Science: Jet Stream for Jetliners

At New York City’s International Airport one day last week, the official report was that the visibility and ceiling were unlimited, and the wind on the surface was blowing from the north at a mild 11 m.p.h. But Louis Harmantas, the Weather Bureau’s chief meteorologist at the airport, had a very different report on the invisible weather six miles up. There the wind was roaring out of the south-southwest at 104 m.p.h. At the same altitude and about 100 miles east-southeast of the airport, the great jet stream itself, flanked by belts of turbulence, hurtled toward Newfoundland at 160 m.p.h.

Such winds do not directly concern the pilots of conventional propeller planes, whose normal ceiling is in the quieter air below 20,000 ft. But the great new jetliners cruise most efficiently in the high, thin air above. Their crews and dispatchers need detailed, fresh information about the tremendous high-altitude winds that crisscross the middle latitudes. To meet their need, the Weather Bureau has announced plans for a new high-altitude forecasting service, hoped to have it in full operation in time for next month’s start of jetliner service across the U.S.

Twisty Center. The bureau will forecast flying weather up to 42,000 ft. Most of the information will come from weather balloons launched every six hours and reported to seven main stations (Suitland, Md., New York City, Miami, San Juan, P.R., San Francisco, Honolulu and Anchorage, Alaska). Electronic instruments dangling under the balloons will report temperature and humidity at the various levels. As the balloons climb through the air layers, their motion will be tracked electronically, revealing the direction and speed of the high-altitude winds. At the National Weather Analysis Center at Suitland, the data will be digested, plotted on charts and sent by facsimile transmitters to airports across the country.

Chief target of the bureau’s new service will be the jet stream itself, which is generally found around 30,000 ft., sometimes blows faster than 230 m.p.h. The jet stream is not easy to keep track of; it snakes and thrashes around like a whipping rope, changing both speed and altitude. A jetliner that gets into its core may arrive at its destination hours ahead of schedule with its tanks still heavy with unburned fuel. But judging by the experience of Air Force pilots, whose jet bombers have been flying the unfamiliar highways of the upper air for years, commercial pilots will probably not find it worthwhile to try for this maximum joyride. The stream’s twisting center is hard to follow, and it often takes the airplane far from its course. Most pilots will be content to pick up 50 to 100 miles of free speed by flying in the stream’s vicinity.

The general direction of the jet stream is from west to east; jetliners flying westward will usually pick their courses and altitudes to avoid it. But sometimes the jet stream or the current associated with it loops into a westerly direction. When the charts reveal such a shift, an alert pilot might get a jet stream assist both coming and going.

Cobbled Turbulence. Besides keeping tab on the jet stream, the Weather Bureau’s new service will chart the ever-changing altitude of the tropopause, the varying boundary around 30,000 ft. between the troposphere (lower atmosphere) where the temperature generally decreases with altitude, and the stratosphere above it, where the temperature remains relatively constant.

The tropopause is a tough neighborhood, where violent winds jostle each other, break into swirls and eddies or porpoise up and down. This churning air (one kind is called “cobblestone turbulence”) is often clear of clouds and therefore invisible, but it can seriously shake up a jetliner that slams through it unwarned at 690 m.p.h. Guided by the new Weather Bureau forecasts, a pilot can fly under, over or around the bumps, keeping the gravy off his passengers’ clothes.