Clouds

Clouds play a critical role for effective small-format aerial photography. The nature and optical properties of clouds vary enormously from high, thin cirrus clouds of ice crystals to dense ground fog. In addition to ice and/or water droplets, clouds may consist of dust, smoke, and other minute debris in the atmosphere, which are derived from both natural and human sources. The particles of clouds range in size from <1 mm diameter to a maximum of ~ 100 mm. Clouds are intrinsically white, as they absorb practically no visible light and all wavelengths are scattered equally (Lynch and Livingston, 1995). The impact of clouds for SFAP depends on their altitude relative to the camera. Clouds positioned above the camera mainly affect the amount and quality of incoming radiation and how it illuminates the ground; whereas, clouds below the camera influence both incoming radiation and light reflected upward from ground.

Beginning with clouds above the camera level, cloud cover has two main impacts on the visual appearance of aerial photographs, namely reduction of shadows and poor color definition, which result overall in low-contrast images (Fig. 4-23). As clouds become thicker and less transparent, the ground beneath becomes increasingly dark and uniformly illuminated, such that shadows of individual objects disappear. Shadows are visual clues to

FIGURE 4-23 Effect of cloud cover on image quality for a Scandinavian coastal scene. (A) High-oblique view under mostly sunny conditions. Colors are well defined and the image has good brightness contrast. (B) Similar view taken a few minutes later under cloud cover. Colors are dull and image has low contrast. Kite aerial photographs by SWA and JSA; Fegge, island of Mors, Limfjord district of northwestern Denmark, September 2005.

FIGURE 4-23 Effect of cloud cover on image quality for a Scandinavian coastal scene. (A) High-oblique view under mostly sunny conditions. Colors are well defined and the image has good brightness contrast. (B) Similar view taken a few minutes later under cloud cover. Colors are dull and image has low contrast. Kite aerial photographs by SWA and JSA; Fegge, island of Mors, Limfjord district of northwestern Denmark, September 2005.

relief and texture of features in the photograph, and without any shadows these objects may be difficult to identify. This affects vertical aerial photographs in particular, which often have a "flat" appearance without shadows.

The color in shadowed zones below clouds is rich in blue light and poor in green and red, which gives rise to the dull blue-gray color palette of cloudy weather. Under heavy cloud cover, color largely disappears from the landscape. Nonetheless, even in the case of moderate cloud cover good SFAP can be conducted in many situations, if the ground is uniformly illuminated. In the case of wispy cirrus clouds, however, the visual impact for most SFAP is minimal. High, thin clouds slightly reduce solar brilliance without significantly degrading shadows or color. This could be advantageous, in fact, for scenes that have high intrinsic brightness contrasts for ground objects, for example dark green conifer forest next to white beach gravel. Thus, altitude and

FIGURE 4-24 Dark cloud shadow obscures the center of this high-oblique view of wind turbines standing 90 m tall. Kite aerial photograph by SWA and JSA; Gray County, southwestern Kansas, United States, April 2006.

FIGURE 4-25 The foreground is well illuminated, and the background is shadowed by clouds. This visual contrast directs the viewer's attention to the main subject—the recreational development adjacent to the lake. Helium blimp aerial photograph by A. Uttinger and JSA; Lake Wabaunsee, Kansas, United States, June 2006.

FIGURE 4-25 The foreground is well illuminated, and the background is shadowed by clouds. This visual contrast directs the viewer's attention to the main subject—the recreational development adjacent to the lake. Helium blimp aerial photograph by A. Uttinger and JSA; Lake Wabaunsee, Kansas, United States, June 2006.

FIGURE 4-26 High-oblique view over the Gulf of Finland with patchy cloud cover. Note dark cloud shadows on water surface as well as bright zones from cloud reflections. This effect is noticeable over homogeneous surfaces, such as water, but is not recognized so often on land. Kite aerial photograph by SWA and JSA; island of Vormsi, Estonia, August 2000.

FIGURE 4-26 High-oblique view over the Gulf of Finland with patchy cloud cover. Note dark cloud shadows on water surface as well as bright zones from cloud reflections. This effect is noticeable over homogeneous surfaces, such as water, but is not recognized so often on land. Kite aerial photograph by SWA and JSA; island of Vormsi, Estonia, August 2000.

FIGURE 4-27 Poor visibility is typical of the eastern United States during the summer. (A) Nearby tow plane is clearly defined, but the ground below is depicted poorly through the haze, and the background is hardly visible. Hand-held photograph taken by JSA from a manned glider; Elmira, New York, July 2005. (B) Afternoon ground fog moves in from the ocean in this hazy view over Martha's Vineyard, Massachusetts. Kite aerial photograph by SWA, and JSA, July 2005. (C) Fog drifts in from the Atlantic Ocean over the Laudholm beach and the mouth of Little River; Wells, Maine. Helium blimp aerial photograph by JSA, SWA and V. Valentine, August 2009.

FIGURE 4-27 Poor visibility is typical of the eastern United States during the summer. (A) Nearby tow plane is clearly defined, but the ground below is depicted poorly through the haze, and the background is hardly visible. Hand-held photograph taken by JSA from a manned glider; Elmira, New York, July 2005. (B) Afternoon ground fog moves in from the ocean in this hazy view over Martha's Vineyard, Massachusetts. Kite aerial photograph by SWA, and JSA, July 2005. (C) Fog drifts in from the Atlantic Ocean over the Laudholm beach and the mouth of Little River; Wells, Maine. Helium blimp aerial photograph by JSA, SWA and V. Valentine, August 2009.

thickness of clouds render considerable variation in colors displayed at the Earth's surface.

Discontinuous or patchy cloud cover results in a mosaic of illuminated and shadowed zones on the ground. This is usually the worst possible lighting situation for effective SFAP, as the bright areas may be overexposed and the dark spots are underexposed with little of the image in the mid-range of brightness (Fig. 4-24). Still, patchy cloud cover may be utilized successfully in some instances to draw attention to illuminated foreground and central portions of the scene, which are of particular interest, while leaving the periphery and background in cloud shadows (Fig. 4-25). In addition to casting shadows, clouds also may reflect white light to brighten the ground in places. This effect is visible on homogeneous surfaces, such as lakes and seas (Fig. 4-26).

The examples of clouds discussed thus far are positioned above the camera. Clouds located below the camera—between the camera and the ground—may have all the same effects as higher clouds by limiting incoming solar radiation. In addition, low clouds scatter light reflected upward from the ground, thus further degrading image quality. Although such conditions should be avoided normally for SFAP, it may be necessary to cope with low clouds in some situations. For example, summer in the eastern United States and southeastern Canada is a time of hazy sky due to a combination of high humidity and industrial pollution. Thus, poor visibility is frequently the case, even on ''clear'' days (Fig. 4-27).

Smoke and smog are widespread around the world. Natural fires are started by lightning strikes, spontaneous combustion, and volcanic eruptions. However, most fires

FIGURE 4-28 Regional smoke from widespread burning of agricultural waste and crop stubble creates a hazy appearance in this high-oblique view over a bog landscape. Kite aerial photograph by SWA and JSA; Mannik-jarve Bog, Estonia, October 2000.

FIGURE 4-29 Controlled prairie burning is a spring ritual to maintain tallgrass habitat in the Flint Hills of eastern Kansas. (A) Smoke billows up from a wall of fire. Note shadow from smoke cloud. (B) Smoke surrounds the camera rig. In spite of the smoke, a diffuse hot spot is visible in upper center of picture (*). Kite aerial photographs by M. Lewicki, SWA, and JSA; Lyon County, Kansas, United States, April 2000.

FIGURE 4-29 Controlled prairie burning is a spring ritual to maintain tallgrass habitat in the Flint Hills of eastern Kansas. (A) Smoke billows up from a wall of fire. Note shadow from smoke cloud. (B) Smoke surrounds the camera rig. In spite of the smoke, a diffuse hot spot is visible in upper center of picture (*). Kite aerial photographs by M. Lewicki, SWA, and JSA; Lyon County, Kansas, United States, April 2000.

and smoke nowadays are related to human activities— clearing forests, prairie fires, burning agricultural waste, oil and gas fires, burning fossil fuels for transportation and industry, etc. Far from being an isolated problem, smoke is a common seasonal or perennial condition that may affect small-format aerial photography. In some cases, smoke spreads regionally from many small fires, for example from burning agricultural waste and crop stubble at the end of the growing season (Fig. 4-28) or spring burning of the tallgrass prairie to maintain grassland habitat (Fig. 4-29). In other situations, smoke has a significant point source, such as a coal-fired electric power plant or cement factory.

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