The Scientific Basis for Defining Seasons

Updated: Nov 8, 2016

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The Scientific Basis for Defining Seasons

Ancient and Modern Meteorology

Meteorological phenomena such as storms, lightning, and thunder are often considered dangerous and frightening for many people. The ability to predict their occurrence in the past was valued in many cultures. In fact, there is a large body of literature that spans the ancient period dedicated to the prediction and explanation of meteorological and astronomical events including animal behavior.  The division of the year into seasons enables the ancient culture to select the proper time plowing, sowing, reaping, and harvesting, making voyages with ships, prepare and protect themselves for against the natural severity of the seasons.

However, ancient meteorology was largely based on past experience or preservation of the events of past weather and their sequence, speculations on the relation between events and proximate causes, philosophy, appeal to the deities, and belief in personal weather-control and magic.  The practice was only changed in the mid 19th century when a collection of facts has been operated by electric–telegraph and now radiotelegraphy.  Although recording observation and discussing them from a philosophical point of view still remains to this day, meteorologist today have a multitude of facts and records to base their forecast and in creating a weather map.   

Weather and Seasons

Weather maps are important tools for developing and explaining weather forecasts. They vary in scope as there are national, continental, and global weather maps. Surface weather maps commonly contain identified fronts; high and low-pressure areas and temperatures, wind strength and air pressure and often used to provide information on the present weather condition.  Forecast weather maps, as the name suggests, are computer generated maps to predict changes in the weather.  Meteorologist also developed short-term (18-36 hours) and long-term (5 days) maps that are updated daily.


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However, since weather condition is rapidly changing, the weather forecast may not always be accurate and mere honest guesses of the weather condition for a certain period of time. The above surface map illustrates four different fronts – cold, warm, stationary, and occluded.  Note that these fronts separate different air masses and associated with lower pressure. A meteorologist using this map will likely forecast that weather along the stationary front (points A and B) will be clear to partly cloudy. The weather condition in the cold front (points B to C) will be cold, rainy in green-shaded areas, and snowy in the white shaded areas.

Contrary to common belief, modern meteorology recognized that the beginning of any season is the day on which the sun passes over particular latitude rather than how cold or warm the following will be.  For instance, the first day of summer in the Northern Hemisphere is June 21 or when the sun is at its highest position in the noonday sky or 23 ½ degrees north (N) latitude. Each day past June 21, the noon sun is slightly lower in the sky as 12 hours or more of daylight in the Northern Hemisphere begins to shorten.

By September 22, the sun is directly above the equator and the astronomical beginning of fall officially begins in the Northern Hemisphere.  Three months after the autumnal equinox, the sun on December 21, is further away from the tilted Northern Hemisphere where days are shorter and nights are longer.  On this day, the sun shines directly above latitude 23 ½ degrees S (Tropic of Capricorn) or the first day of winter in the Northern Hemisphere. The first day of spring occurs around March 20 or when the sun crosses the equator and moving northward.

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