Fast Facts: Red Sky at Night, Sailors’ Delight…

UNDATED一 Many know the back half of this age-old adage, red sky at night, sailors’ delight, red sky in the morning, sailors take warning. Many may also know that the saying is more than just lore, atmospheric science actually supports this old bit of sailing wisdom.

Those in mid-latitude locations such as Oregon, Northern California, Wyoming, Virginia, North Carolina, northern Germany, and Poland can actually take advantage of this. There are two atmospheric-related factors that make this possible. One, storm systems in the middle latitudes generally move west to east and the red color in the sky is the result of sunlight reflecting off clouds. Low pressure is associated with bad weather and high pressure with good weather. Low pressure causes air to converge to try to “fill” the low, and converging air causes upward motion, which in turn produces clouds and precipitation. The reddish color results from scattering of sunlight by suspended particles and aerosols in the atmosphere, and the sun’s rays pass through a greater length of atmosphere at sunrise and sunset than at any other time of day. In addition, aerosol, dirt, and dust concentrations are maximized in the lowest layers of the atmosphere when the atmosphere is dominated by sinking air associated with high pressure. Therefore, when under high pressure we can see vivid red sunsets and sunrises.

If the sky is red in the morning, the eastern horizon must be clear, and the clouds foretelling a coming storm are in the west, indicating the potential for bad weather. By contrast, a red sky at night can be produced only if the western horizon is clear and the clouds overhead are moving east, or clearing out, foretelling good weather.

Picture yourself on a ship off the coast of North Carolina with a wind blowing from west to east. It is morning and you are watching the sunrise and the sky is red. Since it is morning, you are looking east, and the red sky indicates that there is high pressure there. Since you are in the mid-latitudes, the high is moving eastward, away from you. That could only mean that a low, and very likely an associated storm, is moving toward you from the west. Take warning!

Now picture yourself watching the sunset from the ship, and the western sky is red. That means that an area of high pressure is to your west, the westerlies are moving it toward you, and good weather is on the way. Sailor’s delight!