Season

A season is a division of the year based on changes in weather, ecology, and daylight hours. These cycles help us understand the rhythm of our planet. In many temperate regions, we recognize four distinct seasons: spring, summer, fall, and winter.

However, the way we define seasons depends on where you live. Tropical regions often experience only two seasons: a wet season and a dry season.

Some ecologists use a more complex six-season model to describe nature. These models include stages like prevernal, vernal, estival, serotinal, autumnal, and hibernal.

Seasons are caused by the Earth's axial tilt and its orbit around the Sun. The Earth's axis is tilted at an angle of approximately 23.4 degrees. This tilt is also known as the obliquity of the ecliptic. As the Earth moves along its orbital plane, it exhibits axial parallelism. This means the direction of the axis remains parallel to itself throughout the year. The axis points toward Polaris, the North Star, all year long.

Because of this tilt, different parts of the Earth receive different amounts of sunlight. When a hemisphere leans toward the Sun, it experiences summer. The opposite hemisphere leans away and experiences winter.

The intensity of sunlight changes based on the angle at which it hits the Earth. During summer, the Sun is higher in the sky. This increases the solar flux, which is the amount of solar energy reaching a surface. In winter, the Sun's rays fall at a shallow angle. This causes the light to spread over a larger area. Consequently, the light is more indirect and has lower intensity. This change in light density and daylight hours accounts for most seasonal climate variations.

There are two ways to measure seasons: astronomically and meteorologically. Astronomically, a season is the time between an equinox and a solstice. An equinox occurs when the Sun is directly overhead at the Equator. During an equinox, day and night are equally divided between the hemispheres. A solstice marks the maximum or minimum amount of daylight. Meteorologically, seasons are defined as quarters of the annual temperature cycle. These meteorological seasons are centered on characteristic temperatures. They actually trail behind the equinoxes and solstices due to seasonal lag.

Seasonal lag is a phenomenon where the warmest months occur after the summer solstice. This happens because oceans take time to absorb and release heat. In the Northern Hemisphere, the warmest months are usually June, July, and August. In the Southern Hemisphere, the warmest months are December, January, and February. This lag explains why the peak temperature does not align perfectly with the peak sunlight. The thermal absorption of large bodies of water creates this delay in the temperature cycle.

Humanity has tracked these cycles for thousands of years. Ancient Egyptians defined seasons by the flooding of the Nile. They used three seasons: flood, growth, and low water. In India, six seasons called Ritu are used for agriculture and trade.

Even in Roman times, people used a four-season model. The writer Varro described specific dates for the start of each season. These historical definitions were vital for agrarian societies. They relied on seasonal changes to know when to plant and harvest crops.

While the tilt is the main cause, Earth's elliptical orbit also plays a small role. Earth reaches perihelion, its closest point to the Sun, in January. It reaches aphelion, its farthest point, in July. This creates a 7% variation in solar intensity. However, this effect is small and often opposes the temperature trends in the Northern Hemisphere. The Northern Hemisphere has more land, which warms more quickly than the sea. In contrast, the Southern Hemisphere has more water, which helps moderate its seasons.