“Friday Night Lights” definitely took on new meaning this week in parts of the southeastern United States as spectacular pictures of sunsets flooded social media timelines. Many of them were juxtaposed with high school football games throughout the area. Meteorologists from across the region like Birmingham legend James Spann shared breathtaking snapshots from Friday evening. Friends were posting photographs throughout my social media timelines. The amazing “art show” in the sky caused people to reflect on the pace of their day, spiritual peace, or the pure beauty of the moment. Social media timelines can be an endless rehash of memes, political articles, and forgettable banter. However, the almost communal response of this particular sunset was different. Here’s the science behind why it happened.
Many people are familiar with the old saying, “red sky at night sailors delight; red sky in the morning sailors take warning.” However, the sunset last night runs counter to the logic of the saying, which actually does have meteorological validity (some weather lore does not). Because weather systems typically move from west to east in the Northern Hemisphere, unless you are in the tropics, the light from the Sun rising in the east interacts with approaching mid-to-high level clouds (and vice versa for a storm exiting your area). In this particularly case, Tropical Storm Nestor was approaching Georgia from the Gulf of Mexico. Nestor is a rather lop-sided storm with much of its cloud cover on the east and north side of the center. As you see from the Friday afternoon satellite image below, upper level clouds were already streaming over Georgia as sunset approached. Nestor, the 14th named storm of the 2019 Atlantic hurricane season, is expected to bring much needed rainfall to the drought-plagued Southeast but could also feature significant winds, falling trees, and a stray tornado threat.
Okay Dr. Shepherd, “so what makes for the brilliant colors?” To answer this question, I have to remind everyone that the Sun sends energy to Earth in multiple forms such as ultraviolet radiation (UV), infrared radiation (IR), and visible light. The “visible light” part of the so-called Electromagnetic Spectrum allows us to see the wonders of our atmosphere. The blue and violet colors have shorter wavelengths than the red-orange colors. Ahead of Nestor, the molecules and constituents in the atmosphere were sufficiently small that they selectively scattered the smaller wavelengths of “visible light” such that purplish colors were being mixed in with the more traditional reddish-orange colors associated with the sunset. This interaction of light and scattering constituents in the air is called Rayleigh Scattering and is also the reason the sky appears blue on most days. If larger particles like dust, particulates of pollution or water is in the atmosphere, the Mie Scattering regime is more prevalent and the sky takes on more of a milky white color as no particular color is selectively scattered. The reddish-orange colors seen in the photographs above were enhanced by the upper level clouds from Tropical Storm Nestor. Former NOAA scientist and colleague Stephen Corfidi has an excellent discussion of these processes at the Storm Prediction Center (SPC) website. He writes,
To produce vivid sunset colors, a cloud must be high enough to intercept “unadulterated” sunlight…i.e., light that has not suffered attenuation and/or color loss by passing through the atmospheric boundary layer. (The boundary layer is the layer near the surface which contains most of the atmosphere’s dust and haze). This largely explains why spectacular shades of scarlet, orange, and red most often grace cirrus and altocumulus layers, but only rarely low clouds such as stratus or stratocumulus.