Ribbons of fast moving air drive weather and affect flight times.
Cirrus clouds reveal the position of a strong polar jet over the Canadian coastline in this 2003 image captured from the Space Shuttle. Air moving through the jet can produce vertical motion that may generate thin clouds that are shredded by the strong windshear around the jet core.
All along the vertical boundaries between the cells there is a pressure gradient, and winds are flowing in a general westerly path along the boundary. But, near the top of the troposphere, there is a much stronger density difference and therefore pressure discontinuity as the warmer air folds over the top of the colder air.
This creates a region where the pressure gradient can become very intense over a very short distance, producing a core within which the geostrophic wind can accelerate to over 100 kts in a very short distance. Speeds within the jet core have been measured at as high as 215 kts.
The strongest pressure difference is found between the Polar cell and Ferrell cells, an area that's home to the polar jet – often labeled the polar vortex by the popular press. It tends to occur between about 23,000 and 39,000 ft MSL. Because of the additional heating and smaller pressure differences, the subtropical jet is both weaker and higher, occurring between about 33,000 and 52,000 ft MSL. Both jets occur at around 250 hPa of atmospheric pressure, just below the tropopause where air temperature stops dropping with increased altitude.
Out of contact with the ground, the paths of the polar and subtropical jets are determined by synoptic scale upper air pressure patterns that result in a change in the Coriolis effect and produce a deviation from the original path. High pressure aloft poleward of the jet will produce a trough, while an upper air high equatorward of the jet will create a ridge. These waves, called Rossby waves, once established, tend to migrate in the same direction as the jet, but at a slower speed. In some cases, semipermanent highs will keep a Rossby wave in place for days or even weeks.
A jet that encounters an upper level low, however, often will split, with its main branch going poleward, but a troughing branch moving around the equatorial side of the low. It is often these troughing branches that become the most intense.
As the jet stream flows from ridge to trough, air is converging. As with any fluid, convergence requires acceleration to maintain density. Coming out of the trough, the air is able to diverge and slow. This process means that the strongest jet stream winds are normally found within the base and exit region of the trough as jet streaks.
Places where the jet changes direction, splits, or joins are also locations where strong to severe turbulence may occur. A good rule of thumb is that the greater the change in the jet, the stronger the turbulence. Clear air turbulence (CAT) is also occasionally encountered on the cold air side of the jet where the thermal discontinuity can be greatest, as well as just beneath the jet axis where strong eddies can occasionally shear off.
An important point to make about the polar jet is that not only is the exit region of a trough a place where pilots will often find fast winds and moderate to severe turbulence, but they are also where the jet is supporting vertical motion of the atmosphere. If you compare a 300 hPa map with a surface map, you'll often find a surface low sitting beneath the jet trough exit region.
The divergence of air in this part of a Rossby wave supports the strengthening of that surface low and may destabilize the air throughout the region, producing significant thunderstorm activity which may include hail and icing at the jet stream flight levels.
Entering and exiting the jet
In general, the pressure gradient surrounding the jet region is such that the jet region begins relatively abruptly. While the jet may be over 60 miles horizontally, it may be less than 2000 ft thick in places, and winds outside the jet may be many knots slower. This means that pilots should expect some shear turbulence any time they approach the jet edge.
In some places and times, such as over the north Atlantic and during the winter, the pressure gradient discontinuity may be more gradual, allowing entry or exit of the jet core with minimal shear. But in other places, such as the north Pacific or during the spring and fall months when temperature differences are highest, the pressure gradient and thus winds can go from weak to substantial over a few hundred feet and make for a rocky entry or exit from the jet stream.
Low level jets
Not all jet streams occur above FL300, however. The term jet stream simply refers to any current of air that is moving appreciably faster than the air around it. This situation can set up near the ground as well as in the middle troposphere. Mid-level jets often become established in the region between surfaces of significantly different temperatures. This can create a regional circulation pattern fueled by a weak thermal low that develops above the warmer area.
The low and circulation strengthen as the warm air is forced aloft, but a capping mid-level temperature inversion produces a high pressure at around 10,000 to 18,000 ft MSL. Such circulations are often found where large subtropical deserts abut cooler coastal water.
One very important mid-level jet is the East African Jet. This jet is formed by the circulation produced by the hot Sahara desert and the colder water of the Gulf of Guinea. As with most jets, the East African and other mid-level jets migrate with the Sun, moving northward during the months of June through September and southward between December and March.
The East African jet is important to aviation because it's part of the African southwest monsoon, which brings much needed rain, but also low ceilings and poor visibility to west equatorial Africa around July. Additionally, pressure variations aloft produce the shorter-wavelength tropical easterly waves that move west across the central Atlantic to become a key component in the production of tropical cyclones.
Closer to the surface, nighttime cooling can stabilize and decouple the atmospheric surface layer from the free atmosphere above. This separation creates a pseudo-surface at around 3000 ft or so (around 850 hPa), above which the free atmosphere still maintains a pressure gradient. Places where this becomes most pronounced are regions where terrain is rising gradually over a large distance, such as the US Great Plains. The higher altitude terrain cools the air in the free atmosphere more than lower altitude terrain can.
The result is a flow of air along this free-atmosphere boundary that may be anywhere from 25 to 50 kts. These low level jets are often responsible for quickly transporting much of the moisture that fuels the afternoon thunderstorms of the central US in summer. Normally, however, pilots climbing or descending through the low-level nocturnal jet barely notice the effect.
Terrain also plays a role in other, more localized jets. One in particular is the valley jet, which often forms at night as the surface atmosphere cools and the increasingly dense air begins to converge in the valley center and flow toward the valley mouth. When the draining air exits the valley, it encounters stiller air and will carve a current through it.
This effect is most pronounced in narrower valleys and where a valley abruptly terminates into a flat plain. Because they occur at low altitude and often in close proximity to rising terrain, valley jets can be dangerous to pilots attempting to depart or land at airports near the valley mouth.