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. However, 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.) 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. 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".
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.
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.
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 in the winter of a non-leap year to 92 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, 1999, The American Meteorological Society.