The lowest level of the atmosphere, the troposphere, contains almost all of Earth's weather. In the stratosphere above, moisture drops to almost zero. The boundary that separates these two layers—the tropopause—is defined as the point at which water ceases to cool as altitude increases. In the tropics, the tropopause is exceptionally cold and usually occurs at a higher altitude (around 17 kilometers, or about 10 miles) than in polar regions. The atmospheric conditions at these heights play an important role in global climate, as they determine the limits of cloud formation and water vapor dynamics.