"They" Say it's the First Day of Fall - Do You Believe Them?

The fall equinox, the start of fall, is officially 8:44AM EDT on Sunday, September 22nd this year. Although I've noticed sunrise getting later and sunset getting earlier for well over a month now, I'm not convinced that it's not still August, which would make it August 53rd. There have been one or two days that were cloudy and rained all day long, and the temperatures felt more like it was easing into fall, but our current forecast through the last of month shows consistent temperatures that are higher than I'd like to see. 

Screen capture from the Weather Underground. 

You'll notice that the daily high temperatures never get as low as 85, but on the positive side, they never touch 90. On the way to the grocery store Thursday, the car's under the hood temperature sensor read 97. Yeah, that's because it's stagnant air under the hood with car sitting in direct sunlight. Once we drove a few miles that dropped to 91. 

The night time temperatures shown never get below 75 and both of those are the two last days of the month.  All in all, it's not unusual for this time of year, and even the kids and homeless guys you hear talking in the park are talking about "when it cools off in November."

The thing most of you have never thought of

The word equinox translates (roughly) as "equal nights" and the equinox essentially has an equal length night all around the world.  The "fun fact" here is that the sunrise and sunset times are changing the fastest around the equinox. At any given location, the day-to-day difference in day length (as measured by the difference between sunrise and sunset day to day) is greatest around the days of the equinoxes.  An easily seen aspect of that is the number of hours of daylight is close to the same now around lots of the US, while in three more months at the next solstice (winter solstice here in the northern hemisphere), the northernmost latitudes will be in 24 hour long nights and zero hour long days while the lowest latitudes will have changed much less.  

What that means is that terminator line dividing day and night are the closest to vertical that they'll get on a map. This line is usually called the grayline in ham radio circles and it is dramatically important for determining where signals can come from and go to at any given moment. The effect is greatest at the lowest amateur bands and is justifiably famous on 160 meters (1.8 to 2.0 MHz). The effect is also important on 80 meters (3.5 to 4.0 MHz) becoming less important as frequency increases.  

Here's an example of today's grayline mapped out a little while ago Friday evening as I write:

This particular map is showing reported contacts on the 6m ham band (50.0 to 54.0 MHz) and grayline propagation doesn't show up here. What does show up is the more common ionospheric propagation - and pretty much everything except grayline. The two different darker colors designate the twilight area (the lighter area with almost straight North/South lines) and the fully night area (the darker oval on the right).  The shape of those areas changes radically as we get closer to the solstice and over the course of a year there are dramatic differences in the areas you can contact due to the moving grayline.

If you're on 160 to 40 and even 30 meters, remember these are the bands that tend to be open to remote locations at night so your chances of working places in daylight are pretty close to zero.  The bands from 20 meters up through the rest of HF tend to be open more in the daylight and the opposite of the lowest four bands. Since sunrise and sunset are pretty much the defining aspects of night and day, knowing where the day/night terminator is can be the first step in trying to figure out where to try to contact.

Oh. As is often the case, if you go searching for grayline with a search engine, you'll find gray and grey as well as one word or two words.