OFTEN we sit in an airport’s departure lounge to see the flight crew boarding ahead of us whilst we patiently wait until called. What we do not realise is that the pilots and flight engineers started work long before, in meteorological briefings at our departure airport. The reasons for these meetings will be evident later.
Why does a flight from Kuala Lumpur (KLIA) to London (LHR) take one hour 75 minutes longer than our return flight from LHR to KLIA? Why does a flight from LHR to New York (JFK) take one hour 20 minutes longer than a return flight from JFK to LHR? Your correct informed guess is right, but it is only partially connected to the spinning of our planet in its orbit from west to east.
On Jan 8, a British Airways Boeing 777-200 passenger airline approached near supersonic speed recording a ground speed of 1,200 km per hour (kph). The speed of sound at ground level is just 25 kph faster. The total flight time for this aircraft in its journey from JFK to LHR was five hours and 16 minutes, arriving in London some 90 minutes ahead of schedule.
This record flight was the result of a very powerful jet stream, allowing the aircraft to reach near supersonic speed through the knowledge gained by the pilots at their earlier meteorological briefing.
Undoubtedly flying with a tail wind in a jet stream of air reduces flight times to the happiness of passengers and reduces fuel costs for the airline. No, British Airways did not compensate passengers as a result of this record flight.
I liken a jet stream to the effect seen in modern gymnastics when a gymnast performs in most artistic and athletic ways whilst thrashing a hand-held ribbon through the air.
The ribbon twists and twirls in undulations and meanderings much like the path of a jet stream.
Between 10km and 15km above ground level, in the upper atmosphere, such snake-like meanderings twist and turn as they are fuelled by the warm and cold air masses below. Jet stream air tunnels are 80km to 640km wide and the inner air tunnel can move at 4,000kph.
It is the pilot’s skill to take an aircraft into the core of a jet stream without causing passengers discomfort during the entry and exit from these tubes of air. Clearly, once entering the core, an aircraft is given extra momentum.
Jet streams are not new phenomena. The 1920s saw a Japanese professor Ooishi tracking high altitude balloon movements near Mount Fuji on Honshu Island. The results of his research, written in Japanese and later translated into Esperanto, escaped the rest of the world.
In World War II, the Imperial Japanese Forces launched incendiary devices (called fugos) attached to high altitude balloons, which would enter the west to east-flowing jet stream, to then explode upon impact amongst the pine clad forests of the north western coastal states of the United States.
Fortunately only one ninth of the total of balloons launched ever reached their targets. In 1944, a B29 US reconnaissance aircraft in its flight to photograph Tokyo found that its ground speed was reduced to121kph as the headwind was 282kph. Later, bombing attempts on Tokyo fortunately saw bombs blown well away from the city as aircraft faced headwinds of 270kph.
It was in 1945 when a German meteorologist, Carl Rossby, coined the term jet stream. There are between two and three jet streams in each hemisphere at altitudes of 10,000 to 15,000 metres, lying above the lower atmosphere where temperature contrasts between warm and cold air fuel the strengths of each jet above.
This is well exemplified in the northern hemisphere with colder air masses to the north and warmer air masses to the south. Naturally, the temperatures of these air masses are reversed in position in the southern hemisphere.
The Polar Jet meanders between 40 and 60 degrees north of the Equator and is responsible for directing rapidly moving cyclones over Western Europe and the west coast of North America in the winter months. In these latitudes in the summer months, this jet weakens as anticyclonic (high pressure) patterns prevail, creating higher temperatures.
During the United States’ winter weather, a polar jet stream can lift warm Pacific air on its northerly route towards Alaska to give much higher than average temperatures there and on its meandering southern course sweep across the colder middle North American continent to bring severe frosts as far south as Florida where the blossoming orange groves receive frost bite.
The westerly blowing Subtropical Jet lies between 30 degrees north and south of the Equator and is at its strongest over India. This jet is considered to be the most powerful of all wind systems yet, surprisingly, does not affect the underlying weather as much as the Polar Jet.
The third jet stream rides at about 10,000 metres above sea level and directly affects Borneo’s monsoonal weather patterns. This is the Tropical Easterly Jet from October to April but from May to September it becomes the Tropical Westerly Jet when it is at its strongest.
Nearly a century after jet streams were first discovered, we are still not fully sure of their full effects upon our weather. We are almost there in research and the vast collection of daily weather recordings worldwide and upwards to the upper atmosphere and beyond will soon provide the answers.
Climatic change will continue to dog our weather patterns and in doing so affect the strength, movements and timings of these overhead jet streams wherever we live on Earth.
Recently, I saw snow and hail fall at night only to melt in the morning sun in temperatures which on that day reached 10 to 12 degrees Celsius. One night there were gale force winds after a totally calm day and the following night more snow was forecast, yet in the following two days temperatures again rose to well above freezing point. The Polar Jet is doing its very best to blast me into space!
Just one thought: when next on a long haul flight, watch your video screen and observe live your plane’s speed and whether there are tail or head winds, which will affect the time of arrival at your destination.
If your flight is much quicker than expected, then thank your lucky stars that your pilot managed to get you to surf a jet stream.