Humans have used the idea of time to mark past experiences and anticipate future happenings. To quantify time, we have traditionally used observable periodic natural phenomena, such as the daily and yearly paths of the apparent sun through the sky and the monthly phases of the moon. The sun is especially important as its uneven heating distribution drives weather systems.
Various calendars have been devised to mark time for practical purposes, to include agriculture, commerce, taxation and religious observances. These calendars attempt to use 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 the earth's spin axis with respect to the normal 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 two cycles are not commensurate, since twelve complete lunar months of a somewhat nonuniform number of days (because the lunar orbit about the 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 feature that humans would like to have when 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 with a sequence of twelve months, along with the inclusion of an extra day to 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 make the calendar more closely aligned with the a more precise length of the solar year was 365.244 days (or 365 days, 5 hours, 48 minutes and 46 seconds). This Gregorian calendar requires that only those centurial years divisible evenly by 400 would be leap years, while 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 exact but will result in an error of less than 1 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 the 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 the earth travels around the sun in 365.24 days, necessitating the insertion 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 hour drift for three years before they revert to an earlier time.) This identification scheme focuses upon the "astronomical seasons". The astronomical seasons are those portions of the year marked for the 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 spring in the northern hemisphere is the elapsed time between the vernal or spring equinox and the summer solstice. From the viewpoint of the astronomical seasons, we are in early spring, since astronomical spring started almost 3 weeks ago with the passage of the vernal equinox. Summer follows and continues until the autumnal equinox, and so forth. These astronomically determined cardinal points are defined in terms of the orientation of the earth's spin axis with respect to the sun as the earth 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 the thought 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. To 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. Monthly statistics of various weather elements are compiled. A separate set of records 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 the "leap year day" to account for the slight difference in length between the civil year and the solar year. Furthermore, the elliptical orbit of the 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 meant to fit our calendar as well as the temperature cycle. The public typically thinks of winter as being 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, "spring", between the year's coldest and warmest portions of the year can be closely linked to the calendar months of March, April and May. We can also have Winter 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 locations in mid-latitudes typically lags the solar illumination cycle by about one month over the continents and by about 6 weeks over the oceans, a "summer like" weather event may occur well before the summer solstice. In many locales, summer-like weather can begin early in June.
By international convention, meteorologists have defined "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 contains 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. The seasonal length is more uniformly divided, ranging from 90 days in the winter of a non-leap year to 92 days in 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, 2001, The American Meteorological Society.