Astronomical fall arrived at 8:30 Tuesday morning with the autumnal equinox, when the sun crossed the equatorial plane from the northern to southern hemisphere.
For the last 3 months since the summer solstice, the most direct rays from the sun have shined north of the equator. For the next 6 months the most direct rays from the sun will drift south of the equator. You’ll notice the afternoon sun getting lower and lower in the southern sky until the winter solstice in December.
This begins the half a year where the southern hemisphere receives more incoming solar radiation than the northern hemisphere. While we’re cooling down and heading toward winter, folks south of the equator are enjoying spring-time conditions and getting ready for the heat of summer.
Here in the ArkLaTex we’ll see a steady cool down. Average highs on this first day of fall are still in the mid 80s. By the last day of fall those average highs are in the upper 50s. Average lows in the low 60s this time of year are down into the 30s by the 3rd week of December.
We’ll not only start cooling off, but the hours of daylight will continue to dwindle until the winter solstice in December. The days have been getting shorter since the summer solstice in June, but in the weeks just before and just after the autumnal equinox we’re losing daylight the quickest. Right now we’re shedding about 2 minutes of sunshine each day. By the time we hit the 3rd week of December we’ll have almost 2 hours less of daylight! Sunset today was at 7:10pm. Between the shorter days and the end of Daylight Saving Time November 1st, the sun will set at 5:09pm on December 1st.
The autumnal equinox is one of 2 days out of the year when at almost all points on the Earth, outside of the north and south poles, the sun rises due east and sets due west. Despite the term equinox, which is Latin for ‘equal night’, the time between sunrise and sunset is not exactly 12 hours yet. The discrepancy is due primarily the way the Earth’s atmosphere affects sunlight. The sun’s rays are refracted, or bent, by the atmosphere which allows us to see it rising and setting even when it is still just below the horizon. We won’t see exactly 12 hours between sunrise and sunset until September 26th.
The KSLA First Alert Weather team will keep you updated on the day-to-day changes as we head through the fall season. Here’s how you can get the First Alert on the forecast:
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