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Similarly, scientists who study human vision have determined that weather transitions are sometimes too subtle for the limits of the visual system.  Like other sensory organs, the eye responds best to changes.  It adapts to circumstances that do not change, or those that change in a gradual or subtle way, by reducing its response.  Just as the skin becomes so acclimated to the “feel” of clothing that it is generally not even noticed, the eye can become so accustomed to progressive small changes in light, color, and motion that it no longer “sees” an accurate picture. In deteriorating weather conditions, the reduction in visibility and contrast occurs quite gradually, and it may be quite some time before the pilot senses that the weather conditions have deteriorated significantly.  In essence, you have to learn how to look past the visual illusion and see what is really there.
Certain weather conditions also make it particularly difficult to accurately perceive with the eye. For instance, a phenomenon called “flat light” can create very hazardous operating circumstances.  Flat light is a condition in which all available light is highly diffused, and information normally available from directional light sources is lost. The result is that there are no shadows, which means that the eye can no longer judge distance, depth features, or textures on the surface with any precision.  Flat light is especially dangerous because it can occur with high reported visibility. It is common
in areas below an overcast, and on reflective surfaces such as snow or water.  It can also occur when blowing snow or sand create flat light conditions accompanied by “white-out,” which is reduced visibility in all directions due to small particles of snow, ice or sand that diffuse the light.  

Awareness is important in overcoming these challenges, but you can also develop your visual interpretation skills.  Appendix 8 provides tips and techniques you can use to estimate in-flight visibility and cloud clearance, thus enhancing your ability to evaluate in-flight weather conditions accurately.
ATIS/ASOS/AWOS. In-flight weather information obtained from ATIS and ASOS/AWOS broadcasts can contribute useful pieces to the en route weather picture, but it is important to understand that this information is only a weather “snapshot” of a limited area. ATIS and ASOS/AWOS broadcasts are primarily intended to provide information on conditions in the airport vicinity. The information reported is derived from an array of sensors. While these systems are designed to be as accurate as possible and are increasingly sophisticated, the automated system is actually monitoring only a very small area on the airfield and that it reports only what it can "see." For example, sensors that measure visibility are actually measuring a section of air less than 24 inches wide. Even a dense fog on a portion of the airfield will go undetected by the system unless the fog actually obscures the sensors.  The system will not “see” an approaching thunderstorm until it is almost directly over the automated site’s ceiling instruments.
　
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