The Seasons

Astronomical, Meteorological and Phenological Seasons

 The duration of a season and when it commences are both defined in different ways for different purposes.

Astronomical seasons are based on the position of the Earth in relation to the Sun. The orbit of Earth around the Sun forms the basis for the astronomical calendar in which seasons are defined with two solstices and two equinoxes. Earth’s tilt and the Sun’s alignment over the equator determine both the solstices and equinoxes National Centers for Environmental Information (NCEI) (noaa.gov).

Meteorological seasons are based on the annual temperature cycle. Meteorologists and climatologists break the seasons down into groupings of three months based on the annual temperature cycle as well as the Gregorian calendar. In Australia, meteorological autumn includes March, April, and May; meteorological winter includes June, July, and August; meteorological spring spans September, October, and November; and meteorological summer is over December, January, and February.

People have also used observable natural phenomena to mark time for thousands of years. Phenology is the study of cyclic and seasonal natural phenomena, especially in relation to climate, plant and animal life. Depending on where you are in the world different phenological markers can be used to define the seasons, such as when the first leaf falls, the first frost or the first spring growth appears.

The dry season occurs between May and September in the southern hemisphere. During this time of the year, the Sun is located over the northern hemisphere, so the southern hemisphere receives less heat and starts to cool down. Tropical regions closer to the equator experience smaller variations in temperature so it is not the temperature that distinguishes the wet and dry seasons but the amount of rainfall The wet and the dry: seasons in the tropics - Social Media Blog - Bureau of Meteorology (bom.gov.au).   

Australia’s national science agency, CSIRO, reports Australian rainfall is highly variable and is strongly influenced by phenomena such as El Niño, La Niña, and the irregular oscillation of sea surface temperatures in the Indian Ocean. This natural variability now occurs on top of a background warming trend with varied changes across different regions being expected.

Seasonal cycles as described by Indigenous peoples differ substantially according to location. Natural barometers are not uniform across the land, so the reaction of plants and animals are used to gauge what is happening in the environment and the names of seasons are often dependent on localised events or resources.

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