Background Information
The Sun is the primary source of energy for the Earth’s climate system. Click Sun to see images of the Sun taken today by the National Aeronautics and Space Administration. The Sun is huge and hot: it has a diameter of approximately 1,400,000 km, and it has an average surface temperature of approximately 5,500° C. For comparison purposes, the Earth has a diameter of approximately 12,700 km and an average surface temperature of 14° C. Therefore, the Sun’s diameter is 109 times larger than Earth’s diameter and the Sun is nearly 400 times hotter than Earth. The Sun’s extremely high temperature means that it emits an enormous amount of radiation: the luminosity (the total amount of energy emitted) of the Sun is 3.846 x 1028 W. The distance from the Sun to the Earth is approximately 150 million kilometers; at this distance, Earth intercepts a tiny amount of the radiation emitted by the Sun. Just above the Earth’s atmosphere, the average amount of solar energy intercepted by a surface perpendicular to the sunlight is approximately 1,367 W m-2. This value is known as the solar constant. Instruments on-board NASA satellites have been measuring the solar constant since 1978; click on the left-hand image below to see an example of such a satellite. Next to the satellite image is a graph showing changes in the solar constant. You should notice at the value for the solar constant changes from year to year, but you should also recognize a long-term pattern in the values which repeats approximately every eleven years; this is notice the solar cycle (this is an important cycle because peaks in the cycle can affect communication instruments). The third image below is a graph of the solar spectrum, which shows a breakdown of the radiation making up the solar constant. Most of the solar radiation is in the visible, near-infrared, and ultraviolet portions of the electromagnetic spectrum; a representation of the electromagnetic spectrum is the fourth picture in the set below.
For the Earth as a whole, only about half the solar radiation present at the top of the atmosphere reaches Earth’s surface. The solar radiation must pass through Earth’s atmosphere where it is absorbed by gases and particles and reflected back into space by clouds and particles. For example, the next lab focuses on the absorption of solar radiation by a gas, ozone, in a layer of the atmosphere known as the stratosphere. The left-side portion of the image below shows the absorption and reflection of solar radiation by the atmosphere and Earth’s surface. Note that the incoming solar radiation is 341.3 W m-2, rather than the solar constant value of 1365.2 W m-2. The solar constant is averaged across the entire surface of Earth; therefore, the value is divided by four. The 341.3 W m-2 value is the total solar radiation reaching the top of the atmosphere per second (174,226,942,644,300,000 W) divided by the surface area of Earth (510,000,000,000,000 m2).
At the present time, Earth’s axis of rotation is tilted 23.5 °away from the perpendicular to its orbital plane. This is what causes changes in solar radiation received by locations on Earth over the course of a year. Examine the image below to visualize this tilt. Therefore, the Earth’s tilt is the cause of the seasons. It takes one day for Earth to complete a full rotation, and the Earth orbits around the Sun, which is typically 150 million kilometers away, once every 365.2564 solar days (i.e. one year). The axis remains tilted in the same direction towards the stars (particularly the North star Polaris) throughout a year. The tilt causes day-today changes in the duration and intensity of solar radiation at all latitudes.
Play the video below which shows the Earth revolving around the Sun and how the tilt of the Earth determines what latitudes receive the most solar radiation.
The seasons pertain to the Northern Hemisphere. Watch the animation several times and notice how the Earth’s axis is always pointed the same direction (i.e. it is always pointed towards the North Star). The distance from the Earth to the Sun is not the cause of the seasons: Earth has a nearly circular orbit with perihelion (i.e. Earth closest to the Sun) occurring on January 5 and aphelion (i.e. Earth farthest from the Sun) occurring on July 5.
When solar radiation intercepts a portion of Earth at a lower angle (i.e. the Sun is closer to the horizon), the energy is spread over a larger area, and is thus weaker than if the Sun were higher overhead with solar energy concentrated on a smaller area. Examine image below help visualize the effect of this ‘beam spreading’. A location with the Sun directly overhead (i.e. an altitude or angle of incidence of 90°) receives 40% more solar radiation per square meter (e.g., W m-2) than will a location with the Sun at an altitude of 45°. The lower Sun angle (45°) causes the solar radiation to be received over a much larger surface area, which decreases the total amount of solar radiation in W m-2.
Below is a graph showing the change in the intensity of solar radiation with a change in angle of incidence. Notice how solar radiation is 40% more intense (i.e. 100 units compared to 70 units) when the Sun is directly overhead as opposed to being at an altitude of 45°.
Another factor to consider is daylight hours; more daylight hours leads to more solar radiation. The intensity and duration of sunlight is greatest during the summer and least during the winter.
