Insolation (sunlight) reaching the Earth's surface unmodified by any of the atmospheric processes just discussed is termed direct solar radiation. Solar radiation that reaches the Earth's surface after being scattered is called diffuse solar radiation. Not all the direct and diffuse radiation incident on Earth's surface is absorbed. As in the atmosphere, some of the sunlight received at Earth's surface is redirected back into the atmosphere or into space by reflection. The reflectivity of the Earth's surface varies considerably. Quantitative measurements of a surface's reflectivity are called albedo. Table 5.3 lists the albedos of some common surface types on our planet.
Visually, we can estimate an object's surface albedo from its tone or color. This method suggests that albedo increases as an object gets lighter in shade. The data in Table 5.3 verifies this idea. Light-toned surfaces like snow do have high albedos. Low albedos are associated with surfaces that appear dark-colored to our eyes. Some dark-colored surfaces include blacktop roads, coniferous forests, and dark soil. Table 5.3 also shows that water's albedo varies with the Sun's angle. When the Sun's angle is high, water tends to absorb more than 95% of the incident insolation. At low Sun angles, the water's surface becomes much more reflective.
The Earth's surface albedo varies by geographic region and time of year. Most of the Earth's surface is either covered by water, vegetation, bare soil, or rock. Generally, these surfaces have albedos between 5 to 45%. Over time, the albedo of vegetation can vary significantly. Plants can shed their dark-colored leaves during the cold, dry seasons. This process often exposes a surface with a different albedo than bare ground, low-growing plants, and decaying leaves cover it. The color of plants also changes with moisture conditions. Drought often causes plant leaves to become dried out and lighter in color. Subsequent rainfall after a drought can reduce the albedo of vegetated surfaces due to new leaf growth. In the middle and high latitudes, the albedo of ground surfaces can be significantly altered by temperature changes and snowfall. When temperatures drop below freezing, precipitation falls as highly reflective snow (Figure 5.22).
Satellite Measurements of Albedo
Global measurements of Earth's surface albedo can be best obtained with the aid of sensors aboard orbiting satellites. NASA's Earth Radiation Budget Experiment (ERBE) was one of the first attempts to make such measurements. This experiment used a variety of satellite sensors aboard Nimbus-7, NOAA-9, and the Earth Radiation Budget Satellite (ERBS) to monitor the Earth's albedo for about four years. Figure 5.23 shows the monthly average surface albedo of the Earth for January and July 1987. In this figure, most of the atmosphere's reflective properties have been removed. The patterns seen here are probably representative of most other years. For both January and July, the lowest surface albedos occur over oceans in a zone that covers more than 100 degrees of latitude. Albedo values of this zone are between 8 and 13%, and the center of this zone shifts seasonally. In July, the low albedo zone is located approximately at the Tropic of Cancer (23.5°N), while in January, it migrates to the Tropic of Capricorn (23.5°S). At the higher latitudes, the albedo of the ocean surface increases significantly because of low Sun angles or the presence of sea ice. In the July image, the Arctic Ocean region has an albedo of 45 to 60%. Vegetated areas have an albedo of 15 to 25% on Earth's terrestrial surface. Non-vegetated regions like the Sahara Desert reflect about 30 to 40% of the Sun's incoming light. Other land surfaces with high albedos are glaciers and seasonal snowfields. The large glaciers covering Greenland and Antarctica reflect as much as 75% of the insolation falling on their surfaces. Comparing the January and July images, we can see that the albedo of areas north of 45°N varies annually due to seasonal snowfall. In these areas, summer albedos typically are around 20%, while winter values jump to as high as 70%.
Figure 5.23 shows the surface reflectivity of the Earth for January and July 1987. Comparing this figure to Figure 5.24, which measures combined surface and atmospheric reflectivity (or planetary albedo), illustrates the contribution that clouds make to reflecting incoming sunlight back to space. Significant reflective cloud bands exist at the equator and in the mid-latitudes. Skies are generally clear of clouds over the major deserts, subtropical oceans, and the large continental glaciers of Greenland and Antarctica.
FIGURE 5.22 Image of highly reflective snow. A winter snowfall has quickly changed the albedo of these trees from low (dark green colored) to high (white colored). Image Copyright: Michael Pidwirny.
FIGURE 5.23 Surface reflectivity of the Earth for January and July 1987 as measured by sensors aboard various satellites for NASA’s Earth Radiation Budget Experiment (ERBE). Cells with missing data are colored white. Image Source: NASA - Earth Radiation Budget Experiment.
FIGURE 5.24 Combined surface and atmosphere reflectivity (or planetary albedo) of the Earth for January and July 1987 as measured by sensors aboard various satellites for NASA’s Earth Radiation Budget Experiment (ERBE). Cells with missing data are colored white. Image Source: NASA - Earth Radiation Budget Experiment.