Today -- Tuesday, March 19 -- is officially the last full day of winter. For you trivia buffs, tomorrow is the Spring (Vernal) Equinox, with spring beginning at 6:02 AM (CDT). The Equinox: 12 hours of daylight and 12 hours of dark, right? Well, not exactly. If you want to read the nuts and bolts, we go into a little detail at the bottom of today’s write-up.
As for our weather, it was a very pleasant day today and we’ll have no complaints about this evening. But clouds will be on the increase overnight as a mid/upper-level disturbance to our west moves towards us through the night and over the lower Mississippi Valley during the first part of Wednesday. Our most reliable models are sending mixed signals about the rain chances with this system, so we’ve “split the difference” and are calling for isolated-to-scattered areas of light rain during Wednesday morning, so be ready for patches of wet streets for the morning commute.
Skies should be clearing from west-to-east by mid-day, leaving us with a nice Wednesday afternoon. Temps will be near 50° for metro BR at sunrise, with highs only climbing to about 70° or so for the afternoon, several degrees cooler than Tuesday’s high. Canadian high pressure will maintain a north-to-NNE wind through the day and also keep humidity on the low side.
That Canadian surface ridge slides to the east on Thursday. At the same time, a warm front will slide northward through the Bayou State, linked to a low-pressure center over the Southern Plains.
From here into the weekend, the weather picture is still a little fuzzy. Let’s try this . . . the Plains low moves east across the lower Mississippi Valley on Friday, dragging a cool front with it. That front then stalls across southern Louisiana from the latter half of Friday into Saturday, keeping our local weather unsettled for both days.
For the time being, we’re calling for mainly light to moderate scattered showers on both days, with t-storms possible, especially closer to the coast. At this stage, we see no serious threat for severe weather and rain totals over the two days are expected to remain below 1” for most, if not all, locations in the WAFB area.
A surface low riding along that stalled frontal boundary from west-to-east reaches the mouth of the Mississippi by early Sunday while a second surface ridge of continental high pressure builds-in from the north and northwest of the coastal front. Those two features will work together to push the front out into the Gulf. That should leave us with a decent latter half of the day for Sunday, followed by noticeably cooler but dry days for Monday and Tuesday.
So, about that equinox: why do the sunrise/sunset tables show a few minutes more daylight on the Equinox than exactly 12 hours? Which means “nighttime” is less than 12 hours.
The presumption that Equinox means exactly “12 and 12” is based on astronomical geometry (the relationship between the Earth and Sun) … and it would be essentially correct if there were no atmosphere. First off, this “12 and 12” rule ignores the indirect sunlight we get before sunrise and after sunset -- the period officially referred to as “twilight.” And twilight is a result of sunlight “scattered” by the atmosphere even when the solar disk is below the horizon. (The moon, for example, with no atmosphere, has no twilight periods.)
In fact, there are three types of twilight -- Civil Twilight, Nautical Twilight and Astronomical Twilight -- each effectively defined by the amount of indirect lighting available. (We’ll leave you to dig up the details of these three definitions.)
But even if we exclude “twilight,” the daylight period during the equinox (Remember, there are two each year: Spring/Vernal and Fall/Autumnal) is longer than 12 hours. There are two basic reasons.
First, since the Sun is a “disk” and not a simply a point in the sky, the official “12 and 12” would occur when the solar disk was evenly split through the middle while “sitting” on each horizon -- in the morning and the evening. But the official definition of sunrise/sunset is the time when the upper-edge of the solar disk is just touching the horizon -- when the upper edge of the solar disk just touches the horizon in the morning as it “rises” and when it just slips to even with the flat horizon as it “sets” in the evening. The time it takes for the Sun to “move” from the disk’s mid-point to the upper edge (relative to the horizon) adds a couple of minutes to the official “daylight” period at the start and the end of the day.
Second, the Earth’s atmosphere refracts (bends) the Sun’s direct-beam light (the light from the solar disk). Because of the refraction, we actually see the top edge of the solar disk before that edge has actually risen above the horizon from a purely geometric perspective. So sunrise “appears” before it would actually occur if there were no atmosphere. Likewise in the evening, atmospheric refraction keeps the solar disk above the horizon a little longer, delaying the time when the solar disk actually disappears from view.
For Baton Rouge, the combination of these two factors extends the daylight period (excluding twilight) about 8 minutes or so -- not exactly something to get all that worked up about, eh? The added daylight time (still measured in a handful of minutes) gets a little a shorter as you get closer to the equator and a little longer as you move towards the poles.
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