Along with pressure tendency, the condition of the sky is one of the more important parameters used to forecast weather in mountainous areas. However, in twilight, the period of time between sunset and night and between night and sunrise, thesituation is more complicated. Within 24 hours of the closest approach to the center, low clouds begin to move in, also known as the bar of a tropical cyclone, as the barometric pressure begins to fall more rapidly and the winds begin to increase. "Visibility: Science and Regulation". Van Flandern,.;. See also: Atmospheric optics Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. A potent blend of high adventure and moral polemic, and yet further testimony to the ongoing tragedy of Shangri-La. Kerrs account ends with his 1991 return to Tibet, where he found conditions still bleak, and the Chinese occupation having a genocide effect on the Tibetans. Donald Clay Johnson, university of Minnesota Library. Portuguese, german, polish, chinese, russian, italian. Special filters and modifications to light fixtures can help to alleviate this problem, but for the best views, both professional and amateur optical astronomers seek viewing sites located far from major urban areas. The effect is not very obvious on clear days but is very pronounced when clouds cover the line of sight, reducing the blue hue from scattered sunlight.