Officially spring! March 20 marks winter’s end, even if winter weather still looms

The relentless winter of 2014 continues for much of the U.S., but there is at least some (symbolic) hope if you’re desperate for warmer weather: Today marks the spring equinox, which signals the end of astronomical winter in the northern hemisphere.

At 12:57 p.m. EDT today, the sun can be seen directly overhead on Earth’s equator before its direct rays migrate northward toward the Tropic of Cancer for the next three months.

On the equinox, all latitudes receive equal periods of daylight and darkness. (timeanddate.com)
On the equinox, all latitudes receive equal periods of daylight and darkness. (timeanddate.com)

Astronomically, the March equinox is equivalent to September in terms of incoming solar energy, daylight, and darkness. As the Earth’s axis is tilted neither away from nor toward the sun, all latitudes experience roughly 12 hours of daylight and darkness and see sunrise and sunset occur at due east and west, respectively, along the horizon. The only exception to this rule is the North and South Pole, where the sun rises (or sets) for the first time in six months, each year at a different point on the horizon.

(Justin Grieser)
(Justin Grieser)

With daylight in the northern hemisphere increasing at its most rapid pace of the year, average temperatures also move steadily upwards. In Washington, D.C., the average high temperature rises a full 10 degrees during the month of March, and daylight increases by 2 minutes and 32 seconds each day shortly before and after the equinox.

Yet even though average temperatures are on the upswing, the spring equinox is considerably colder than its fall counterpart in September. A comparison of average high temperatures on the March and September equinoxes shows just how much colder early spring is, on average:


Daylight and incoming solar energy are equivalent to what we see on the September equinox, but average temperatures are much colder. (Image by Justin Grieser, data from NOAA)

As discussed last year, this seasonal temperature lag occurs because the atmosphere responds slowly to changes in Earth’s incoming solar radiation. The oceans, which cover more than 70 percent of the planet, play a major role in regulating seasonal temperature changes. Water takes longer than air to heat up and cool down, which means that when the northern hemisphere begins absorbing more solar energy than it loses, most of this heat goes into warming the upper oceans first.

Regardless of the temperature outside, the spring equinox serves as a reminder that the sun’s intensity is much stronger than it was in midwinter. The sun now appears 23.5º higher in the sky at midday than it did on the winter solstice, therefore the sun’s harmful ultraviolet (UV) rays are also significantly more intense.

By late March, the UV Index already approaches moderate to high levels in most of the continental U.S. on a clear day, so remember to use sunscreen if outside for an extended period of time. This is especially true in any areas with lingering snow cover, since snow easily reflects the sun’s UV rays off the surface.

Of course, given the latest 6-10 day temperature outlook from the National Weather Service, many won’t be seeking the outdoors just yet. While seasonable temperatures align with the first day of spring today, winter is staging another comeback. On the bright side (literally), the sun is up for more than 12 hours now, so it’s only a matter of time before spring arrives in earnest.

More from CWG:

Autumn Equinox 2013
Spring Equinox 2013
Autumn Equinox 2012
Spring Equinox 2012
Autumn Equinox 2011

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David Streit · March 20, 2014