Why do high altitude areas have brighter sunlight and lower temperatures as compared to the plains?

High altitude areas have brighter sunlight and lower temperatures because of thinner atmosphere and absence of dust particle.
Brighter sunlight: There is little diffraction of incoming sunlight so that the latter is brighter.
Lower temperature: Atmosphere is unable to block the outgoing infra red rays. There is little retention of heat. The light appears to be brighter. Due to low atmospheric pressure the temperature also remains low.