In recent decades studies all around the world have shown an increasing trend of visibility degradation. In Hong Kong, the visibility has shown a deteriorating trend from 1968 to 2003 (Leung et. al 2004). The level of visibility has been proven to be related to the level of air pollutants causing air pollution. Therefore, visibility degradation is a sign of deteriorating air quality and a potential threat to human health. In addition, visibility degradation will blur the vista of the skyline that tourists are looking for in their trips. The degraded visibility will harm the tourists’ enjoyment at popular tourist sites such as the Peak, Hong Kong. This study aims at finding the standard of visibility violation of the lay public and the tourists in Hong Kong and their perception on visibility degradation. The study designs a questionnaire based on a judgement study on photograph of perception and justification of visibility standards by Pryor (1995) and the conceptual model of Sell et al. (1988) to unveil the reason behind the human standard on visibility degradation and the relationship between people’s background and their perception. The study suggests the standard of visibility violation is 4.5km in the Peak, Hong Kong and the standard varies from place-to-place while it is very much based on the environmental setting of the vista. A modified model based on the conceptual model by Sell et. al (1988) is introduced to explain the relationship between environmental change, individual, perception and their responses. It is deduced that human perception on visibility degradation will start when the environmental degradation exceeds the acceptance level of people. The finding proved that the standard of visibility violation could be determined. The standard would change according to a place-to-place format and poor visibility was interpreted according to the aesthetic characters and the aesthetic integrity of the vista. The demographic background of different individuals only determined the perception on visibility degradation but not the standard of visibility degradation. Meanwhile, according to the findings, a standard of low visibility based on either health concern or aesthetic concern and an early warning system of low visibility were proposed. / published_or_final_version / Geography / Doctoral / Doctor of Philosophy