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Spring arrives. What really is an equinox?

Happy first day of spring! The spring or vernal equinox is one of only two times out of the year when the Earth’s axis is tilted neither toward nor away from the sun, resulting in equal daylight and darkness at most latitudes.

The spring equinox occurs at 5:50 p.m. The second equinox is at 3:50 a.m. Sept. 23, which will signify the first day of fall or the autumnal equinox.

Full disclaimer: we generally do not see 12 hours of daylight and 12 hours of darkness on the actual equinox. This generally happens a few days before in the spring and a few days after in the fall.

On Sunday, March 17, we measured equal light and dark — referred to as the equilux — with the sun coming up at 7:17 a.m. and going down at 7:17 p.m. The reason is that the equinox is measured by the position of the center of the sun, but the sun has an angular size. Therefore when we see the sun come up, we still get daylight even if the center of the sun has not passed beyond the horizon. This is obviously true with the sunset. The Earth’s atmosphere also plays a part by bending the sun’s rays, making the daylight a little longer.

Daylight hours will continue to get longer after Wednesday, resulting in more sunshine and more warmth for us due to the high angle of the sun. We are gaining about 2 minutes 30 seconds of sunlight each day in the D.C. region. The sun officially rose at 7:12 a.m. Wednesday and the sunset is expected at 7:20 p.m. By the end of March, the sun will rise at 6:54 a.m. on March 31 and set at 7:30 p.m.

Wednesday morning saw a beautiful sunrise to start off the first spring day of the year. Temperatures will rise to the mid-50s in most parts of the D.C. area; this is just a little below normal for this time of year, as normal temperatures should be around 57 degrees. Clouds increase through the day on Wednesday, with some rain arriving after 8 p.m. Spring showers resume for Thursday, the first full day of spring, but we are doing pretty well in terms of weather compared to last year.

Remember the first day of spring of 2018? Some parts of the region received a nice little coating of snow on the ground, making us hold onto the idea of winter that much longer. While there is no snow in our spring forecast for 2019, I wanted to look back over the last 20 years and see how first days of spring around the D.C. area have shaped up.

The coldest first day of spring over the last 20 years was just last year, with daytime highs only reaching to around 43 degrees. The warmest day that we have had on the vernal equinox was 2010 when we hit 74 degrees. We have only had measurable snow (small amounts) three times on March 20, 2018, 2016 and 2015. Spring saw a wet start for six consecutive years beginning in the year 2000. The most measurable rain came down in 2003, when 1.95 inches of rain were recorded; that record still holds today.

Forecast:

Wednesday: Becoming cloudy by evening with few showers after sunset.
Highs in the low to mid-50s.

Thursday: Chilly and wet with heavy rain at times.
Highs in the high 40s to mid-50s.

Friday: Mostly cloudy with midday showers.
Highs in the low to mid-50s.

Saturday: Mostly sunny but breezy and chilly.
Highs in the low 50s.

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