lower latitudes, and that the tropics are warmer than the poles. This temperature difference shapes global atmospheric and ocean circulation patterns. Additionally, Earth’s tilt affects how much sunlight is received and absorbed by different parts of the Earth at various times of the year, and is why we experience the seasons. The amount of solar radiation received and absorbed also influences process in the biosphere by directly affecting plants and other organisms that photosynthesize and are the primary food source in most ecosystems (see species interactions).

If light is not absorbed by a surface, it is mostly reflected. Reflection occurs when incoming solar radiation bounces back from an object or surface that it strikes in the atmosphere, on land, or water, and is not transformed into heat. The proportion of incoming solar radiation that is reflected by the Earth is known as its albedo. Overall, Earth reflects about 29% of the incoming solar radiation, and therefore, we say the Earth’s average albedo is 0.29.

Snow and ice, airborne particles, and certain gases have high albedos and reflect different amounts of sunlight back into space. Low, thick clouds are reflective and can block sunlight from reaching the Earth’s surface, while high, thin clouds can contribute to the greenhouse effect.

The proportion of sunlight that’s reflected vs. absorbed, the re-radiation of heat, and the intensity of the greenhouse effect influence the amount of energy in the Earth system and global processes such as the water cycle and atmospheric and ocean circulation.

This diagram shows the percentage of sunlight that is reflected by different Earth surfaces or clouds.

Earth system models about the absorption and reflection of sunlight

This Earth system model is one way to represent the essential processes and interactions related to the absorption and reflection of sunlight. Hover over the icons for brief explanations; click on the icons to learn more about each topic. Download the Earth system models on this page.

This model shows some of the changes to Earth’s surface and atmosphere that can affect the amount of sunlight that is absorbed or reflected. These changes influence the amount of heat that is re-radiated, and can also greatly influence the biosphere by altering the amount of sunlight available for photosynthesis.

How human activities influence the absorption and reflection of sunlight

The Earth system model below includes some of the ways that human activities directly affect the amount of sunlight that is absorbed and reflected by Earth’s surface. The development and spread of urban areas, especially using asphalt and other dark colored materials, can dramatically increase the absorptivity of the surface. This creates urban heat islands, where cities experience higher temperatures than surrounding areas. Hover over or click on the icons to learn more about these human causes of change and how they influence the absorption and reflection of sunlight.

The Earth system model below includes additional ways that human activities directly affect the amount of sunlight that is absorbed and reflected by Earth’s atmosphere. Hover over or click on the icons to learn more about these human causes of change and how they influence the absorption and reflection of sunlight.

The Earth system model below shows how human pollutants and waste affect the ozone layer and the amount of ultraviolet sunlight that is absorbed by Earth’s upper atmosphere (the stratosphere). Hover over or click on the icons to learn more about these human causes of change and how they influence the absorption and reflection of sunlight.

Explore the Earth System

Click the icons and bolded terms (e.g. re-radiation of heat, airborne particles, etc.) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you.

To learn more about teaching the absorption and reflection of sunlight, visit the Teaching Resources page.

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From Citizendium

Figure 1
This is a diagram of the seasons. Regardless of the time of day (i.e. the Earth's rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also arctic winter. In addition to the density of incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle.

The angle at which sunlight strikes the earth, which varies by location, time of day, and season, is an important factor in the amount of energy received at any location on the globe. Seasonal change in the angle of sunlight, caused by the tilt of the earth's axis, is the basic mechanism that results in warmer weather in summer than in winter (see Figure 1). (See also season.)

Geometry of sun angle

Figure 2
One sunbeam one mile wide shines on the ground at a 90° angle, and another at a 30° angle. The one at a shallower angle covers twice as much area with the same amount of light.

When sunlight shines on the earth at a lower angle (sun closer to the horizon), the energy of the sunlight is spread over a larger area, and is therefore weaker than if the sun is higher overhead and the energy is concentrated on a smaller area. (See Figure 2.)

Figure 1 shows the angle of sunlight striking the earth in the Northern and Southern hemispheres when the earth's northern axis is tilted away from the sun, when it is winter in the north and summer in the south.

Figure 2 depicts a sunbeam one mile wide falling on the ground from directly overhead, and another hitting the ground at a 30° angle. Trigonometry tells us that the sine of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the sunbeam hitting the ground at a 30° angle spreads the same amount of light over twice as much area (if we imagine the sun shining from the south at noon, the north-south width doubles; the east-west width does not). Consequently, the amount of light falling on a given area is only half as much. The mathematical relationship between incidence angle and energy received per unit area is known as Lambert's cosine law.

See also

• Axial tilt
• Season
• Declination