From: Subject: Thursday Weather Topic in Greater Depth Date: Thu, 8 Oct 2009 10:02:31 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w04r_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Thursday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Thursday, 1 = October 2009=20

METEOROLOGICAL SEASONS

The public typically thinks of winter as being the coldest time of = the year,=20 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=20 winter officially begins at the winter solstice, which occurs on or = about 21=20 December and that the official summer season begins at the time of the = summer=20 solstice, on or about 21 June. (The exact date varies because the earth = travels=20 around the sun in 365.24 days, necessitating the insertion of an extra = day every=20 fourth year.) This identification scheme focuses upon the "astronomical=20 seasons". The astronomical seasons are those portions of the year marked = for the=20 earth's passage by four cardinal points in its orbit about the sun. = These=20 cardinal points consist of the two solstices and the two equinoxes.

Thus, the astronomical spring in the northern hemisphere is the = elapsed time=20 between the vernal or spring equinox and the summer solstice. Summer = follows and=20 continues until the autumnal equinox, and so forth. These astronomically = determined cardinal points are defined in terms of the orientation of = the=20 earth's spin axis with respect to the sun as the earth moves around the = sun.=20

However, this scheme for identifying the seasons is not necessarily = the most=20 satisfying for describing the seasonal variations in many natural = phenomena. For=20 example, the word summer typically conjures the thought of long days and = short=20 nights. If the astronomical definition were followed, summer would = commence only=20 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=20 "Mid-summer day".

While the "normal" annual temperature cycle at most locations in=20 mid-latitudes typically lags the solar illumination cycle by about one = month=20 over the continents and by about 6 weeks over the oceans, a "summer = like"=20 weather event may occur well before the summer solstice. In many = locales,=20 summer-like weather can begin early in June.

Another problem arises, especially when many seasonal weather or=20 climatological records are considered. Monthly statistics of various = weather=20 elements are compiled. A separate set of records would have to be = produced for=20 the astronomical seasons that begin on the solstice and equinox dates. = These=20 calendar dates vary slightly from year to year because of the inclusion = of the=20 "leap year day" to account for the slight difference in length between = the civil=20 year and the solar year. Furthermore, the elliptical orbit of the earth = about=20 the sun causes the lengths of the astronomical seasons to vary between = 89 and 93=20 days.

By international convention, meteorologists have defined = "meteorological=20 seasons" in terms of three-month intervals that are centered upon the = typical=20 occurrence of the warmest and coldest months of the year. By this = convenient=20 definition, meteorological spring consists of the months of March, April = and=20 May; summer contains the warmest months of June, July and August; autumn = is=20 September, October and November; and meteorological winter consists of = the=20 coldest months of December, January and February. The seasonal length is = more=20 uniformly divided, ranging from 90 days in the winter of a non-leap year = to 92=20 days in spring and summer. Seasonal statistics can then be determined = easily=20 from the monthly statistics.

OTHER TYPES OF "SEASONS"

Several other designations of seasons have been developed by = meteorologists=20 to aid in the handling and interpretation of weather data for specific = purposes.=20 Some of the commonly used seasonal designations include:

Return to the Thursday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =