A factor of inanimate nature that gives impetus to seasonal changes in the life of plants and animals.

In spring and autumn, under the same weather conditions (temperature, humidity, etc.), opposite phenomena occur in nature. For example:
– in the spring, the buds of the plants bloom, and in the fall at the same temperature, the leaves fall off;
– in the spring, bears, hedgehogs, badgers wake up from hibernation, and in the fall – prepare for it;
– in May, swallows return from the south, and at the beginning of autumn, when there is still a lot of food and warmth, they gather in flocks and fly away to the wintering place. The signal for seasonal changes is the length of daylight hours.