Three Weeks Into Spring

Illumination of the Earth by the Sun
at the March equinox, as seen from the Sun.

The vernal equinox was at 6:02 AM CDT this morning, marking the start of astronomical spring. The minutes of sunlight will be increasing each day until the summer solstice on June 21st. As we all know, spring is the transition between the coldest period of the year (winter) and the warmest (summer). For those of us in the weather and climate field, spring actually started on March 1st and will continue through May 31st. Why do we use the calendar seasons instead of the the astronomical seasons?  Generally it is much easier to compute averages and other statistics for a fixed number of days. The occurrences of the solstices and equinoxes can vary by a day or so each year making the seasons different lengths. And, it turns out that the meteorological seasons are closer to reality in general.. A study done in 1983 by a researcher at the University of Illinois (now with the National Center for Atmospheric Research) found that, for the mid-latitudes of North America, the seasons (in terms of temperature) more closely follow the calendar than the astronomical events.  In fact, the astronomical seasons are only appropriate over the oceanic areas of the Southern Hemisphere. The response of the atmosphere to solar radiation lags by about 27 1/2 days at mid-latitudes. This means that during summer, for example, the peak heating would tend to occur during mid-July, 27 1/2 days after the summer solstice.  Defining summer as June 1 to August 31 makes sense since mid-July is the midpoint of the meteorological summer (June, July, August). However, the calendar alignment isn't perfect for a number of reasons. For example, if we were to base the seasons in the Northern Hemisphere strictly on mean temperature, spring would be the period from March 9 to June 8. 

March 1st or March 20th - this year some will argue, with good reason, that spring still isn't here!

If you have an interest in the astronomical you will find the U.S. Naval Observatory (USNO) web site a wealth of information. The USNO serves as the official standard of time for the entire United States. You can determine sunrise or sunset for any location in the country or find out when the phases of the moon will occur at any point in the future, or get the dates and times of the equinoxes and solstices. The Sky This Week describes what planets can be viewed and what other astronomical phenomena can be observed.

Trenberth, Kevin E., 1983: What Are the Seasons? Bulletin of the American Meteorological Society, Vol. 64, No. 11, pp. 1276-1282.