Mobile Menu - Fr Mobile Search

Low-Level Jet

L
Weather Terminology Search
Alphabetical Search 

Legend

Legend

METAR Code

L LVL JET

Weather Symbol

HAZARDS

  • Frequently associated with strong wind events that can produce moderate to severe wind shear and mechanical turbulence.
  • Produces head and tail winds at different heights that can affect aircraft spacing on arrival.
  • Produces cross winds which affect runway selection and airport arrival rates.
  • Can play a role in sustaining convection.
  • May enhance the possibility of freezing precipitation due to the transport of warm air into the mid levels.
  • Reduction in visibilities (due to heavy rain, snow, and blowing snow) can be more severe in the presence of a low-level jet.

About

Definition

A core of relatively high wind speeds, typically centered at altitudes ranging between 300 and 3000 feet above the ground, on occasion as high as 7000 feet.

Associated terms coming soon:

Inversion and low-pressure system are terms associated with a low-level jet that will be coming soon to the Aviation Meteorology Reference.

Associated Terms

2

Low-Level Jet

Visualization

Image 1 of 3
  • Strong low-level jets are generally well forecast by numerical weather models, as they are a result of large-scale horizontal temperature difference. Weaker, more localized low-level jets pose more of a forecasting challenge, because they are dependent on many of the small-scale factors described in the Necessary Ingredients section.
  • Additionally, low-level jets can be difficult to observe, as they can pass over regions of Canada that are data sparse. While there are some satellite/radar indicators of the jet’s presence, it can be hard to follow their movement in more northern sites.
Image 1 of 3
Image 1 of 3

MAIN CONCERNS

Possible turbulence, abrupt changes in windspeed/direction, compression and loss of airport efficiency, loss of lift if enhancing freezing precipitation in the area.

Service Providers

Operations Duty Managers

CYYZ/CYUL- The low-level jet is always included in the supervisor briefing at the start of the shift and often covers multiple sectors.

  • It is used mainly for situational awareness but at major airports it can impact the arrival rate depending on orientation, altitude, and intensity.

Is it performance increasing or decreasing (slowing or speeding up aircraft on downwind or final approach) and will it increase or decrease the number of aircraft per hour? Also known as “compression.”

  • At which altitude are the strongest winds? Where are we seeing the biggest changes? These two questions are critical to understand and plan for impacts in the terminal. Knowing when conditions are expected to improve is also a question asked of the forecaster.

In winter months, low-level jet questions also revolve around any events that include risk of freezing precipitation.

This phenomenon can have serious safety implications for Flight Operations if aircrew are not made aware of its presence. Aside from the turbulence that low level jets can create, the velocity of wind may induce fuel consumption beyond the flight planned amount, resulting in a potential insufficient fuel event. Terrain can also alter the impact from a low-level jet by causing significant wind shift/shear on transition from cruise to arrival. Surface winds can be nearly calm from one direction, but not far aloft the wind can be 40-60 knots from the opposite direction.

FIC

When PIREPs or forecasts indicate the presence of low-level jet(s), this will be included in the “Significant Threats to Aviation Safety” introduction at the start of the Pilot Briefing, after the Pilot indicates that their proposed flight will be operating within the approximate location of the low-level jet. PIREPs will be solicited, including the absence of any low-level jet – this is to confirm the motion of the low-level jet, and to verify that operations in that area have stabilized.

AAS

Advisory specialists rely solely on PIREPs to verify and monitor low-level jets and the associated L LVL WS. We will tailor our runway determination and advisories to account for it. Light winds at the surface may favour one runway, but not far aloft the strong winds favour another runway, which could alter the runway determination.

Strong winds below 10,000ft have an influence on the spacing required between aircraft on final. More space equals less capacity.

What most affects terminal ATC with this are tailwinds on approach.

  • When we vector aircraft to final, we must respect minimum spacing (3 nautical miles or wake turbulence separation, whichever is greater), and in "regular winds" (with no special weather), we "lose" about 1nm of spacing between aircraft when starting at the same speed.
  • In other words, if we aim to finish with as close as possible to 3nm at the threshold, we should start with around 4nm (with both aircraft assigned the same speed). This 1nm loss is caused by the aircraft slowing down from our assigned speed (usually 160 knots) to their final approach speed right before landing.

When there is a tailwind, the amount of mileage we "lose" can go up to 2nm or even 3nm. This means that to finish with 3nm at the threshold, we must start with 5nm or 6nm between all our aircraft, same speed assigned. This greatly increases the workload for multiple reasons:

  • It's more difficult to gauge our timing, our eyes are "used" to vectoring 3-4nm in trail, doing it for 6nm takes more concentration.
  • During the entire approach, we witness aircraft catching up on each other, so we must monitor the "closing rate" even more precisely.
  • The wind is never constant, we might be losing 2.5nm and then suddenly we are losing 3nm. We must be able to recognize this and readjust as soon as the winds change.
  • Monitoring or scanning an approach path that has aircraft spacing constantly diminishing takes energy and concentration.

If there is a tailwind on final, there is a headwind on the downwind. Aircraft arriving "straight in" are way faster than usual, and aircraft on the downwind are way slower than usual.

  • Timing our base turn properly in order to start with the required spacing (5 or 6nm) takes a lot of concentration, and due to the speeds involved a slightly wrong timing (turning a few seconds too late or too early) can have a huge impact on the final spacing. We must constantly be readjusting ourselves.

The impact of low-level jets is two-fold.

Low-level jets can enhance the duration and intensity of freezing rain. If this occurs, an airport can see significant delays including temporary closure, and this will cause delays (see freezing rain for more details). If the consequence is strong winds, a couple of considerations must be taken into account:

If surface winds are in the same direction as winds within the core of the low-level jet, conditions are favorable in terminal for smooth operations and little compression.

  • However, strong winds aloft will increase the speed at which aircraft transit our sectors, leaving less time to sequence them for arrival into a major airport like CYUL. In CYUL, En-route ATC may tactically request earlier handoffs from adjacent sectors to help sequence aircraft for approach to the major airport.

If surface winds are opposite to winds in the low-level jet, this creates a much more complex situation. This can impact the distance between aircraft on final approach as traffic can slow down dramatically when they encounter the wind.

An example is in CYUL when winds are northeasterly at the airport but southwesterly aloft. Impacts:

  • Compression on final in the terminal area as aircraft move into northeasterly headwinds and slow considerably compared to their upstream counterparts. This requires additional mileage in trail in the en-route and terminal areas to maintain adequate spacing.
  • Sequencing becomes even more complicated with this required added mileage in the en-route while aircraft are flying relatively fast.

If the wind is a crosswind on the primary runway, it may cause some overshoots, or some aircraft types may not be able to land at all.

  • If there are strong winds or crosswinds, smaller aircraft will often overshoot their approach and proceed to their alternate destination.

En-route controllers deal with low-level jets indirectly. If there is reduced capacity due to strong winds, controllers will have to provide extra spacing between incoming aircraft as the terminal sectors will generally not be able to handle as much volume as normal.

Users

  • In general a low-level jet infers wind shear near the surface with abrupt changes in speed and/or direction which jet aircraft (rather than turboprops) are susceptible to.
  • A risk of a low-level jet would be identified in a GFA and/or a TAF and is associated with moderate-severe turbulence.
  • Mountainous terrain can interact with a low-level jet to produce turbulence.
  • Aircraft equipped with wind shear detection systems can automatically warn of wind shear while on approach, a warning that often necessitates go-arounds.
  • Wind shear at a major airport can be problematic due to the volume of aircraft involved along with a reduction in arrival rates due to compression of aircraft in strong head wind conditions.
  • Pilot reports are helpful and are heavily relied upon in validating the extent of WS involved.
  • Low-level jets tend to be present over large geographical areas necessitating longer alternate airports and increased fuel carriage (cost).

A forecast for a low-level jet (or wind shear) at a regional airport is difficult to manage as it is often associated with severe turbulence SIGMETs.

  • With less traffic moving in and out of an airport there is less access to PIREPs and any flights that operate are often pathfinders to determine whether conditions are safe to operate in.
  • In some cases a forecasted low-level jet or WS may lead a flight to not operate at all based purely on the forecast with no PIREPs to downgrade the weather phenomenon.

Numerous East Coast and British Columbia airports have a reputation for low-level jet/wind shear events.

If we encounter a turbulence SIGMET associated with a low-level jet, it is a factor that we will take into consideration in our planning. Once again, the goal is to make sure we have a reliable alternate airport.

A common practice when departing an airport is to use a reduced thrust setting in order to reduce wear and tear on aircraft engines. This is not permitted when wind shear is present.

Along the St-Lawrence valley, a low-level jet commonly generates a strong tailwind in the first 3000ft of the initial climb.

  • Knowing this phenomenon is present will lead us to use full takeoff thrust. GFA is a product which is not used for an overseas operation, mainly because we need a more global source of information.
  • A low-level wind shear forecast in the TAF will draw our attention, PIREPs are heavily relied upon to indicate the presence of wind shear.

A typical scenario in the St-Lawrence would be to have a northeasterly wind at the surface, with a strong tailwind at 3000ft. By experience, we expect that kind of condition with a warm front. A common verification that pilots will do when leveling off around 3000ft-4000ft is to note the wind at that altitude.

  • If it is assessed that a strong tailwind will be present on the approach, we will plan to make sure we extend flaps and gear earlier to have more drag.
  • A tailwind at that altitude will artificially steepen the descent on the glideslope, requiring a higher vertical speed.
  • Also, if the wind shifts from a 40KT tailwind at 3000ft to a 10KT heading on ground, we must lose that inertia created from the 50KT difference, on an artificially steeper approach.

If a General Aviation (GA) pilot is aware of the presence of the low-level jet, they should also be aware of the presence of turbulence associated with it.

  • Depending on the direction of flight the low-level jet may provide a beneficial tailwind component allowing the pilot to take advantage of increased ground speed, or on the other hand, a significant headwind component which will slow the aircraft considerably, requiring more time and/or fuel to reach the destination.

Even though upper wind forecasts can give an idea of conditions, GA pilots that have underestimated the effects of the low-level jet have often found a need to divert to another destination or sometimes even decide to return to their point of departure because the winds aloft were higher than predicted.

  • Some simple math to put it in context; A typical C172 with a cruising speed of 100KT would see a 50% reduction in ground speed with a low-level jet producing 50KT headwind at the cruising altitude.

Pre-flight weather checks should include GFA and a weather briefing from the local FIC. Not following these crucial steps can lead to unwanted and even dangerous surprises in-flight.

  • Many GA pilots don’t check the GFA or winds aloft and only base their decision-making on the METAR/TAF, especially if it’s just a local flight and not a navigation.
  • Sometimes low-level jet results in stronger surface winds, G25KT and more. Many of these pilots will be comfortable with a 25KT headwind, but without checking the GFA and upper winds, they don’t realize the impact of a decision to go flying.
  • The movement of the low-level jet as it approaches the airport may also come with a change in wind direction and if a pilot is unaware of the approaching low-level jet, they may elect to takeoff and will then be met with winds that are beyond their abilities for landing. If they are returning to the same airport, or if they have an emergency and need to return, this could present a hazard.
  • Some pilots do check the GFA and notice the low-level jet depiction, but from lack of experience are unaware of the visual depiction, this results in a similar impact to the decision making process.
  • For pilots that check the GFA and notice the low-level jet, they can then check the upper winds in the locations they will be flying to make an assessment of how it will affect the en-route portion of flight, and to what extent the low-level jet presence at the airport will increase risk for the flight.

Most GA aircraft and experienced pilots are capable of handling 30-35KT of headwind and around 15-20KT of non-gusting crosswind, however presence of gusts and variability in wind directions can create extra hazards and that’s why GA pilots are encouraged to develop a system of personal limits with respect to wind conditions.

Strong surface winds that may be related to a low-level jet aloft will also create hazards for small aircraft taxiing – they can be easily pushed around by the wind and less experienced pilots will have trouble controlling the aircraft.