Lighter Than Air

(relaxing music) The Earth and its yearly trip around the Sun tracks a seasonal cycle we all know well. The currents of air high above us or jet streams circling in our atmosphere we may not be so familiar with, but like many cycles in nature, one affects the other. They are interconnected and they remind us that whether we're conscious of it or not, changes are continuously happening. Nothing stays the same. That's impermanence. The Earth is surrounded by an atmosphere made up of five layers. Water and dust are found in the layer closest to the surface of the Earth, which is where clouds form and weather occurs. This layer extends approximately 10 miles up from the surface of the Earth. Above that are layers where ozone abounds, where meteors burn up in, where the Aurora Borealis effect occurs, and finally, where our atmosphere touches outer space. In the upper part of that innermost layer seven miles high or so, there are ribbons of wind called jet streams, two main ones in the Northern Hemisphere and two in the Southern Hemisphere. All of them travel like waves with peaks and troughs, continually circling the Earth, and all flowing west to east, traveling in the opposite direction to the Earth's rotation on its axis. Take, for instance, the polar jet stream, traveling over North America, moving masses of air and moisture and storm systems along with it. When the jet stream dips southward, it brings a pocket of cold air down. And if conditions nearer the ground are right, precipitation like snow, hail, or sleet can occur. Conversely, when it rises northward, it brings a pocket of warm air up, and again, with the right conditions, it can lead to a heatwave. If the jet stream motion slows down or temporarily stalls out, then a long cold snap can happen in winter, or the hazy, hot, and humid weather of summer occurs. It can last day after day after day. Our seasons are determined by the relationship of the Northern Hemisphere to the Sun. Because our Earth tilts on its axis, the Northern Hemisphere alternates between being further from the Sun in winter and closer to the Sun in summer. Jet stream activity and its speed are determined by air temperature differences high above the North Pole and the Equator. So in the winter, when the air is colder above the North Pole and that temperature difference is therefore greater, the jet stream moves faster and weather events can be more active. That means that the jet stream will have steeper peaks and troughs, bringing larger cold air masses southward to some parts of North America, and larger warm air masses northward elsewhere. Jet streams move fluidly around the Earth, and the air masses that they move along with them are in a dynamic balance. Whenever a more northerly mass of air dip south, a southerly air mass rises north. There is a rhythm to the motion, a lead...