APAD: Red sky at night

Red sky at night

Red sky at night, sailor's delight" is a weather proverb that, with a scientific basis, suggests that a red sunset  typically brings fair weather. The red color is because dust particles in a high-pressure, dry atmosphere scatter the sunlight, allowing the longer, reddish wavelengths to be seen more prominently by the time the sun's light reaches your eyes in the west.  

The proverb is most reliable in regions with prevailing westerly winds, where weather systems tend to move from west to east. 
When the sun sets in the west, its light travels through a longer path of the atmosphere. 

A red sky at night means the atmosphere to the west (where the sun is setting) contains dust and dry air, which are characteristic of a high-pressure system. 
This high-pressure system is moving into the area from the west, indicating that the low-pressure storm systems to the east are retreating, leading to stable, clearer skies ahead. 

A red sky in the morning, on the other hand, means high pressure has passed, and the approaching weather systems from the west (where the sun rises) are bringing moisture and storm clouds. Thus, "red sky in the morning, sailor's warning" suggests an approaching storm.