WX BRIEF

Jet streams

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


Finding the jet

Jet stream winds at 300 hPa (around FL350) over the North Atlantic. White area represents a jet streak in which winds at the core exceed 150 kts. Eastbound traffic may find a route through the jet stream to be more advantageous than a great circle route, while westbound traffic is better advised to use a polar route.

Low level jets can often be forecast by looking at winds aloft for temperature inversions at around 850 hPa at night, coupled with an increase in wind speed directly above the inversion, generally flowing in the direction of rising terrain.

Similarly, a look at 700 and 500 hPa weather maps will help pilots to quickly identify the location and strength of mid-level jets that may exist between areas of higher and lower pressure aloft.

However, most pilots will be interested in finding the polar and subtropical high altitude jet streams. These high speed corridors are very important to long-haul flights, as they can add or subtract hours and hundreds of gallons of fuel from a transcontinental flight. These jet streams are so important to aviation that they are factored into the twice-daily assignment of North Atlantic Tracks System (NAT-OTS) and Pacific Organized Track System (PACOTS).

NAT-OTS and PACOTS are systems that ATC uses to assign optimal routing across the heavily trafficked air routes of the North Atlantic and North Pacific. The primary role of these tracks is to keep aircraft separated, but they are adjusted twice each day to ensure that eastbound traffic can use the jet stream's tailwinds effectively, while westbound aircraft will not have to fight strong headwinds.

Thus, when pilots are assigned a track across one of these oceans, they can be reasonably confident that the headwinds and tailwinds have already been taken into account by ATC.

Elsewhere, pilots can refer to the 300 and 250 hPa weather charts to find the high altitude jets. In winter months, you may need to look at lower levels (400 & 300 hPa), while in the summer, 250 or even 100 hPa charts may be appropriate. Most of the time these currents are well defined meanders within which the wind speed barbs indicate winds that are some 30–50 kts stronger than the adjacent winds. Routings that maximize tailwinds or minimize headwinds can then be determined.

In the absence of high altitude weather charts, a surface chart that shows fronts will suffice. The jet stream will normally rest close to the position of the warm and cold fronts surrounding a surface low and will ridge over the top of a surface high. In flight, a ribbon of cirrus often gives away the position of the jet as the vertical motions within them can generate condensation and the strong winds shred the thin clouds.

As always, if actual conditions don't live up to the maps or forecasts, or if you find better winds at a different altitude, please file a pirep.

Karsten Shein is a climatologist with NOAA in Ashe­ville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.


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