2.8: Variations in Radiation

Radiation Variations Across Earth

The Earth maintains a radiative balance between the energy it receives from the Sun, the energy in its atmosphere, and the energy it radiates into space. But that does not mean that different regions of the Earth receive the same amount of energy every day. The energy received by various parts of the Earth depends on the season and latitude.

Seasonal Changes

The Earth revolves around the sun in an elliptical orbit in 365.25 days, or one year. Since the orbit is elliptical, the actual distance between the Earth and the sun varies throughout the year. It is a common misconception that this is the reason for our seasons. However, that is not true. The Earth is closest to the sun in January (147 million km), while it is furthest from the sun in July (152 million km). This would suggest that summer occurs in January and winter in July, but that's not how we experience our seasons in the Northern Hemisphere.

The Earth's tilt is responsible for our seasons. If the Earth did not have a tilt, we would not have seasonal changes. The Earth is tilted by 23.5° on its axis. Earth's tilt with respect to space does not change throughout the year; thus, the Northern Hemisphere is tilted toward the sun in June and away from it in December, as illustrated in Figure \(\PageIndex{1}\). The tilt does not significantly change the distance of each hemisphere from the sun. But it does change the angle at which sunlight strikes the ground. This is called the angle of insolation.

Why should this angle of insolation affect our seasons? To understand this, take a piece of paper and a flashlight. Shine the flashlight directly onto the paper so you see a circle illuminated. All the light from the flashlight is in that circle. Now, slowly tilt the paper so that the circle elongates into an ellipse. All the light is still in that ellipse, but the ellipse is spread out over more paper. The density of light drops. In other words, the amount of light per square centimeter drops (the number of square centimeters increases, while the total amount of light stays the same). The same is true on the earth. When the sun is overhead, the light is falling straight on you, and so more light (and more heat) hits each square centimeter of the ground. When the sun is lower in the sky, the light gets more spread out over the surface of the earth, and less heat (per square centimeter) can be absorbed. Since the Earth's axis is tilted, the sun is higher when you are on the part of the Earth where the axis points more towards the sun, and lower on the part of the Earth where the axis points away from the sun. The angle of the Sun’s rays and the length of the day change because of the Earth’s tilt.

Figure \(\PageIndex{1}\): Due to the Earth's tilt, the northern hemisphere is tilted towards the Sun in June, and tilted away from it in December. The northern hemisphere experiences summer when it is tilted towards the Sun and winter when it is tilted away. (Public Domain; The Seasons via NWS).

During the summer, each hemisphere faces the sun. For example, during Northern Hemisphere summer in June, the sun shines more directly on the Northern Hemisphere than on the Southern Hemisphere. On June 20-22, the sun is directly above the Tropic of Cancer, at 23.5° north latitude. This day, known as the Summer Solstice, marks the longest day of the year in the Northern Hemisphere and the shortest in the Southern Hemisphere. While it seems like a small change, the angle of insolation and the amount of solar radiation absorbed (per area) vary significantly throughout the year. This is because, in addition to the Sun's angle, the Earth's position changes the length of the day throughout the year. On September 22-23 and on March 20-21, the sun is directly above the equator. These days are called equinoxes (equal days), since both hemispheres enjoy nearly 12 hours of daylight and 12 hours of night. March 20-21 is called the Vernal (Spring) Equinox, while September 22-23 is called the Autumnal (Fall) Equinox. On December 21-22, the sun is directly above the Tropic of Capricorn, at 23.5° south latitude. This day, known as the Winter Solstice, is the shortest day of the year in the Northern Hemisphere but the longest in the Southern Hemisphere. Figure \(\PageIndex{2}\) depicts the summer season in the Southern Hemisphere, where there is a higher density of incident rays due to the higher Sun angle. The Northern Hemisphere is experiencing wintertime.

Figure \(\PageIndex{2}\): Distribution of solar rays on Earth during the Northern Hemisphere winter. The Southern Hemisphere, experiencing summer, receives high-intensity incident solar radiation, while the Northern Hemisphere, experiencing winter, receives low-intensity solar radiation. (Public domain; Rhcastilhos via Wikimedia Commons).

Radiation vs Latitude

In the “summer hemisphere,” the combination of more direct sunlight and longer days means the pole can receive more incoming sunlight than the tropics, but in the winter hemisphere, it gets none. Even though illumination increases at the poles in summer, bright white snow and sea ice reflect a significant portion of incoming light, reducing the potential for solar heating. Differences in albedo and solar illumination at different latitudes lead to net heating imbalances across the Earth system. At any place on Earth, the net heating is the difference between the amount of incoming sunlight and the amount of heat radiated by the Earth back to space. Figure \(\PageIndex{3}\) depicts the relationship between the average annual incoming solar and outgoing terrestrial radiation with latitude. The blue line indicates the net shortwave radiation received by the Earth. In contrast, the red line indicates the net longwave radiation emitted annually by the Earth and the atmosphere. In the tropics, there is a net energy surplus because the amount of sunlight absorbed exceeds the amount of heat radiated. In the polar regions, however, there is an annual energy deficit because the amount of heat radiated to space exceeds the amount absorbed from sunlight.

Figure \(\PageIndex{3}\): Latitudinal variation in Earth's radiation balance. The tropics receive surplus heat energy, while the polar regions have an energy deficit. The surplus heat energy is transferred from the tropics to higher latitudes by the atmosphere and the oceans. (CC BY-NC 4.0; Dr. Michael Pidwirny & Scott Jones via PhysicaGeography.net)

Figure 2.8.3 is a graph showing Earth’s energy balance as a function of latitude. The horizontal axis represents latitude from 90° north to 90° south, and the vertical axis shows energy in watts per square meter. Two curves are plotted: net incoming solar radiation and net outgoing longwave radiation. In low latitudes near the equator, incoming solar radiation exceeds outgoing radiation, creating an energy surplus. In high latitudes toward both poles, outgoing radiation exceeds incoming solar radiation, creating an energy deficit. The diagram highlights that excess heat from low latitudes is transported toward higher latitudes by the atmosphere and oceans.

The net heating imbalance between the equator and poles drives an atmospheric and oceanic circulation that climate scientists describe as a “heat engine.” In everyday use, we associate the word engine with automobiles, but for a scientist, an engine is any device or system that converts energy into motion. The climate is an engine that uses heat energy to sustain atmospheric and oceanic motion. Evaporation, convection, rainfall, winds, and ocean currents are all part of the Earth’s heat engine.