It still feels like winter in many parts of the country, but Earth’s orbit says otherwise.
At 7:02 a.m. EDT this Wednesday (March 20), astronomical winter ends as spring officially begins in Earth’s northern hemisphere.
Astronomically, the spring equinox marks the moment at which Earth’s axis tilts neither away from nor toward the sun, resulting in nearly equal periods of daylight and darkness across the globe. An observer on Earth’s equator will see the sun at zenith before its direct rays shift northward toward the Tropic of Cancer over the next three months.
Equinox: What’s equal and what’s not?
Sun observers and astronomers may think of the spring equinox as an astronomical transit point when the Earth-sun relationship stands in perfect balance. This is true when we consider that all locations on Earth experience slightly over 12 hours of daylight, as well as the position of sunrise and sunset with respect to the horizon.
Link: Day and night world map
The most “equal” aspect of the equinox is the location of sunrise and sunset. Except at the north and south poles, all latitudes on Earth see the sun rise at due east and set at due west on the March equinox. So whether you live in Miami, northern Alaska, or anywhere in the southern hemisphere, the sun will rise and set within 90 degrees of due north on a compass.
Another unifying trait of the March equinox is that it marks the time of year when daylight most rapidly increases in the northern hemisphere (or decreases for those south of the equator). While the days grow longer more quickly at northern latitudes, all northern hemisphere locations see their maximum daily increase in sunlight right around the spring equinox.
Equal daylight but unequal solar strength
Apart from the location of sunrise and sunset, however, most other aspects of the equinox are not the same. In fact, even the equal daylight concept is only partly true. Due to atmospheric refraction, most locations see the sun above the horizon for about 12 hours and 10 minutes on the spring equinox. The exact date of 12 hours from sunrise to sunset occurs anywhere from March 12-18 and depends largely on latitude (read more).
More important, while the March equinox brings roughly equal hours of daylight to the globe, the intensity of the sun’s rays will never be the same at all latitudes. In other words, equal daylight does not imply equal incoming solar radiation.
Comparing U.S. cities in the table above, we see that day length says little about average temperatures in early spring. Though daylight increases fastest in the northern United States, a lower sun angle at higher latitudes warms the ground less effectively. This results in colder soil and air temperatures and is why spring vegetation is still several weeks away from blooming in northern climates.
Beware of increasing UV
But whether or not it feels like spring, the equinox is a reminder that the sun’s declination – or height with respect to the horizon – is increasing at its fastest pace of the year. For humans, this means the sun’s ultraviolet rays are now significantly more intense than they were in midwinter. As we move into April the intensity of the sun’s rays will continually increase, so remember to keep sunscreen handy if you’re out gardening on a warm spring day.
Most of the lower 48 states now see at least moderate UV levels on a clear day. In all but the northernmost states, the UV index reaches high intensity when skies are clear at local noon (around 1 p.m. on daylight saving time).
The table above shows the maximum UV index around the March equinox as well as the declination of the sun at solar noon. Because the sun is seen directly overhead (90º above the horizon) at the equator (0º latitude), calculating the maximum sun angle on the equinox is straightforward: simply subtract your city’s latitude from 90 degrees.
On the March equinox in Washington, D.C. (38.9º north latitude), the sun climbs to 51.2º above the horizon on the March equinox, delivering high UV levels on a sunny day. In Anchorage, however, the UV index is still low, as the March sun there shines at the same low angle seen in the nation’s capital around mid-December.
Unlike a year ago, when record March warmth accompanied the astronomical start of spring, temperatures in 2013 remain below average for much of the central and eastern U.S. Yet no matter what the weather outside, the spring equinox is a reminder that incoming sunlight is significantly stronger as the Earth’s northern hemisphere begins to tilt toward the sun. Even as winter overstays its welcome for some, we now see more daylight than darkness. That means spring can’t be far behind.