Objectives: Students will:
Life on this planet is supported by energy from the Sun. The Earth's energy budget is a balance between energy inputs and outputs. Earth's climatic system has an important role in maintaining the balance between the energy that reaches the Earth from the Sun and the energy going back into space from the Earth. Radiation received from the Sun is absorbed and scattered by molecules of gases, liquids, and solids in the atmosphere, by clouds, by the Earth's surface. Radiation that is absorbed causes the planet to heat up. Earth retransmits as much energy as it absorbs from the Sun via reflection and emission. However, the heat emitted by Earth is in the form of long wave radiation, rather than in the shorter wavelengths that it received. The air will absorb some of the radiation emitted by Earth, some of the energy is radiated back to Earth, and some radiated into space. See figure titled Earth's Energy Budget from http://ess.geology.ufl.edu/HTMLpages/ESS/GLY1033_Notes/Radiation_Budget.gif .
The amount of solar energy that is reflected back to space is called the albedo. Earth's average albedo is .3, meaning about 30% of incoming solar energy is reflected back to space.
A potentially important effect in climate change is a variation in the solar irradiance reaching Earth. Scientists monitor solar variability and use models--mathematical representations--of the Earth system to understand how changes could affect climate. For example, an increase or decrease in global cloud cover could increase or decrease Earth's albedo, which would increase or decrease the amount of solar radiation reaching Earth.
Earth's energy balance is described by the following equation:
Oceans and forests reflect only small amounts of solar radiation, and thus have low albedos. Deserts and clouds have high albedoes because they reflect large portions of the Sun's energy. Clouds reflect large portions of the Sun's energy before it can reach and heat the Earth's surface, thus causing the Earth to be cooler. Clouds also trap some of the long wave radiation emitted by Earth and radiate it back to Earth's surface, causing the Earth to be warmer. Generally, low, thick clouds tend to reflect radiation--meaning less radiation reaches Earth's surface, resulting in cooler temperatures. High, thin clouds will reflect less radiation and allow more to reach the surface, warming Earth. Cloud surfaces can reflect up to 50 W/m2 of solar radiation.
The temperature, thickness, and types of particles making up a cloud will impact the cloud's albedo and its transmission of long wave radiation. The position of the continents, biological innovation (vegetation type and density), mountains, deforestation, and the rise and fall of sea levels also impact the albedo of Earth. If the albedo goes down, more energy is added to the system.
Albedo is influenced by dust and water in the atmosphere, and snow and ice. Albedo is also a function of the angle of incident. The larger the angle of incident the smaller the fraction of reflection.
Low-energy ultraviolet light -- sometimes called long-wave UV -- penetrates to Earth's surface. This low-energy ultraviolet light cause Day-Glo paints to give off spectacular colors and white clothing to glow brightly when washed in detergents that contain fluorescent dyes (advertised as making clothes "whiter than white"). Most of the high-energy ultraviolet light -- sometimes called short-wave UV -- is blocked by the ozone layer in Earth's upper atmosphere. High energy ultraviolet light causes skin tanning. Extended exposure can lead to eye damage and skin cancer in light-skinned people. Skin cancer is rare in dark-skinned people.
Although transparent to lower-energy ultraviolet light, glass blocks higher-energy ultraviolet light. Lotions that are advertised as Sun blockers also block higher energy ultraviolet light. When the glass is inserted between the lamp and the fluorescing material in this demonstration, the fluorescence diminishes or stops. Some materials fluoresce with lower energy ultraviolet waves as well as the higher energy waves, and any continued fluorescence is the result of lower energy waves.
Ultraviolet light tells astronomers several things. For example, the local neighborhood of our Sun -- within 50 light years -- contains many thousands of low-mass stars that glow in the ultraviolet. When low-mass stars use up all their fuel, they begin to cool. Over billions of years, the internal heat left over from stellar fusion reactions radiates into space. This leftover heat contains a great deal of energy. These stars, called white dwarfs, radiate mostly ultraviolet light. Until astronomers could make observations with ultraviolet telescopes in space, they had very little information about this phase of a star's evolution.
Visible light, passing through a prism at a suitable angle, is dispersed into its component colors. This happens because of refraction. When visible light waves cross an interface between two media of different densities (such as from air into glass) at an angle other than 90 degrees, the light waves are bent (refracted). Different wavelengths of visible light are bent different amounts and this causes them to be dispersed into a continuum of colors. (See diagram)
Diffraction gratings also disperse light. There are two main kinds of gratings. One transmits light directly. The other is a mirror-like reflection grating. In either case, diffraction gratings have thousands of tiny lines cut into their surfaces. In both kinds of gratings, the visible colors are created by constructive and deconstructive interference. Additional information on how diffraction gratings work is found in the Analytical Spectroscope activity and in many physics and physical science textbooks.