The heavy water to light water ratio in ice cores increases as temperature increases. This occurs because heavy water and light water evaporate and condense at different rates. Light water evaporates more readily than heavy water, so water vapor in the atmosphere generally has a low heavy water to light water ratio. However, as temperature increases, evaporation of both heavy water and light water occurs more readily, so the abundance of heavy water in the atmosphere increases. Then, because heavy water condenses more readily than light water, the ratio of heavy water to light water in precipitation begins to increase. Some of this precipitation that contains a greater amount of heavy water falls on glaciers where it becomes incorporated into the ice that is eventually taken as part of an ice core sample. Therefore, the section of the ice cores from the time when the precipitation fell will contain water molecules with a higher heavy water to light water ratio, indicating warmer temperatures.