The Seasons
One
finds disagreement on the starting dates of the seasons at both the
scientific and the lay level. There are however three basic ways in
which starting dates may be assigned. South Africa does not really
experience four distinct seasons. Throughout South Africa the
transitional seasons of Autumn and Spring tend to be very short. Most
analysis of climate is done using the assumption that January is
mid-summer and July min-winter.
Astronomical basis
The
instances at which the solstices and equinoxes occur can be accurately
calculated. Earlier astronomical textbooks often defined the four
seasons as starting on the dates of the corresponding equinoxes and
solstices. But more recent books avoid defining the seasons in any way:
two new editions of earlier books have in fact deleted their previous
definitions. Moreover, expressions such as spring equinox and summer
solstice are no longer used in astronomy. Instead, the four astronomical
instances are identified as the ascending and descending equinoxes in
March and September respectively, and the northern and southern
solstices in June and December. One reason for these changes is to avoid
the inevitable confusion of northern and southern hemisphere seasons:
the ascending equinox in March is the spring equinox in the northern
hemisphere, but the autumnal equinox in the southern hemisphere.
Thus
the earlier astronomical definitions of the starting dates of the
seasons are no longer relevant, and their use should be discouraged.
Climatological basis
A
further failing of the earlier astronomically defined seasons is that
they simply did not describe the real seasons as actually experienced.
As one of the contributors to this article expressed it, summer does not
start four days before Christmas. A climatologically definition of the
seasons would obviously be more realistic. In the temperate latitudes of
Europe and North America the climatological seasons are conventionally
defined as shown in Table I. The equivalent seasons in the southern
hemisphere are of course six months out of phase with those in the
northern hemisphere, and are also given in Table I.
The
use of intervals of exactly three calendar months for the conventional
temperate latitude seasons is a matter of convenience rather than
climatological reality. For example, in England the latter part of
November is wintry rather than autumnal, with cold, foggy days occurring
fairly frequently. Elsewhere in the world the disparity becomes even
worse. Climatologists therefore ignore the conventional seasonal
nomenclature and use labels which are more appropriate to the climate of
a particular region: for example hot season, cold season, post-rainy
season, etc. The durations of these seasons depend on the climate of the
region, and have no direct relationship to either the astronomical
seasons or the calendar months. Thus September is spring-like in
Gauteng, with cool mornings and warm afternoons, whereas it is still
winter-like in the Western Cape, with the possibility of snow on the
Eastern Cape mountains. Unfortunately the lay public would find it too
confusing if a different set of seasonal dates was adopted for different
parts of the country in order to encompass this variability.
Phenological basis
Phenological
phenomena (this is, those relating to the natural seasonal behaviour of
plants and animals) are the most fundamental markers of the changing
seasons. This can be seen from the etymology of the names of the seasons
in various languages. Thus in English, spring, from Anglo-Saxon for
rise or burst forth, is the season when sap rises and plants put out
buds. Autumn, from early Latin for ripen, is the season when crops reach
maturity and can be harvested.
Unfortunately,
an appeal to the phonological seasons merely confounds the confusion.
In parts of Europe the phonological seasons are taken to occur one month
earlier than the conventional climatic season, where in other parts and
in the USA the two systems coincide. In any case, May Day (1st May)
rather than the first day of spring however that may be chosen seems to
be the preferred date for celebrating this season in Europe.
In South Africa, the wide range of climatic regions and phenotypes (compare coastal KwaZulu-Natal with the south-western Cape) also adds to the difficulty of defining a clear-cut seasonal calendar based on phenology.
In South Africa, the wide range of climatic regions and phenotypes (compare coastal KwaZulu-Natal with the south-western Cape) also adds to the difficulty of defining a clear-cut seasonal calendar based on phenology.
Conclusion
Apart
from rejecting the astronomically-based seasons in compliance with
modern astronomical usage, there are no firm grounds for choosing one
set of dates rather than another for the starting dates of the seasons.
There is certainly no official designation of the starting dates. On
broad climatological and sociological grounds, however, choosing the
dates in Table I would have the advantage of conformity established
conventions.
On the basis of these conclusions, the following recommendations are suggested:
Adopt the seasonal calendar given in Table I for the southern hemisphere.
Emphasize strongly that these are conventional or traditional dates, and that an official calendar does not exist.
Adopt the seasonal calendar given in Table I for the southern hemisphere.
Emphasize strongly that these are conventional or traditional dates, and that an official calendar does not exist.
Bureau of Meteorology, major climatic zones
Approaching
cyclonic storm east of the Serpentine Lakes, Great Victoria Desert, on
the border of South Australia and Western Australia. Image courtesy of
CSIRO Land and Water.
Mataranka tropical pool, Northern Territory
Cyclone Yassi hits Townsville, 2011, Climate Commission
Trees
shaped by the weather along a track near Margaret River crossing,
c1920. Image courtesy of National Library of Australia: an24431083.
