Partner Series
Equinox: Why Spring Weather Can Keep Us Guessing
On Tuesday, March 20 at 12:15 p.m. EDT the sun crosses the celestial equator traveling north, marking the vernal equinox in the northern hemisphere and the beginning of spring. Days and nights are of equal length. The sun rises due east and sets due west.
Credit: Starry Night software

Next Tuesday, at 12:15:21 p.m. EDT (16:15:21 GMT), the winter season will officially come to an end in the Northern Hemisphere. How can we be so sure? At that moment, the sun will arrive at one of two positions where its rays will shine directly down on the equator. Indeed, if you were standing on the equator at a point just to the west of the Itapará River of the Roraima State in northern Brazil, the sun would appear directly overhead, even as we in the Northern Hemisphere make the transition from winter to spring. At that time, the sun will also be shining equally on both halves of the Earth. 

Spring at last!

Many look upon the arrival of spring as an end to cold — and, in northern climes — snowy weather. That, of course, is simply not true. In some years, unseasonably cold temperatures and accumulating snows can linger well into April, just as warm weather sometimes hangs on well into October. [Night Sky, March 2018: What You Can See This Month (Maps)]

 

The annual astronomical changes that cause the weather to vary in the planet's temperate zones are far from simple. The plane of the Earth's equator is tilted 23 1/2 degrees to the planet's orbital plane around the sun. During the year, varying amounts of sunlight strike different regions of our planet. Both the angle of incidence of the solar radiation (the angle at which the sun's rays strike the Earth's surface, which provides a measurement of the intensity of solar radiation) and the length of daylight change dramatically.

While these are indeed the basic reasons for the temperature differences of the seasons, a number of meteorological factors produce the immediate, day-to-day variations we experience. And these variations are determined by our atmosphere's heat-retaining ability and its circulation, which interact in a very complex way each day to determine the weather. But the seasons' lag after the equinox can be appreciated by many people, just from their own experiences with weather in the United States and Canada.

For instance, summer will officially arrive with the solstice on June 21. If the insolation — the total energy received from the sun — alone governed the temperature, we should then experience the year's hottest weather. But the atmosphere in temperate regions continues to receive more heat than it gives up to space — a situation that lasts a month or more, depending on the latitude. In New York City, for example, the stretch of time with the highest daily mean temperature of the year (77 degrees Fahrenheit, or 25 degrees Celsius) runs from July 11 through Aug. 2 (43 days after the solstice) before it finally begins to decline. A reverse process occurs after the northern winter solstice in December. 

The solar heating effect depends directly on the sun's declination in the sky, which also controls its daily path across the sky and the number of hours the sun is above the horizon. On April 13, the insolation is the same as it is on Aug. 29, but in New York City, for example, you can see a big difference because of the seasonal lag. On April 13, 1940, for example, it was as cold as 26 degrees F (minus 3 degrees C), and on April 13, 1875, the same city saw as much as 10 inches of snow — whereas the mercury soared as high as 99 degrees F (37 degrees C) on Aug. 29, 1953. [Vernal Equinox: First Day of Spring Seen from Space (Photo)]

If you ask someone to give you the date of the first day of spring, the response almost certainly will be March 21. But since 1980, for North Americans, spring has actually begun no later than March 20. And it will continue this way through the year 2102. 

In 2016, for the Central, Mountain and Western time zones, spring begins on March 19. And in 2020, it will fall on March 19 for all of the United States for the first time since 1896. This shift in dates happens because the Earth's elliptical orbit changes the orientation of its axis, and because our year does not contain an even number of days. The vagaries of the Gregorian calendar, such as the inclusion of a leap day in century years divisible by 400, also help contribute to the seasonal date shift. 

In fact, had 2000 not been a leap year, the equinox would be occurring next Wednesday, not next Tuesday.

Another complexity involving the vernal equinox concerns the axiom, "equal days and equal nights on the equinox." Yet each year, I always get at least one or two inquiries asking why that isn't so. Perhaps someone who was skimming through the weather page of their newspaper on the day of the equinox, looked at the almanac box — which provides the local time of sunrise and sunset — and noticed that the length of day and night is not equal at all. In fact, on the equinox dates in both March and September, the length of daylight is actually longer than the period of darkness by several minutes.

Check out the situation for New York City. As the table below shows, days and nights are equal in length, not on the equinox, but on St. Patrick's Day:

Date Sunrise Sunset Length of Day
March 17 7:05 a.m. 7:05 p.m. 12 hrs. 00 min.
March 18 7:03 a.m. 7:06 p.m. 12 hrs. 03 min.
March 19 7:02 a.m. 7:07 p.m. 12 hrs. 05 min.
March 20 7:00 a.m. 7:08 p.m. 12 hrs. 08 min.

One factor to consider is that when we refer to sunrise and sunset, the time refers to when the very top edge of the sun appears on the horizon. Not its center, nor its bottom edge. 

This fact alone would make the time of sunrise and sunset a little more than 12 hours apart on equinox days. The sun's apparent diameter is roughly equal to half a degree.

But the main reason that this happens is due to our atmosphere; it acts like a lens and refracts (bends) its light above the edge of the horizon. In its calculations of sunrise and sunset times, the U.S. Naval Observatory routinely uses 34 arc minutes for the angle of refraction and 16 arc minutes for the semi/half-diameter of the sun's disk. In other words, the geometric center of the sun is actually more than eight-tenths of a degree below a flat and unobstructed horizon at the moment of sunrise. 

As a result, viewers actually see the sun for a few minutes before its disk actually rises and for a few minutes after it has actually set. So you can thank our atmosphere for making our days a bit longer; the length of daylight on any given day is actually increased by approximately 6 or 7 minutes.

In other words, when you watch the sun either coming up above the horizon at sunrise or going down below the horizon at sunset, you are actually looking at an illusion — the sun is not really there, but actually below the horizon! 

Now you see it ... when you don't!

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Fios1 News in Rye Brook, New York. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.