Acclimatization generally refers to the ability of living things to adjust to changes in climate. Acclimatization usually occurs in a short time, within the organism's lifetime. Many plants, such as maple trees, irises, and tomatoes, can survive freezing temperatures if the temperature gradually drops lower and lower each night over a period of days or weeks. The same drop might kill them if it occurred suddenly. This process is called hardening.
Scientists believe several events cause hardening.
Hardening may result from a decrease in the water content and an increase in the sugar content of the plant. This keeps the plant from freezing. Animals acclimatize in many ways. Sheep grow very thick wool in cold, damp climates. Most human beings become short of breath and tire easily when they move to higher altitudes.