People have used the concept of time to mark past experiences and anticipate future events. To quantify time, we have traditionally used observable periodic natural phenomena, such as the daily and annual apparent path of the sun through the sky and the monthly phases of the moon. The sun is especially important because its uneven heating of the Earth-atmosphere system drives atmospheric circulation and weather.
Various calendars have been devised to mark time for practical purposes, including agriculture, commerce, taxation, and religious observances. These calendars rely on some recognizable recurring event, such as the return of the sun to its high position in the sky or a full moon.
Solar calendars track the periodic movements of the sun across the sky through its annual cycle, as a consequence of the tilt of Earth's spin axis with respect to the perpendicular to its orbital plane about the sun. The solar year represents the time elapsed between one vernal equinox and the next. The solstices are other important events in the solar calendar, marking the point where the sun's path is either the highest or lowest in the sky. The lunar calendar, from which the month originated, is based on the lunar cycle of recurring phases of the moon.
Unfortunately, the solar and lunar cycles are not commensurate, since twelve complete lunar months of a somewhat nonuniform number of days (because the lunar orbit about Earth is not exactly circular) do not fit the solar year exactly. Furthermore, these cycles do not have periods that fit nicely into an integer number of days--a desirable feature for constructing a civil calendar with simple whole numbers to identify days. Various early calendars used elaborate adjustment schemes that were unsatisfactory. In the first century B.C., Julius Caesar decreed calendrical reform with a 365-day year consisting of a sequence of 12 months, along with the inclusion of an extra day at the end of February (the last month of the old Roman year) every fourth year. In the 16th century, an adjustment, called the Gregorian calendar, was made to align the calendar more closely with the more precise length of the solar year of 365.244 days (or 365 days, 5 hours, 48 minutes and 46 seconds) . The Gregorian calendar requires that only those centurial years divisible evenly by 400 would be leap years, whereas the other centurial years (e.g., 1800 and 1900) would not. So, the year 2000 marked the first time since 1600 that a centurial year was a leap year. This Gregorian scheme is still not precise but produces an error of less than one day in 3000 years.
The orbital points of the solstices and equinoxes are used to define portions of the year in terms of the solar input into Earth's energy balance, yielding the so-called astronomical seasons. Most people have been taught that winter officially begins at the winter solstice, which occurs on or about 21 December and that the official summer season begins at the time of the summer solstice, on or about 21 June. (The exact date varies because Earth travels around the sun in 365.24 days, necessitating the addition of an extra day every fourth year. The calendrical corrections just described explain why the exact times of the solstices and equinoxes do not occur at precisely the same time every year, but undergo a 6 hr drift for three years before they revert to an earlier time.) This scheme focuses upon the astronomical seasons. The astronomical seasons are those portions of the year marked for Earth's passage by four cardinal points in its orbit about the sun. These cardinal points consist of the two solstices and the two equinoxes.
Thus, the astronomical fall in the Northern Hemisphere is the elapsed time between the autumnal equinox and the winter solstice. From the viewpoint of the astronomical seasons, we are in mid-autumn, since astronomical fall started about 7 weeks ago with the passage of the autumnal equinox. Winter follows and continues until the vernal (spring) equinox, and so forth. These astronomically determined cardinal points are defined in terms of the orientation of Earth's spin axis with respect to the sun as the planet moves around the sun.
However, this scheme for identifying the seasons is not necessarily the most satisfying for describing the seasonal variations in many natural phenomena. For example, the word summer typically conjures up thoughts of long days and short nights. If the astronomical definition were followed, summer would commence only when the daylight length is waning following the summer solstice. For the British, this day with the longest daylight of the year is more aptly called "Mid-summer day."
Another problem arises, especially when many seasonal weather or climatological records are considered. A separate set of records of the various weather elements would have to be produced for the astronomical seasons that begin on the solstice and equinox dates. These calendar dates vary slightly from year to year as a result of the inclusion of leap year day to account for the slight difference in length between the civil year and the solar year. Furthermore, the elliptical orbit of Earth about the sun causes the lengths of the astronomical seasons to vary between 89 and 93 days.
Similar to the astronomical seasons, we can define meteorological seasons that are geared to the annual temperature cycle as well as to fit our calendar. The public typically thinks of winter as the coldest time of the year, summer as the warmest time of the year, with spring and fall (or autumn) representing the transition seasons. These seasons are for meteorological observing and forecasting purposes and they are more closely tied to our monthly civil calendar. The current transition interval, "autumn," between the warmest and coldest portions of the year can be closely linked to the calendar months of September, October, and November. We can also have Fall Outlooks and monthly and seasonal averages and records. This information is useful for agriculture, commerce and other purposes.
While the "normal" annual temperature cycle at most middle latitude locations typically lags the solar illumination cycle by about one month over the continents and by about 6 weeks over the ocean, summer-like weather episodes often occur well before the summer solstice. In many middle latitude Northern Hemisphere locales, summer-like weather can begin as early as mid-May.
By international convention, meteorologists define meteorological seasons in terms of three-month intervals that are centered upon the typical occurrence of the warmest and coldest months of the year. By this convenient definition, meteorological spring consists of the months of March, April and May; summer encompasses the warmest months of June, July and August; autumn is September, October and November; and meteorological winter consists of the coldest months of December, January and February. Seasonal length is more uniform for meteorological seasons (than for astronomical seasons) ranging from 90 days for winter of a non-leap year to 92 days for spring and summer. Seasonal statistics can then be determined easily from the monthly statistics.
Several other designations of seasons have been developed by meteorologists to aid in the handling and interpretation of weather data for specific purposes. Some of the commonly used seasonal designations include:
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2005, The American Meteorological Society.