When commercial airplanes break through the clouds to reach cruising altitude, they have typically arrived in the stratosphere, the second layer of Earth’s atmosphere. The air up there is dry and clear, and much calmer than the turbulent atmosphere we experience on the ground.And yet, for all its seeming tranquility, the stratosphere can be a powerful conveyor belt, pulling air up from the Earth’s equatorial region and pushing it back down toward the poles in a continuously circulating pattern. The strength of this circulation can significantly impact the amount of water vapor, chemicals, and ozone transported around the planet.