WX BRIEF

Jet streams

Ribbons of fast moving air drive weather and affect flight times.

By Karsten Shein
Comm-Inst, Climate Scientist


Streamline map of the polar and weaker subtropical jets over North and Central America. The strongest winds, or jet streaks, are normally found in the exit region of troughs in the flow, while weakest winds typically occur in the entrance to ridges.

Taxiing out from the FBO, the flightcrew saw the worry on their boss's face. The early morning fog had been thick enough to shut down their planned departure to the closing of a major deal for the company. Though they'd still have an hour or so to spare when they landed, the airport was well outside downtown and the late departure would mean heavier traffic on the highway.

The copilot had gotten a good weather briefing that morning, and the fog delay had given the crew additional time to pour over the upper air charts. They saw that if they cruised up to FL350 and took a more southerly route from what they intended, the extra climb and distance would be more than made up as they hooked into a 180-kt jet streak that had strengthened since they had looked at the maps the night before. Despite a nearly 2-hr delay in taking off, they'd only arrive about 30 minutes after they'd planned.

Few words conjure up the idea of fast travel more than the term "jet stream." Theorized since the 1800s, the concept of a strong current of air high in the atmosphere gained attention after the 1883 eruption of Krakatoa, which produced a massive ash cloud that circled the equator.

Though Japanese Meteorologist Wasaburo Oishi was able to prove the existence of jet streams by launching balloons from Mt Fuji, it wasn't until the 1930s that high altitude flight started to occur. Wiley Post, wearing pressurized flight suits of his own invention, made several flights to altitudes at which he noted his ground speed was significantly greater than his indicated air speed.

The term "jet stream" was coined in 1939 by a German meteorologist. But ironically it was American and British ferry pilots who most advanced our understanding of these "Strahlstömung" or "jet currents" as they used them to speed aircraft from US factories to the European theater of WWII over the next few years.

By 1944, US Army Air Corps meteorologists on Guam were able to forecast the jet stream over the Pacific which had been reducing the range of American bomber flights headed for Asian targets. Today, the accurate forecasting of jet stream position and strength is critical to most commercial and business aviation operations.

What causes jet streams?

Side view of the 3 main circulation cells operating in the northern hemisphere. The polar and subtropical high-altitude jet streams occur in the upper levels where the cells meet.

Jet streams, or simply jets, are currents in the atmosphere that are produced where air density changes quickly over a short distance. Jets can occur in most parts of the troposphere – the lowest and densest layer of the atmosphere. Depending on the characteristics of the density discontinuity, jets can be weak or strong, short or long, low or high. And they can become interrupted, split into divergent branches, come together, and meander across the sky in waves of varying amplitude and wavelength.

The cause of the density differences is very often due to temperature and the uneven heating of the Earth by the Sun. Heat excites the air molecules, forcing them farther apart, therefore reducing air density. Cold does just the opposite. The difference in density is measured as a difference in pressure, another related variable.

In a given volume, higher density equates to higher pressure, and lower density to lower pressure. The difference in pressure drives the movement of air molecules (or wind) from areas of high pressure to areas of low. The rate at which that happens (wind speed) is directly related to the pressure difference, which is known as the pressure gradient force.

The Coriolis effect is another factor at play in winds. Because Earth is rotating, any object on a straight trajectory through the air will exhibit a curved trajectory relative to the movement of Earth below. The end result is that, though the movement of an air molecule might start out heading directly toward lower pressure, the Coriolis effect diverts the path to the right in the northern hemisphere and to the left in the southern.

At the surface, friction ensures that the pressure gradient force always exceeds the Coriolis effect and the air eventually spirals into the lows. But aloft, the 2 forces balance each other until the air is flowing perpendicular to the pressure gradient in what is known as geostrophic wind.

Upper air jets

View of the northern hemisphere polar jet stream in a 300 hPa model forecast. Diverging flow and areas where large waves are moving through the current are also places where pilots should expect turbulence and, at lower levels, a potential for adverse weather.

Near the top of the troposphere, there are 2 main regions of density discontinuity in each hemisphere. These areas are the boundaries of 3 primary vertical circulation cells circling the planet.

On either side of the equator are the Hadley cells. Equatorial heating lofts air that must diverge poleward when it hits the stratosphere's temperature inversion.

As it moves poleward, it converges with air from the Ferrell cell operating over the middle latitudes. The Ferrell cell is opposite to the Hadley cell in that it is driven by air being forced aloft by overriding the polar front – the boundary between the warmer midlatitude air and the cold polar air that is being driven equataorward at the surface by a high near the poles.

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