Ground illumination. On overcast or cloudy days the ground is uniformly illuminated in "gray" light--strongly scattered light that is rich in blue and deficient in red and green components. The lower level of illumination means either slower shutter speed, larger aperture (smaller f-stop), or faster film will be needed for proper exposure, any of which may result in degradation of photo quality. Furthermore such scenes have a black-and-white character in which good color definition is lacking.

Shadows. Shadows are an important element in aerial photography. Shadows create a 3-dimensional effect that helps for identification of ground features. Sharply defined shadows aid in recognition of objects that may appear unfamilar when viewed from above, and shadows may indicate the heights or slopes of features. An aerial photograph without shadows appears "flat" and lacking in definition. Shadows may be enhanced by taking photographs with a low sun position--early morning, late afternoon, or winter.

Lighting conditions include many factors beyond the presense or absence of clouds. The direction of photography in relation to the sun is another important variable to take into account. Photographs in oblique views are taken normally in directions away from the sun (or at right angles to the sun). This provides for good illumination and some shadowing. An exception to this general rule is the opposition effect, where the camera is in direct alignment with the sun and ground area being photographed. In most cases, oblique photographs taken toward the sun result in high brightness contrast but poor color, because of specular (mirror) reflections and strong shadows. Sun glint may create dramatic effects over water bodies.

Temperature and Exposure

At low temperature, wind chill is an important factor for KAP flyers. For example, at 45°F (7°C) with a wind of 25 mph (40 km/h), the wind chill is 22½°F (-5°C); the same wind at 25°F (-4°C) feels like -7°F (-22°C). Note: wind chill applies only to people and warm-blooded animals; it has no effect on cameras and other mechanical devices. For cold weather KAPing, wear well-insulated clothing with wind-breaker outerwear. Wind is a relief on hot days, as it helps cool the body by evaporation of sweat. Drink plenty of fluid to prevent rapid dehydration. Bright sun means strong ultraviolet (UV) radiation at any time of year, especially at high altitude or low latitude, near water bodies, or with snow cover. Strong sun screen and good sunglasses are most important.

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