Seasons: Meteorological Versus AstronomicalSeasons: Meteorological Versus Astronomical

Seasons are periods in a year marked by specific weather conditions, temperatures and length of day. Most modern day calendars around the world divide the year in 4 seasons: spring, summer, fall (autumn) and winter.

The dates of when these seasons begin and end can vary depending on who you ask. People in Australia and New Zealand for example, consider September 1 as the beginning of spring. The Irish on the other hand believe that spring begins on February 1, when they celebrate St Brigid's Day. Some cultures, especially those in South Asia have calendars that divide the year in six seasons, instead of the four that most of us are familiar with.
Note: The following information applies only to the Northern Hemisphere. Seasons in the Southern Hemisphere are opposite to the seasons in the Northern Hemisphere.

Astronomical Seasons

Astronomers and scientists use the dates of equinoxes and solstices to mark the beginning and end of seasons in a year.
There are 4 astronomical seasons in a year:

• spring - March Equinox to June Solstice,
• summer - June Solstice to September Equinox,
• fall (autumn) - September Equinox to December Solstice, and
• winter - December Solstice to March Equinox.

Because the dates of the equinoxes and solstices can change, the length of astronomical seasons within a year and between years can vary, making it difficult to properly compare and study seasons in different years.

Equinoxes and solstices dates worldwide

Meteorological Seasons

In order to be consistent and to make forecasting easier, meterologists divide the year into 4 meteorological seasons of 3 months each:

• spring - starting March 1 and ending May 31,
• summer - starting June 1 and ending August 31,
• fall (autumn) - starting September 1 and ending November 30, and
• winter - starting December 1 and ending February 28 (Februrary 29 in a Leap Year).

Summer in the North, Winter in the South

Seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere. For example, under the definition of astronomical seasons, the June Solstice marks the start of summer in the Northern Hemisphere but the start of winter in the Southern Hemisphere. The same rule applies for the other three seasons. Similarly, the meteorological seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere.

Earth's Rotational Axis

Astronomers use equinoxes and solstices to set the start and end dates of the seasons.

Seasons occur because of the 23.5 degree tilt of the Earth's rotational axis. Around the June Solstice, the Northern Hemisphere is titled towards the sun. This causes summer there. The Southern Hemisphere on the other hand, is tilted away from the sun and therefore, experiences winter. The opposite occurs around the December Solstice, when the Southern Hemisphere is tilted towards the sun, while the Northern Hemisphere is tilted away