A season is a division of the year marked by distinct changes in weather patterns and the duration of daylight. The exact definition of when one season begins is not universally fixed, leading to confusion over which months belong to which season. This variation exists because seasons can be defined either by fixed calendar dates or by specific astronomical events in Earth’s orbit.
The Scientific Reason for Seasonal Change
The Earth’s orbit around the Sun is the primary factor, but the cause of seasons is not the changing distance between them. Seasons occur because the Earth rotates on an axis tilted approximately 23.5 degrees relative to its orbital plane, a characteristic known as axial tilt.
As the Earth revolves around the Sun, this tilt causes each hemisphere to receive varying intensities of sunlight and differing lengths of daylight. When the Northern Hemisphere is tilted toward the Sun, it experiences summer due to more direct solar radiation and longer days. Conversely, when tilted away, the solar rays strike at a lower angle, resulting in the lower temperatures of winter.
How We Define the Start and End of Seasons
The two most common methods for defining seasonal dates are the astronomical and meteorological systems. The astronomical definition is rooted in specific celestial events that reflect the Earth’s position relative to the Sun. These events are the solstices, marking the longest and shortest days of the year, and the equinoxes, when day and night are nearly equal in length. Because the Earth’s orbit is slightly elliptical, the exact timing of these events shifts slightly each year, meaning astronomical seasons do not begin on a fixed date.
The meteorological definition relies on fixed calendar dates, established for easier weather record-keeping and statistical comparison. Meteorologists group the seasons into three-month blocks to align with the annual temperature cycle. This approach makes it simpler to compare seasonal data year over year, as the length of the meteorological seasons is more consistent.
Seasonal Months in the Northern Hemisphere
The meteorological calendar for the Northern Hemisphere uses the warmest and coldest three-month periods. Meteorological spring includes March, April, and May, and summer encompasses June, July, and August. Autumn covers September, October, and November, while winter consists of December, January, and February. This system groups months with similar temperature profiles.
The astronomical seasons are tied to the precise moments of the solstices and equinoxes. For the Northern Hemisphere, spring begins around March 20th with the vernal equinox, and summer starts around June 21st with the summer solstice. Autumn begins around September 22nd with the autumnal equinox, and winter starts around December 21st with the winter solstice. The exact day of these transitions may vary annually due to the difference between the calendar year and the astronomical year.
How Seasons Are Measured Globally
The four-season model is most relevant in the temperate zones. In the Southern Hemisphere, the seasons are completely reversed compared to the North. For instance, meteorological summer in the South runs from December 1st to February 28th, while winter is from June 1st to August 31st. The December solstice marks the beginning of astronomical summer in the Southern Hemisphere, while signaling the start of winter in the North.
Near the equator, the Earth’s axial tilt has a much smaller effect on day length and temperature. These tropical areas generally do not experience four distinct, temperature-driven seasons. Instead, the year is often divided into two primary seasons: a wet or rainy season and a dry season. These cycles are governed by shifts in atmospheric patterns, such as the movement of the tropical rain belt, rather than changes in daylight duration.