The complete saying states: RED SKY IN MORNING, SAILORS WARNING; RED SKY AT NIGHT, SAILORS DELIGHT. This saying only applies to mid-latitude locations (winds are easterly in the tropics / in the high latitudes the sun rises and sets at a large deviation from the east-west trajectory). Storm systems in the middle latitudes generally move west to east. A red sky in the morning implies the rising sun in the east is shining on clouds to the west and conditions are clear to the east. Clouds moving from the west (especially upper level cirrus) indicate an approaching storm system. A red sky at night implies the sun (setting in the west) is shining on clouds to the east and conditions are clear to the west (because the sun can be seen setting). If you can see the sunset, the sky will be redder. Clouds to the east indicate an exiting storm system in the middle latitudes. Upper level clouds (especially cirrus) are noted for giving the sky a reddish hue during dawn or dusk. As a mid-latitude cyclone approaches, it is the upper level cirrus that are seen first, followed by lower clouds. The approach of upper level cirrus from the west often indicates an approaching storm system. The sky will not be as red at night if a storm system is approaching because the sun is setting behind the clouds approaching from the west. A red sky at night implies "the storm system moving through has ended!"; The clouds have broken and the sun is shining on and reddening the exiting clouds. The sun will continue to shine on clouds for a period of time after the sun has dipped below the horizon (especially cirrus). Keep in mind this saying was developed before satellite, radar and modern meteorological knowledge. Much of the knowledge of an approaching storm system back then was cloud and wind patterns. Of course, this saying (weather folklore) has some profound problems such as:

(1) The sky can be "reddish" near the sun at dawn and dusk (with or without clouds)
(2) storm systems do not always move straight west to east
(3) cirrus can occur without a storm system approaching or leaving. Clouds can cover one side of the sky or the other without being directly associated with a storm system
(4) the meteorological sailor may delight at an approaching storm system even while taking precautions at the same time
(5) rarely do the clouds from an approaching or exiting storm system only cover 1/2 of the eastern or western sky. The saying represents an ideal case.

SUMMARY: If you can see the sunrise but the west part of the sky is dark: look out for approaching bad weather. If you can see the sunset: the weather conditions will be nice.

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