The jet stream is familiar to most people: It’s a river of air that circles the globe in mid-latitudes at heights of 5,000 to 40,000 feet. The jet stream arises from the need of the atmosphere to balance its energy account, which involves heat moving poleward.
Zonal and Meridional Flow
We can separate the wind into two components: the east-west component and the north-south component. West to east wind is called ‘zonal’ and north to south wind is called ‘meridional.’ The meridional wind transports heat from equator to pole.
The Jet Stream Mechanism
There is no definitive physical explanation for the jet stream other than the following: The equations governing the atmosphere require a solution, and there is no mathematical one. On a spinning globe with differential heating between poles and equator, the empirical solution (the one we observe) is a circumpolar zonal flow at middle and upper levels, with an embedded jet stream in mid-latitudes. At the surface, there are three cells: westerlies in mid-latitudes and easterlies near the equator and poles. All of this creates a convective (caused by movement of air) balance. Warm air is continually transported toward the poles and cold air is transported towards the equator.
Jet Stream Flow: Could We Simplify, Please?
If the flow of the jet stream were just west to east, not much would happen. It would be like two tennis players hitting the ball back and forth on the same trajectory forever. If one of the players hits the ball near the side of the court, things get more interesting. So it is with the jet stream. When there is a dip (trough) or peak (ridge) in the flow, weather starts to happen and the flow starts transporting heat.
The General Circulation of the Atmosphere
In its simplest form, energy transfer is accomplished as follows: When warm and cold air lie side-by-side, the warm air wants to rise and the cold air wants to sink, lowering the potential energy and converting it to kinetic energy of wind. This process begins at the surface, works up to the zonal flow, transfers energy poleward and then back to the surface, where the wind is turned back into heat by friction with the ground. The key to all this is a Maxwell’s Demon that fuels the jet stream.
Maxwell’s Demon is the term used for a method of separating fast and slow-moving molecules in a gas. If you could figure a way to let only the fast-moving molecules flow, you could theoretically make a perpetual motion machine. Within a closed system, it’s impossible (and not for lack of people trying). But the atmosphere, not a closed system and fueled by constant differential heating (because of the angle of the sun) between equator and pole, has found a way to do it. The alignment of troughs and ridges is such that the air with a faster zonal component is moving poleward, keeping the jet stream zooming along.
Think of a letter ‘U’ (in the northern hemisphere) tilted so that the top is to the right (east). As air passes through the U, the molecules with the greatest zonal components are transported northward.
Venusian and Jovian Zonal Flow
The weather on Earth can be inconvenient at times, but we should be thankful that the energy transfer mechanism is relatively benign. It needn’t be. It’s not bad on Venus: The jet stream races at close to 300 miles per hour, but at the surface there’s barely a breath of air. And you wouldn’t want to take one if there were.
On Jupiter, by contrast, the zonal winds are over 300 miles per hour, but more importantly they give rise to storms with winds up to 400 miles per hour. You wouldn’t want to live there.