The so-called jet stream is a strong wind belt from west to east, with a height of 8- 12km and a speed of 320km per hour. It can be thousands of kilometers in length and hundreds of kilometers in width. The jet stream flows along the boundary between cold air and hot air. Because the dividing line between cold and hot air is most obvious in winter, the jet stream in winter is the strongest in both the northern hemisphere and the southern hemisphere.
Northern hemisphere cross section showing jet stream and tropopause height
Image source: /2020/ 10/
The rapids also change with the height of the sun. In spring, with the daily increase of the height of the sun, the average latitude of the rapids moves to the polar regions. In summer, the rapids in the northern hemisphere usually appear near the US-Canada border. As autumn approaches, the height of the sun decreases and the average latitude of the rapids moves to the equator.
This "high speed in the air" has a jet bonus!
Knowing the causes of rapids, will you still be unfamiliar with them? However, the role of rapids in the flight journey is no stranger to everyone.
If you have ever paid attention to long-distance international flights, you will find that the time required for the plane to travel back and forth between the two places is generally different, and the farther the distance is, the more obvious this situation is. Take Beijing and London as examples. After taking off from Beijing, the direct flight from Beijing to London will fly westward, with a journey time of 10 hour and 55 minutes, while the flight time from London to Beijing only takes 9 hours and 45 minutes.
Why is there such a big difference between the same distance and the round-trip time? And why do all flights from east to west take longer than those from west to east? Someone once asked: Is the difference in flight time related to the direction of the Earth's rotation?
It really doesn't matter! When the plane flies in the sky, it does not get rid of the gravity of the earth, and it is still in the inertial system of the earth, so it will not be affected by the rotation direction of the earth. Otherwise, the plane can circle the earth 1 day without flying. However, although the rotation of the earth does not directly affect the flight speed of aircraft (the direct influence of Coriolis force is very small), another change brought by the rotation of the earth has become the main factor affecting the flight speed of aircraft, that is, jet stream.
In order to understand this problem easily, let's first understand two concepts-airspeed and ground speed. Aircraft flying in the air has a moving speed relative to the atmosphere, which is also called airspeed. Accordingly, the speed of the plane relative to the ground is the ground speed. In order to ensure the smooth flight of the aircraft in the atmosphere, the aircraft needs to maintain a stable airspeed. So the ground speed at this time is equivalent to the superposition of airspeed and wind speed. So the ground speed will be higher when the plane is downwind and lower when it is upwind.
A flight east across the Atlantic with a strong westerly wind.
Image source: /2020/02/ 1 1/ Surfing-Rapids-Emission Reduction-Aviation-Radiation/
The mid-latitude jet center where the Atlantic "high speed in the air" is located is located in the stratosphere with a pressure surface of 200 hectopascals, where the wind direction is horizontal and the aircraft is relatively not bumpy, which has become the flight altitude of most civil aviation aircraft. There is a steady westerly wind blowing in the stratosphere in the mid-latitude area all the year round. As mentioned above, the ground speed of downwind flight is higher, and the aircraft in this area fly from west to east along the west wind direction, which can save more time.
The average wind field (vector) and wind speed (color) of 200 hectopascals in 2007.
Image source: https://www.ncep.noaa.gov/ (National Environmental Prediction Center)
Taking new york and London as examples, if the difference of round-trip routes is not taken into account, according to the same distance of 8 175km, the downwind flight time is about 6.4 hours, while the return journey takes 7.5 hours, the difference is more than 1 hour. The shorter the time, the less fuel is used. According to Boeing 777' s fuel consumption of 2,700 gallons per hour, downwind aircraft can save 2,970 gallons of jet fuel. If calculated at $65438 +0.89 per gallon, you can save $5665438 +03 per flight, which is really not a small sum.
It is not difficult to find that it is time-saving and safe to use rapids for long-distance flights. With this advantage, it is not surprising that the "air speed" in the Atlantic Ocean can become the only way!