In the early hours.
Image by Neil. Moralee
A sodium light consists of an evacuated tube containing electrodes and metallic sodium. … In the vapor state, the sodium electrons are excited to higher energy levels When these electrons return to a lower energy level, light is given off. The yellow colour is a product of the light emitted by the sodium electrons.
The yellow colour of low-pressure sodium lamps also leads to the least visual sky glow, due primarily to the Purkinje shift of dark-adapted human vision, causing the eye to be relatively insensitive to the yellow light scattered at low luminance levels in the clear atmosphere. One consequence of widespread public lighting is that on cloudy nights, cities with enough lighting are illuminated by light reflected off the clouds. Where sodium vapour lights are the source of urban illumination, the night sky is tinged with yellowy orange.
