I love the fall season. I am a sports fan so it is the time of the year when football, basketball, and baseball converge. One of the other things that I love about the fall is the cool, crisp mornings. Here in the Atlanta area, a cold front passed in recent days. Nighttime temperatures dipped into the fifties and had me checking the pantry for hot chocolate. Yeah, yeah, I know readers in more northern latitudes are snickering, but that is hot chocolate, thermostat-adjusting, and sweater weather here in the South. I snapped the photo below from my yard just before writing this piece because it is also the time when the sky can appear extremely blue. Here’s why.
In the fall and winter, there are a couple of attributes that contribute to vibrant blue skies. First, the Northern Hemisphere is increasingly tilting away from more direct solar radiation from the Sun as we transition to the fall season. By the winter solstice in December, the tilt is maximized. These times of the year represent the “cool or cold” seasons in the Northern Hemisphere.
Second, the weather patterns feature far more active jet streams and frontal passages during the fall and winter. When a cold front passes, as we experienced here in Georgia recently, colder and drier typically follows. Check out this Facebook post from the National Weather Service – Atlanta on September 22nd, 2021 – “So close we can almost taste it… (pumpkin spice, that is). 🍂🎃 This broad expanse of cloud cover (below) on satellite shows us the location of the cold front that is passing through northwest GA. Expect gusty winds as it mixes through today and a crisp morning tomorrow!”
As a reminder, the gases and particulates in the atmosphere scatter sunlight. Because of its wavelength, blue light is selectively scattered more than other colors within the visible portion of the electromagnetic spectrum. The National Weather Service (NWS) website Jet Stream provides more insight on the process of Rayleigh Scattering that causes our blue sky. It notes, “As sunlight enters the atmosphere much of the violet light waves scatter first but very high in the atmosphere and therefore not readily seen.” Indigo is also scattered away and is mostly visible at higher altitudes, for example, if you are flying in a commercial airplane. The website goes on to say, “Next, blue light waves scatter at a rate about four times stronger than red light waves. The volume of scattering by the shorter blue light waves (with additional scattering by violet and indigo) dominate scattering by the remaining color wavelengths.”
Ok, Dr. Shepherd, but why is the blue so vibrant after a cold front passes or during the cold season. There are three primary reasons. Cooler air tends to have a lower relative humid and hold less water vapor. By default, this typically means less haze and clouds occupying the sky on a mostly clear day. During the fall, the sun angle is also lower, which decreases longer wavelength green and red colors and intensifies the Rayleigh Scattering of blue to our eyes.
Those that artistically inclined (I’m not by the way) would appreciate this fact. During the fall, orange, yellow, and red leaves start to dominate the landscape. A blue sky sharply contrasts with this backdrop, which also enhances the blue appearance of the sky. I think it has something to do with complementary colors, but I am not the expert on that.