(WRDW/WAGT) -- Earlier this week, a weather balloon launched from Upton, New York measured a wind speed of 231 mph, which is a new record for the site. The wind speed was recorded at the 250 millibar (mb) level, which is a measure of pressure, the usual form of gauging height in the atmosphere. Pressure decreases with height in Earth’s atmosphere. At the surface, pressure usually fluctuates between 1000-1030 mb. The 250 mb level is usually between 30,000-35,000 feet up and where the jet stream can usually be found.
Surface level conditions were actually quite nice that day. Winds calm, temperatures in the 40s and skies clear, which goes to show that what you see at the surface doesn’t always tell the full story. To get a better understanding of what drives weather patterns in the mid-latitudes, we need to go about 6 miles above the surface.
According to the National Weather Service, jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere. They form due to differences in air temperature and the rotation of the Earth. Wind inside jet streams flows from west to east, but the overall flow of jet streams shifts north and south. In the northern hemisphere we have the polar jet stream, which is usually located around latitude 50-60° N, and the subtropical jet stream usually located around 30° N.
The impressive 231 mph measured wind speed occurred in a jet streak. A jet streak is a narrow region embedded in the core of the jet stream where the fastest wind speeds can be found. The jet streak occurs from convergence (air piling up) and divergence (air spreading apart), which leads to winds slowing down and speeding up. Convergence and divergence are caused by the rounding of troughs and ridges in the meandering air current.
The jet stream is very important for air travel. Planes use the jet stream as a tailwind when traveling west to east and try to avoid the air current when traveling in the opposite direction. A plane earlier this week hit the record setting jet streak and clocked a ground speed of 801 mph!
The Virgin Atlantic flight was traveling from Los Angeles to London when it hit the jet streak over Pennsylvania. The aircraft was a Boeing 787-9 Twin Jet and has a usual cruising speed around 560 mph and max propulsion of 587 mph. 801 mph is faster than the speed of sound (767 mph), but the plane did not break the sound barrier because it had an 801 mph ground speed, not air speed. The concept can be understood through vector addition. The plane travels at air speeds between 560-580 mph and then hits a tailwind around 230 mph, adding them together leads to a ground speed in the ballpark of 801 mph. In order for the plane to break the sound barrier, it would have to travel an air speed faster than 767 mph, not ground speed.