Temperature inversion in the atmosphere tends to concentrate and trap particles in the air, causing haze. A common type of temperature inversion may be characterized by smoke that rises to a certain height,

Figure 4-11.-Characteristic of temperature inversion.

then flattens out and goes no higher (fig 4-11). This condition makes it difficult to photograph the ground. In these circumstances, you can use film with extended red sensitivity or a red filter to help cut through the haze. There is nothing you can use to cut through the smoke.

Thermal shimmer is another condition with which you should be concerned. Thermal shimmer is the result of heated air moving upward. When thermal shimmer is occurring, distant objects appear to shimmer because the light rays are being refracted by the rising heated air. This occurrence can obscure minute detail in high-altitude photography. When thermal shimmer exists, you should take enough photographs to ensure that subject detail occurs in at least one photograph.

Good weather conditions for aerial photography are generally considered to be clear with 5 to 10 knots of wind (to help blow off smoke and smog) and visibility of 7 miles or more. Minimum conditions are generally considered to be scattered clouds with visibility of 6 miles. When weather conditions are poor, shoot low-oblique photographs from relatively low altitudes. This helps to minimize the effects of smoke and haze.

AIRCRAFT

After you have gained some experience in taking hand-held aerial pictures, you will probably have a preference for a particular type or model of aircraft. Hand-held aerial photographs can be made from almost any aircraft. However, there are some generally accepted preferences, such as high-wing aircraft or relatively slow-flying aircraft. Most slow-flying aircraft have a window, door, or hatch that can be removed or opened to simplify unobstructed photography. A high-wing plane is preferable to a low-wing plane because the wing is out of the field of view of the camera.

Helicopters provide photographers more freedom to work at lower altitudes, but there are certain misconceptions about the fig. 4-12. One of these misconceptions is that the platform can be suspended

Figure 4-12.-Helicopter used in an aerial photographic assignment.

anywhere in a motionless state. Most helicopter pilots prefer to maintain some forward speed-at least enough to make a safe landing in the event of engine failure. An added advantage of maintaining some forward speed is it reduces vibration. While the helicopter is hovering, it vibrates excessively and the engine exhaust oftentimes washes in front of you and the camera. Both of these factors contribute to unsharp photographs. There are two distinct advantages of using helicopters for hand-held aerial photography. First, they have the ability to maneuver in and out of places that are inaccessible to fixed-wing aircraft. Second, they have the ability to fly at low altitudes more safely than fixed-wing aircraft.

If you have a choice of aerial photographic platforms, consider an aircraft that can fly with a window or door removed or open. By removing or opening a window or door, you eliminate the need to shoot through a glass or plastic window. This prevents reflection problems, such as glare, that can result in obscured image detail. Of course, you are unable to open a "window" in a high-performance jet aircraft. Navy helicopters have doors that can be completely removed or opened while in flight. With the door opened or removed from a helicopter, you have fewer obstructions to obscure the view of the camera than in a fixed-wing aircraft. You can generally sit with your legs hanging out of a helicopter fig. 4-13 . This is both convenient and comfortable for shooting your aerial pictures. Additionally, sitting on the floor with your legs out of the helicopter gives you a stable body position. It also enables you to lean out and see the target as you approach it.