“Red sky at night, sailor’s delight. Red sky in morning, sailor’s warning.”
Scientifically speaking, a region of low pressure causes air to move in an upward motion and it will have higher pressure on either side. Low pressure is less dense, less dusty, and sunsets/sunrise will be yellowish. Low pressure is also associated with producing clouds, rain, and bad weather– sailor’s take warning! High pressure is typically cloud free but more dense, dirty, and red at sunset or sunrise.
In the mid-latitude ocean, where this saying comes from, storm paths go from west to east. If the sunrise is red (in the east) there is high pressure there and the high is moving eastward–away. A low pressure, and very likely an associated storm, will be coming in right behind it. Sailor take warning! In reverse, a red sunset, if you are in the westerlies, means that an area of high pressure is to your west, and moving toward you. Good weather is on the way!
Note that this only works from about 30 degrees to 60 degrees latitude in the belt of westerlies (trade winds), in both the northern and southern hemispheres. The region 30 degrees on either side of the equator is characterized by easterlies and the saying would be the opposite.
Adapted from “Red sky at night, sailor’s delight. Red sky in morning, sailor’s warning” by John Augustine and Lisa Smith from the U.S. Department of Commerce National Oceanic and Atmospheric Administration (NOAA)
Earth System Research Laboratory, Global Monitoring Division