Jet Stream

A major weather pattern that happens at or near the tropopause is the jet stream. Because most all airline flights occur within this region, it can greatly affect your travel in a couple of different ways.

Without getting into a heavy discussion of weather, the tropopause is the upper layer of the troposphere which is a major layer of our atmosphere. The troposphere is the layer that starts at the earth's surface and reaches an altitude between 24,000 and 50,000 feet. Temperature and winds can vary greatly in this area and affects your flight's fuel efficiency, ground speed, comfort, and safety. In fact, many airlines have available to your pilots in their preflight planning, charts on paper or the computer that marks the best knowledge of the height and location of the tropopause that will be encountered along your route of flight. Height of the tropopause varies from about 65,000 feet over the Equator to 20,000 feet over the poles and also depends upon the time of year (summer, winter). An abrupt change in the temperature is an indication of the height of the tropopause.

Maximum winds can occur at levels near the tropopause and can be an indication of where you can expect Clear Air Turbulence (CAT), more on this in another section.

The jet stream is like a river of wind that extends across the globe. It occurs at breaks in the tropopause and is defined as wind of 50 knots or greater. It is strongest in the winter months in the mid-latitudes when it shifts toward the south. Besides the obvious comfort and safety considerations when figuring the jet stream, is the speed and fuel burned when getting to your destination. Winds blow from west to east in the mid-latitudes (roughly 30 degrees North to 60 degrees North latitude). When it shifts further south in the winter to occupy much of this area, winds will greatly increase from west to east. That is why the same flight will take longer going east to west (strong headwinds!) than in the summer. Of course, it will be a lot shorter the other way (west to east)!

While timetables and airline schedules take this into account with their winter schedules, no one can predict how strong or the location and direction of the jet stream can be. For example, flying from New York to California can be scheduled for 6 hours from takeoff to touchdown. But if the jet stream that is present over the United States shifts direction and speed, it can vary the time considerably. This is where close cooperation between your Captain and Flight Dispatcher can make the difference. Decisions need to be made in a couple of areas. What is the passenger and cargo weight Weight-Restricted considerations? If the most important thing is to stay close to the scheduled time, then generally speed must be increased to overcome the increased headwinds we now have. This could mean more fuel burned. If we are at maximum weight for takeoff or landing then passengers and/or cargo might have to be taken off to accommodate more fuel. Another option, if it is determined that taking off passengers and/or cargo is not wanted, is a fuel stop along the way so weight limitations are adhered to. But then, time could be added for this fuel stop. However, if you fly faster then normal (and burning more fuel), and your fuel stop has been alerted and is good at getting you fueled in a short time ("short-turnaround"), then this can be overcome.

As you can see, there are a lot of variables to play with. This is where the operational experience of your pilots, flight dispatch, and operations of your respective airline can make the difference between your flight getting to your destination on time or being late because of this jet stream phenomemon. Jet streams can vary widely in strength, location, and height. This is just another variable, that while taken into account on an average basis, reflects the flexibility, experience, and common sense needed for operational efficiency.

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