Warm St. Paddy's day is also the 'Equilux'
Posted March 17, 2015
Temperatures will run well to the warm side of normal for this St. Patrick's Day, as we expect a high around 81 degrees with a mix of sun and clouds, while temperatures top out in the upper 40s with more clouds than sun and some mist for the folks in Ireland's capital.
Of course, this time of year we're in the transition period between wintertime patterns and summer weather, so some big swings between very warm and quite chilly conditions remain quite possible, and in fact we'll be much cooler for the second half of this week, and there are some hints at potentially dipping below the freezing mark a couple of mornings next week.
The transition to Spring is one we mark as meteorologists with the start of March (the beginning of "Meteorological Spring" on March 1st), but the more traditional beginning to the Spring season is an astronomical one, the occurrence of the Vernal Equinox, or one of two instants each year at which the earth's polar axis is neither tilted toward nor away from the sun, but rather crosswise to it (the other moment this happens is the Autumnal Equinox in September). This year, that marker of Spring occurs at 6:45 p.m. this Friday, March 20.
If we somehow lived on an earth with no atmosphere, and we marked our sunrise and sunset by the passage of the center of the sun's disk across the horizon, the day of the equinox would feature very nearly 12 hours each of daylight and darkness, or 12 hours from sunrise to sunset. However, because refraction by our atmosphere bends the sun's rays downward as they pass through the air, with the greatest amount of bending as seen from any location occurring when the sun is near the horizon, we actually see the sun appear considerably higher in the sky than it really is near sunrise and sunset.
That means it is visible well before it would be if no atmosphere was present (in effect, the refraction allows us to see around the horizon). This causes sunrise to occur several minutes before it would otherwise, and sunset several minutes later. In addition, a little more time is added to the "daylight" period, since we mark sunrise and sunset by the upper edge of the sun's disk crossing the horizon, rather that its center.
Taken together, this all means that the day that comes closest to 12 hours between sunrise and sunset is not the equinox, but about three to four days before the vernal equinox and after the autumnal equinox. This day has been referred to as the "equilux," and occurs Tuesday, March 17. In Raleigh, when rounded off to the nearest minute, we'll have about 12 hours and 1 minute from sunrise to sunset, after having 11 hours and 58 minutes Monday and prior to having 12 hours and 3 minutes Wednesday.
By the equinox on Friday, daylight length will exceed 12 hours by about 8 minutes. These times are taken from the table of daylight duration for 2015 from the U.S. Naval Observatory Astronomical Applications Department. I've included a link to that part of their web page, in case any of you would like a table for some other year or a different location.