As a tree grows, it may produce growth rings as new wood is laid down around the old wood. In areas with seasonal climate, wood produced at different times of the year may alternate light and dark rings. In temperate climates, and tropical climates with a single wet-dry season alternation, the growth rings are annual, each pair of light and dark rings being one year of growth. In areas with two wet and dry seasons each year, there may be two pairs of light and dark rings each year; and in some (mainly semi-desert regions with irregular rainfall), there may be a new growth ring with each rainfall.