I love the fall season. I’m a sports fan, so this is the time of year when football, basketball and baseball converge. One of the other things I love about fall is the cool, chilly mornings. Here in the Atlanta area, a cold front has passed in recent days. Nighttime temperatures plunged into the fifties and made me check the pantry for hot chocolate. Yeah, yeah, I know readers in more northern latitudes chuckle, but it’s the hot chocolate, the thermostat setting, and the sweaters weather here in the South. I took the photo below from my backyard just before writing this piece as this is also the time when the sky can appear extremely blue. Here’s why.
In the fall and winter, a few attributes contribute to a brilliant blue sky. First, the northern hemisphere is moving further and further away from the more direct solar radiation from the Sun as we move into the fall season. At the winter solstice in December, the slope is maximized. These times of the year represent the “cool or cold” seasons in the northern hemisphere.
Second, weather patterns show much more active jet streams and frontal passages in the fall and winter. When a cold front passes, as we experienced here in Georgia recently, it usually results in more cold and drought. Check out this Facebook post from the National Weather Service – Atlanta September 22, 2021 – “So close we can almost taste it … (pumpkin spice, that is). 🍂🎃 That vast expanse of coverage cloudy (below) on satellite shows us the location of the cold front crossing the northwest of the GA. Expect gusts of wind as it mixes today and a cool morning tomorrow ! “
As a reminder, the gases and particles present in the atmosphere diffuse sunlight. Due to its wavelength, blue light is selectively scattered more than other colors in the visible part of the electromagnetic spectrum. The National Weather Service (NWS) Jet Stream website provides more information on the Rayleigh broadcast process that is causing our blue skies. He notes: “As sunlight enters the atmosphere, much of the purple light waves scatter first but very high in the atmosphere and therefore are not easily seen. Indigo is also dispersed and is most visible at higher altitudes, for example, if you are flying in a commercial airplane. The website goes on to say, “Then the blue light waves disperse at a rate about four times stronger than the red light waves. The scattering volume by the shorter blue light waves (with additional scattering by violet and indigo) dominates the scattering by the remaining color wavelengths.
Alright, Dr Shepherd, but why the blue so dynamic after the passage of a cold front or during the cold season. There are three main reasons. Cooler air tends to have lower relative humidity and retain less water vapor. By default, this usually means less haze and clouds occupying the sky on a clear day. During the fall, the sun’s angle is also lower, which decreases the longer wavelength green and red colors and intensifies the Rayleigh scattering of blue to our eyes.
Those who are artistically inclined (I am not for that matter) would appreciate this fact. In the fall, orange, yellow and red leaves begin to dominate the landscape. A blue sky contrasts sharply with this backdrop, which also enhances the blue appearance of the sky. I think it has something to do with complementary colors, but I’m not an expert on it.