Some Global Factors Influencing Seasons

Globally, seasons are not so easily defined as for mid-latitude locations, for which astronomical seasons are reasonable. The main reasons for such are that solar elevation angle and day length do not change much at low latitudes. As most everyone is aware of, seasonal temperature variations are small near our equator, increasing greatly toward our poles; as indicated from monthly averages for Jakarta, Indonesia and Dzardzan, Russia (in Siberia) obtained from Lamont-Doherty Earth Observation's interactive climate server. Near the Tropics (i.e., Capricorn & Cancer) over land, enough temperature change occurs such that seasons can be defined by them, as indicated in a similar graph for Alice Springs, Australia.

Thus, tropical seasons are most effectively defined according precipitation; a wet season or alternating wet and dry seasons often occurring. Global surface pressure regimes indicate several features, among which are semi-permanent subtropical Highs (pressure areas), and a semi-continuous Low near the equator, often called the monsoon trof. Thus, a pressure gradient exists, causing the easterly winds toward the equator to prevail in tropical regions, and westerly winds toward the pole in mid-latitude regions. The Intertropical Convergence Zone (ITCZ) is the location of convergence of NE and SE trade winds. It is not a permanent nor continuous feature, but its effects can often be seen on global satellite images (main site) as convection in a semi-continuous cloud and/or precipitation band near the equator (typically much more noticeable during July-December).

A precipitation maximum occurs once during an average year at each location, about a month or 2 after our sun is most directly above a location at more poleward locations (after establishment of monsoons), and near the time when ocean temperatures are maximum at locations near the equator. Thus, rather than 4 seasons, 2 basically occur - a wet season and a dry (or less wet) season. The wet season often occurs for several months, approximately coinciding with astronomical Summer in each hemisphere. Locations in India are well-known for their noticeable wet season as monsoon winds flow from the Indian Ocean.

Such a wet season does not occur at all tropical locations though. E.g., northern coastal Chile is notorious for lack of rainfall, measurable rain not being reported at some locations for decades (and even centuries !). Consider 2 locations near each other, Arica, Chile (A) and La Paz, Bolivia (B). Precipitation difference is remarkable, caused by the consistent easterly winds (SE trades) over the region, flowing up & down the Andes Mountains. Though in the tropics, a small temperature variation is all that can define seasons in Arica, and a very small precipitation maximum (perhaps caused by westerly jet streams very occasionally affecting the region during the Southern Hemisphere Winter).

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Thus, latitude, land/sea areas, and topography largely determine climate and consequently seasons. At middle and high latitudes, Spring and Autumn are transition seasons between much longer warm and cold seasons, the change more abrupt nearer the poles; and tropical seasons tend to be caused by precipitation more so than temperature, wet and dry (or less wet) seasons being typical. Topography and global wind regimes can locally influence such factors quite significantly.

Text and all graphics named seag-___.gif are copyright of Joseph Bartlo, though may be used with proper crediting. <