As the temperatures across the northern hemisphere drop, the landscape is taking on a much more colourful appearance.
So far the most vibrant autumnal displays have been in Europe. In North America and northern Asia, the leaves still have to reach their peak in intensity.
The vivid colours are caused by the disappearance of the green substance chlorophyll, which is produced when the leaves are producing energy for the tree.
As the days get shorter and the temperature drops, a cork-like substance forms at the base of each leaf. This cuts off the supply of water to the leaf, as well as the supply of energy back to the tree.
Without a supply of water, the chlorophyll disintegrates and colours that were in the leaves all along are suddenly visible.
The intensity and the longevity of colourful display depends on the weather.
If the second half of the summer is predominantly dry, and autumn is sunny and cool, then there is a strong likelihood that the leaves will be vivid in colour.
But an early frost could cause the leaves to drop from the trees, and a large storm would strip the trees bare within minutes.
The uncertainty of the autumnal display explains why a bright, vibrant display is such popular spectacle.