Uneven heating can induce unequal flow distribution between the channels, which is undesirable in heat sinks as the channels starved of flow may undergo a premature dry-out, thereby impairing their heat transfer performance, and limiting predictability and reliability. Unequal heating of the Earth's surface also forms large global wind patterns. In area near the equator, the sun is almost directly overhead for most of the year. Warm air rises at the equator and moves toward the poles. At the poles, the cooler air sinks and moves back toward the equator. Because Earth is a sphere and tilted on its axis, different regions on Earth receive different amounts of energy from the Sun. This uneven heating causes Earth's surface and atmosphere to be warmer near the equator than near the poles. In the atmosphere, warmer air rises as cooler air sinks.