JET STREAMS
Jetstreams are simply narrow bands of high-speed upper thermal wind at very high altitudes. They can be found near the tropopause and play a vital role on the weather experienced on earth.
Thermal winds are generated by a difference in temperature (thermal gradient) between two columns of air over large areas and at great upper heights.
The official definition of a ‘jetstream’ is “a strong, narrow current on a quasi-horizontal axis in the upper tropopause or stratosphere characterized by strong vertical and/or lateral wind shear (CAT). The wind speed must be greater than 60 knots for a wind to be classified as a jetstream.”
Jetstreams typically are 1500 nautical miles long, 200 nautical miles wide, and 12,000 ft deep, and their speed is directly proportional to the thermal gradient; i.e., the greater the thermal gradient, the greater is the speed.
Jet streams are driven by thermal gradients and therefore are found wherever the thermal gradient is high enough. There are two bands of rapid temperature changes in each hemisphere that are marked enough to produce a jetstream.
POLAR FRONT JET:
Firstly, at the polar front around 60º of latitude, where the polar air meets the subtropical air. This is a polar front jetstream, and is the most marked thermal gradient to be found, especially when it is over land in the winter.
Secondly, at the intertropical front, where the subtropical air meets the tropical air. This is known as the ‘intertropical front jetstream’. This is also known as the weaker (10–16 km) ‘subtropical jet’, found at roughly 30ºN/S.
The jetstream exists just below the tropopause in the warm air of a pressure system; i.e., the subtropical air at the polar front and in the tropical air at the intertropical front, but appears on the surface chart to be in the cold sector. This is so because of the slope of the front with height.
The jet moves with the front (i.e., south in the winter), and its direction is not always westerly because the jet follows the pressure system. In fact, the polar front jetstream can blow from 190 to 350 degrees around the polar front. However, its overall direction generally is regarded as being from west to east.
The maximum wind shear/clear air turbulence (CAT) associated with a jetstream can be found level with or just above the jet core in the warm air but on the cold polar air side of the jet
The major westerly jetstreams encircling earth are found in both hemispheres. They can be thousands of miles long, hundreds of miles wide, just a few miles deep, and reach speeds over 200mph. Jetstreams are products of atmospheric heating and coriolis force. Pressure difference caused by the temperature gradient produces a pressure gradient force, acting from areas of HIGH to LOW pressure.
Due to coriolis force, jetstreams flow parallel to the temperature gradient, moving from west-to-east, and meandering north-to-south between the boundaries of hot and cold air.
Where the jetstream meanders, air accelerates and decelerates. These areas are ‘developing areas’. Where are accelerates, air leaves faster than it enters, which leads to depletion of air in this area. In order to fill this depletion, air from below rises up. This rising of air leads to the convergence of surface winds to the rising air column, which results in a lower surface pressure.
The rising air expands, cools and condenses into cloud and precipitation.
Low pressure systems then may develop or deepen further.
Pilots must exercise for Clear Air Turbulence (CAT). The CAT is strongest on the cold side of the jet where wind shear is greatest. Can can be encountered anywhere from 7,000 ft below to about 3,000 ft above the tropopause.
HAZARDS ASSOCIATED WITH JETSTREAMS.
- If encountering unexpectedly strong headwinds, flight crew should;
— Record Groundspeed and Fuel Burn and fly 2,000 ft to 4,000 ft lower while comparing Groundspeed and Fuel Burn to those levels.
— Pilots should fly at a level that gives best specific ground range
(lowest kilogram/nautical mile figure).
— When free of winds, consider flying long range cruise/economy speed to conserve fuel.
— Start contingency planning for an intermediate fuel stop.
— Consider consequences of longer flight time?
— Destination weather forecast, opening times, crew duty times, etc. - Aircraft descending into stronger winds should EXERCISE CAUTION not to exceed Vmo (overspeed). Likewise, aircraft flying into increase tailwinds should anticipate a reduction in IAS to not end up in an airspeed low scenario.
- When flying in an area of forecasted CAT; err on the safe side and keep passenger seat belt sign illuminated.
- British Airways recently set a new record for the fastest subsonic flight from NYC to London, reachign a top speed of 825 mph over the North Atlantic by exploiting the jetstreams to increase its groundspeed.
- Flying upstreams would reduce planned fuel reserves and in extreme cases cause PIC to declare a fuel emergency, or initiate a diversion to another airfield where refueling is possible.
- Seek info from other aircraft ahead, north and south of your planned track to find out what winds they are experiencing and consider adjusting your route to avoid the high winds.
- AIRMANSHIP. A rapid change in outside air temperature (OAT) indicates passage from one air mass to another. If an aircraft is flying in the vicinity of the tropopause, this may indicate the likely presence of a jetstream.
- Jetstreams passing over mountains usually produce waves of considerable amplitude, and increases the risk of turbulence.
A SIGMET gives a concise description of the phenomena in abbreviated plain langugage. Such description contains information issued by a meteorological watch office concerning the occurrence, or expected occurrence, of specific en-route weather phenomena which may affect the safety of aircraft operations (ICAO Annex 3).
Apart from giving forecasts of the location and level of CAT, SIGMETs may describe the following weather phenomena:
- Thunderstorms
- Cyclones (tropical revolving storms)
- Severe turbulence
- Severe icing
- Severe mountain waves
- Dust/sand storms
- Volcanic ash
Info on local terrain induced CAT may be contained in the AIP.
RIDING OUT A JETSTREAM:
- SLOW DOWN;
Reducing aircraft speed will reduce the risk of any potential structural damage, as well as reducing vibration for a smoother instrument scan. - STRAP IN;
Notify crew/illuminate seat belt sign. All passengers and crew should sit down immediately and fasten their seat belt/harness. - SWITCH ON ENGINE IGNITION;
It is recommended that engine ignitions switch be turned on, on certain aircraft types, to prevent turbulence airflow from flaming out the engines. - INFORM AIR TRAFFIC CONTROL;
Notify ATC/warn other aircraft on GUARD (121.5/243.0). Request clearance to climb/descend or diverge from track to escape turbulence. - ASSESS DAMAGE/INJURIES;
Carry out a damage assessment and ascertain condition of any injured passengers. Consider precautionary diversion. - SUSPEND CABIN SERVICE;
Obviously the serving of hot drinks and meals during turbulent conditions puts both cabin crew and passengers at risk.
