Autumn

Autumn is one of the four seasons in regions that have a temperate seasonal cycle. In the Northern Hemisphere, it generally occurs between summer and winter, while in the Southern Hemisphere it falls between summer and winter at the opposite time of year. The exact dates vary according to the calendar system and the definition being used.

Astronomically, autumn begins at the September equinox in the Northern Hemisphere and at the March equinox in the Southern Hemisphere. During an equinox, day and night are approximately equal in length. As the season progresses, the amount of daylight decreases in the Northern Hemisphere, temperatures often become cooler, and the Sun follows a lower path across the sky.

One of the most noticeable signs of autumn is the change in many deciduous trees. As daylight decreases and temperatures fall, trees prepare for winter and gradually stop producing chlorophyll, the green pigment involved in photosynthesis. Other pigments that were already present in the leaves can then become more visible, producing yellow and orange shades.

In some species, additional pigments called anthocyanins contribute to red or purple colors. Eventually, many leaves dry out and fall. Autumn also affects animals and ecosystems: some species migrate, others gather food or store energy, and many plants prepare for colder conditions. The season therefore marks a period of transition between the warmer conditions of summer and the colder conditions that follow.